Isidora Stojanovic
email: isidora.stojanovic@ens.fr

Disagreements about Taste vs. Disagreements about Moral Issues

Keywords: predicates of personal taste, evaluative predicates, moral terms, disagreement, experience-sensitivity 1. Noriko: "This is delicious

The aim of this paper is to argue against a growing tendency to assimilate moral disagreements to disagreements about matters of personal taste. The argumentative strategy adopted in the paper appeals to a battery of linguistic criteria that reveal interesting and important differences between predicates of personal taste and moral predicates. The paper further argues that these semantically tractable differences have an impact on the nature of the corresponding disagreements.

Introduction

The phenomenon of faultless disagreement has been one of the most discussed topics in philosophy of language over the past fifteen years. It has triggered a great deal of interest in predicates of personal taste (henceforth PPTs) such as 'delicious' and 'fun', not only in philosophy but also in linguistics. By 'faultless disagreement', I mean the uncontroversial observation that there are situations in which the two parties appear to be disagreeing, and at the same time there does not appear to be any clear, objective way of telling who is right. This puzzling observation is typically presented via dialogues of the following form:

Let the dialogue be taking place in a restaurant, and let 'this' refer to freshly chopped parsley. 1 Suppose that parsley tastes delicious to Noriko but awful to Suleïman. Then it seems that neither of them is at fault in what they are asserting, or, at any rate, that neither of them is wrong in the same way as one who asserts, say, that Yerevan is in Syria. At the same time, the conversation in (1) looks like a disagreement, which is indicated by the use of the negation particle 'no' followed by Suleïman's assertion of the negation of the sentence asserted by Noriko. This paper is not concerned with the question of how best to account for faultless disagreement. 2 Rather, it is concerned with the question whether disagreements about matters of personal taste are to be explained in the same way as moral disagreements, which are also taken to generate resilient disagreements, disagreements that cannot be resolved in any straightforward way by appealing to matters of fact. Consider: 2. Noriko: "This is morally wrong."

Suleïman: "No, it isn't."

Let 'this' refer to a certain action that is approved of by the system of moral values endorsed by Suleïman, but disapproved of by the one endorsed by Noriko; to make it concrete, let them be talking about Suleïman's enjoying porn movies. Our question is, then, whether the disagreements in ( 1) and (2) ought to be accounted for in the same way.

Of course, (1) and ( 2) differ in that (1) concerns gustatory taste, and (2) morality; but once we abstract over their respective domains, do (1) and ( 2) display the same pattern? Common sense will answer 'no'. Most of us are happy with the idea that our gustatory preferencies may diverge, and that in such cases, there is no point in arguing whether something tastes delicious; as the proverb has it, de gustibus non disputandum. By contrast, we would balk at the thought that our moral preferencies may diverge and that there is no point in arguing whether a given action was morally acceptable or not. Even the phrase "moral preference" may make us uncomfortable, because we take morality not to be a matter of preference. If somebody tells you that they prefer to listen to reggaeton over jazz, you may think that they have bad taste in music, but, after all, it's their choice. But if somebody tells you that they prefer a system that allows for owning slaves and treating them as one pleases, you will not think "It's their choice"; rather, you will consider such a person as a wrongdoer who 1 Faultless disagreement is typically illustrated by artificial examples, and it is actually difficult to obtain naturally occurring dialogues of the form of (1). The closest I got in corpus search is this rant about parsley: "Seriously. Who could legitimately put parsley in their mouth have it chopped up and rubbed all over their taste buds and say "Wow this is delicious". No it isn't and you must be lying." Source: https://www.reddit.com/r/rant/comments/2y1hj2/fuck_parsley/ 2 In previous work [START_REF] Stojanovic | Talking about taste: Disagreement, implicit arguments, and relative truth[END_REF][START_REF] Stojanovic | Emotional disagreement[END_REF], I defended a broadly contextualist approach to disagreement. I have subsequently come to believe that the phenomenon is quite complex, and that the variety of broadly contextualist proposals that have been put forward (including [START_REF] Glanzberg | Context, content and relativism[END_REF][START_REF] Stephenson | Judge-dependence, epistemic modals, and predicates of personal taste[END_REF][START_REF] López De Sa | Presuppositions of commonality: an indexical relativist account of disagreement[END_REF][START_REF] Sundell | Disagreements about taste[END_REF][START_REF] Pearson | A judge-free semantics for predicates of personal taste[END_REF][START_REF] Crespo | Affecting meaning: subjectivity and evaluativity in gradable Adjectives[END_REF][START_REF] Hirvonen | Predicates of personal taste and perspective dependence[END_REF][START_REF] Marques | Disagreeing in context[END_REF], Umbach 2015, Capraru 2016, i.a.) need not compete but often complement each other; see [START_REF] Stojanovic | Context and disagreement[END_REF] for discussion.

has serious problems with morality.

The aim of this paper is to further substantiate the view that disagreements over matters of personal taste pattern differently from moral disagreements. The paper is organized as follows. Section 2 lays out the general structure of the argument, and situates the view vis-à-vis the dominant tendencies in the literature. Sections 3 and 4 provide theoretical and empirical evidence to the effect that expressions of personal taste behave differently from expressions of moral value. Section 5 argues that these linguistically tractable differences have significant repercussions for the nature of disagreement in the two domains.

2

Questioning the motivations for a uniform account

The intended take-home message of this paper is that we should neither assume nor strive for a uniform account of disagreements over matters of taste and moral disagreements. As already pointed out, the position defended here is supported by common sense. One may wonder, then, why there should be any need to argue for such a position. The reason is that the dominant trends appear to go in the opposite direction. This section begins with a rough overview of these trends and continues with an outline of the observations, developed in the remainder of the paper, that raise serious worries for the prospects of offering a uniform account of the two kinds of disagreement.

Simplifying greatly, there seem to be three main tendencies in the literature with regard to whether disagreements about taste and moral disagreements behave alike:

[i] to assume, without further ado, that a uniform account is appropriate;

[ii] to seek a uniform account, based on explicitly stated motivations;

[iii] to express caution regarding the prospects of a uniform account.

My targets in this paper are [i] and [ii]. I hope to demonstrate that the assumption made in [i] is not only unwarranted, but promises to be incorrect. Furthermore, I will argue that the motivations typically appealed to in [ii] are insufficient to warrant a preference for a uniform account, given that there exist important differences between judgments of personal taste and moral judgments. The conclusions at which I will arrive may be seen, then, as vindicating the cautious attitude in [iii].

The assumption that PPTs and moral predicates behave alike, and that, relatedly, disagreements over matters of personal taste and disagreements over moral issues also behave alike, appears to be a dangerously widespread assumption. In his pioneering defense of relativism, Max Kölbel presents us from the outset with the sentences "Licorice is tasty" and "Cheating on one's spouse is bad" as illustrations of one and the same phenomenon (Kölbel 2002: 19). Ever since, there has been a robust tendency to simply assume that PPTs, moral predicates and various other expressions such as aesthetic and epistemic predicates, all belong in one and the same batch of expressions that raise the same sort of challenges and call for the same sort of explanation. The assumption is commonly slid into the common ground without being explicitly stated, as the opening words of [START_REF] Marques | Disagreeing in context[END_REF] illustrate: "When people have disagreements about taste, or about aesthetic or moral values, what is their disagreement about?" (2015: 257).

The danger behind [i] is that we find this assumption in a number of influential proposals. In the relativist camp, beside Kölbel's approach, we find it, for instance, in Richard ( 2008), as well as in [START_REF] Egan | Relativist dispositional theories of value[END_REF][START_REF] Egan | There is something funny about comedy: A case-study in faultless disagreement[END_REF]. And, although MacFarlane (2014) does not explicitly discuss moral predicates, his endeavor to apply his assessmentrelativist framework to PPTs as well as to the deontic modal 'ought' also reveals a tendency towards a uniform account. In the contextualist camp, the assumption underlies, among other, the metalinguistic approach in [START_REF] Plunkett | Disagreement and the semantics of normative and evaluative terms[END_REF] and [START_REF] Sundell | Disagreements about taste[END_REF][START_REF] Sundell | The tasty, the bold, and the beautiful[END_REF], and the discourse-contextualist approach in [START_REF] Silk | Discourse contextualism[END_REF].

To be fair, it is not always clear that an author simply takes the preference for a uniform account for granted, rather than doing so on a motivated basis. In Kölbel's case, the idea that disputes about whether licorice is tasty and about whether cheating on one's spouse is bad are both perceived as giving rise to faultless disagreement could be seen as a motivation for a uniform account. Indeed, in later work (e.g. [START_REF] Kölbel | The evidence for relativism[END_REF], he adduces four types of motivation: (i) what he calls "basic evidence", namely that certain dialogues involving the relevant expressions appear to give rise to faultless disagreement; (ii) object language assessments of what is said; (iii) object language speech reports; (iv) the behavior of a given expression with respect to modification by a 'for'-phrase. Similarly, it could be said that MacFarlane's endeavor to provide a uniform account of PPTs and the deontic 'ought' is motivated by phenomena such as third-party assessment, retrospective assessment, and various norms of assertion and retraction. In the contextualist camp, it can be suggested that the approach in [START_REF] Plunkett | Disagreement and the semantics of normative and evaluative terms[END_REF] and [START_REF] Sundell | Disagreements about taste[END_REF][START_REF] Sundell | The tasty, the bold, and the beautiful[END_REF] draws motivation from the idea that PPTs, moral and aesthetic predicates all behave alike with respect to the phenomenon of metalinguistic negotiation. Last but not least, Crispin [START_REF] Wright | Intuitionism, realism, relativism and rhubarb[END_REF][START_REF] Wright | Truth, objectivity, realism and relativism[END_REF] highlights four features that he takes to be exhibited by both "basic taste" and "basic moral" judgments: Faultlessness, Contradiction, Sustainability, and Parity. He then relies on the assumption that these four features together characterize both kinds of judgments, to propose a unified account of disagreements about taste and disagreements about moral issues. 3 It goes beyond the scope of the present paper to examine the motivations that these different authors adduce and rely on. Fortunately, this will not be needed, for my aim is not to establish that a uniform account is impossible. My aim is more modest. Against the tendency in [i], I want to show that it is wrong to assume without further ado that moral judgments naturally belong in the same broad family as judgments 3 To be sure, [START_REF] Wright | Intuitionism, realism, relativism and rhubarb[END_REF] does not claim that all moral judgments exhibit those features, for he writes: "Assuming that there are indeed disputes as so characterized, it is of course an important and controversial issue how far they extend-whether, for example, certain differences of opinion about ethics, or aesthetics, or justification, or even theoretical science, come within range" (2006: 37). reflecting personal taste; against the tendency in [ii], I want to put pressure on those authors who seek a uniform account of PPTs and moral predicates based on certain types of motivation (such as, typically, intuitions about disagreement) by confronting them with certain significant differences between these two classes of predicates, which, in turn, provide motivation against a uniform account. 4 The core observations that this paper makes are simple and can be summarized as follows. Judgments of personal taste are sensitive to experience in a way that moral judgments are not. This experience-sensitivity is a feature that languages typically encode, and that gets reflected in the semantics of PPTs, without having its equivalent in the semantics of moral predicates. Finally, this semantically encoded feature has repercussions on the expression and the interpretation of claims regarding personal taste, which, in turn, have an impact on our understanding of scenarios in which people disagree about issues of personal taste. By constrast, disagreements about moral issues are not affected by experience-sensitivity in any analogous way.

The remainder of the paper proceeds as follows. Sections 3 and 4 seek to establish the presence of experience-sensitivity with respect to PPTs and its absence in the case of moral predicates. Section 3 relies on standard theoretical methods from semantics, namely tests that reveal systematic differences in behavior between paradigmatic PPTs, such as 'tasty', and paradigmatic moral predicates, such as 'wrong'. These methods, however, have certain shortcomings, since the tests that they use, which look at the behavior of a given expression with respect to other constructions, need not always elicit robust felicity judgments and can be contextually manipulated. This is why, in section 4, I will gesture towards empirical data, combining statistical findings and (disappointingly few) experimental studies, to bolster the claim that PPTs, but not moral predicates, are experience-sensitive. Section 5 discusses the impact that these semantic differences have on disagreements over taste vs. moral issues.

3

Disentangling PPTs from Moral Predicates

Some preliminaries regarding methodology and taxonomy

4 Ironically, one of the four types of motivation that [START_REF] Kölbel | The evidence for relativism[END_REF] discusses, namely behavior in 'for'constructions (which I shall address in Section 3) precisely speaks against a uniform account. The trouble about [START_REF] Kölbel | The evidence for relativism[END_REF] is that he does not explicitly discuss the moral case. He writes: "The aim of this paper is to examine the kind of evidence that might be adduced in support of relativist semantics of a kind that have recently been proposed for predicates of personal taste, for epistemic modals, for knowledge attributions and for other cases. I shall concentrate on the case of taste predicates, but what I have to say is easily transposed to the other cases just mentioned." To the extent that moral predicates are to be counted among those "other cases" (which, given [START_REF] Kölbel | Truth without objectivity[END_REF], is what we would expect), the worry is that he simply fails to establish that the four types of motivation to which he appeals for PPTs are actually transposable onto moral predicates.

Although I have been talking about PPTs vs. moral predicates as if the two were easily distinguishable, it must be acknowledged that the two classes are not separated in any neat way in English and most other languages. An immediate challenge comes from all-purpose evaluatives adjectives such as 'good', 'bad', 'nice', 'horrible', or 'awful', which are used with equal ease in judgments of personal taste, moral judgments, aesthetic judgments, and all sorts of other judgments (see Cepollaro and Stojanovic (2016: 470) for discussion). An equally pressing challenge comes from expressions that belong primarily in one class, but can easily acquire a related meaning in a different class. Adjectives such as 'amazing' and 'disgusting' are a case at point: they are experience-sensitive and, in terms of semantic criteria, they behave like other PPTs, yet they can be, and often are, used to make claims about moral issues and moral values, as this corpus-drawn example illustrates: 5

3. The guy who killed these poor people is a horrible, horrible man and he should be hung in a park. He is a disgusting person.

Polysemy and coercion are mechanisms that allow words to shift their meaning and sometimes even grammatical category; and they are robustly entrenched in human languages (see [START_REF] Recanati | Literal meaning[END_REF][START_REF] Asher | Lexical meaning in context: a web of words[END_REF]. Given how widespread polysemy and coercion are, it comes as little surprise that certain generalizations made about the behavior of a given word, based on the perceived (in)felicity of particular sentences, must involve a grain of idealization. The lack of robust, all-or-nothing linguistic judgment is not peculiar to theorizing about predicates, but arises in virtually any area. 6

Threshold-sensitivity and comparative disagreements

Our main question is whether PPTs behave in the same way as moral predicates. I will argue that there are important differences between them. Recall, though, that we should not expect to find a clearcut demarcation that will completely set apart the two classes of predicates. What we should expect to find is, rather, that paradigmatic predicates of personal taste tend to behave one way, while paradigmatic moral predicates tend to behave another way. Importantly, the differences that I am about to 5 The example comes from COCA -the Corpus of Contemporary American English, from spoken corpus: The Five (FOX, February 13, 2015). 6 For instance, in the semantics of verb aspect, it is generally accepted that the felicity of combining a verb with temporal modifiers such as "in an hour" indicates that the verb denotes an achievement, as in "Suleïman built the raft in an hour". It is also generally accepted that verbs that denote achievements, such as 'build', are infelicitous with modifiers such as "for seven years": "#Suleïman built the raft for seven years". Yet we do find instances of 'build' in combination with the latter, as in "Suleïman built rafts for seven years". The felicity of such sentences is not taken as a counter-example to the claim that 'build' is a verb that normally denotes an achievement. Rather, it is taken as evidence of coercion, to the effect that in a phrase like "build rafts", the verb 'build' acquires an iterative meaning and denotes an activity. See e.g. [START_REF] Pustejovsky | The syntax of event structure[END_REF].

highlight do not preclude that there may also be important similarities between PPTs and moral predicates.

One feature that many PPTs have in common with many moral predicates, but also many predicates of all kinds, is that they are gradable adjectives. Gradability is one of the most discussed features of adjectives (see [START_REF] Morzycki | Modification[END_REF][START_REF] Burnett | Gradability in natual language[END_REF] for recent studies). Let me illustrate it with the help of the adjective 'long'. Consider two train rides, one that takes three hours and another that takes 35 minutes. While we can truly say that the first is longer than the second, neither the meaning of the adjective 'long' nor facts about the world allow us to decide whether, for either of those, we can truly say "That train ride is long". The reason is that, compared to train rides that last many hours or even days (such as the Trans-Siberian) a three-hour ride is short, yet compared to short distance rides that take less than an hour, the same ride is long. One consequence of this phenomenon is that relative gradable adjectives easily give rise to faultless disagreement: if you are used to short distance rides, and I am used to rides that last for hours, then you will be easily led to assert that the threehour ride is long and I will be easily led to deny this, yet there does not seem to be any objective way to adjudicate our dispute.

Because relative adjectives allow their thresholds to vary with the context, they give rise to disagreements that can be accounted for by pointing to the fact that the two parties fail to converge on the relevant context -that is to say, they fail to agree on what the appropriate threshold is. Given that most PPTs, moral predicates and allpurpose evaluatives are also gradable, they can give rise to disagreements that boil down to disagreements about where the threshold lies. Reconsider (2). Noriko and Suleïman's disagreement whether his taking pleasure in watching porn is morally wrong does not actually require that the two of them diverge deeply in their moral judgments. Suppose that for any two given actions, when asked which of the two is a morally better action (or whether neither is better than the other), Noriko and Suleïman give exactly the same answer. We would see Noriko and Suleïman as largely agreeing in their moral judgments. Nevertheless, they could still disagree as in (2), because they can disagree over the extent to which an action must deviate from their shared moral standards in order to count as morally wrong. For example, Suleïman may fully agree that it is morally better to abstain from watching porn than to do so, and still hold, without inconsistency, that the former, although morally worse than the latter, is not yet so bad as to count as morally wrong.

In order to demarcate the subjectivity found in PPTs and in evaluative adjectives from mere threshold-sensitivity, various authors have urged us to shift the focus of attention from the positive form to the comparative form (see [START_REF] Glanzberg | Context, content and relativism[END_REF][START_REF] Saebø | Judgement ascriptions[END_REF][START_REF] Kennedy | Two sources of subjectivity: Qualitative assessment and dimensional uncertainty[END_REF][START_REF] Bylinina | The grammar of standards: judge-dependence, purposerelativity, and comparison classes in degree constructions[END_REF][START_REF] Solt | Ordering subjectivity and the relative-absolute distinction[END_REF][START_REF] Mcnally | The semantics of aesthetic judgment[END_REF]. To wit, single-scale relative adjectives, such as 'long' or 'expensive', no longer give rise to faultless disagreements when used in the comparative form. Consider: 4. Noriko: "The train ride from Paris to Brussels is longer than the one from Brussels to Amsterdam."

Suleïman: "No, the one from Brussels to Amsterdam is longer." 5. Noriko: "The soup is tastier if seasoned with parsley." Suleïman: "No, it is tastier without it!" 6. Noriko: "Taking pleasure in watching porn is (morally) worse than stealing books from a library." Suleïman: "No, stealing books is worse!"

To settle the disagreement in ( 4), all that is required is to measure the length of the respective rides, while no obvious strategy can help in settling the disagreements in (5) or in (6). In this respect, PPTs and moral predicates behave alike -but this is not surprising, since among gradable adjectives, those that are relative to a single scale, such as 'long' (length), 'old' (age), or 'expensive' (price), appear to be the odd ones out.

Experiencer-sensitivity in PPTs: theoretical evidence

The aim of this section is to underscore a crucial aspect in which PPTs differ from moral adjectives. The former, but not the later, are experiencer-sensitive: they encode in their semantic structure an argument place for a subject who undergoes a given experience. There are two main linguistic tests have been proposed to detect the presence or absence of an experiencer. The first is to see whether the adjective may be used felicitously with a 'to' or 'for' phrase, with the phrase modifying the adjective itself (as opposed to modifying the entire sentence). Adjectives derived from verbs that denote events involving experiencers, such as 'exhausting' or 'soothing', but also certain adjectives derived from verbs that are taken to belong among PPTs, such as 'entertaining', 'boring' or 'disgusting', clearly pass this test. Here are two corpus-drawn examples:

7. I don't really care who invented math. It's boring to me, no matter who invented it.7 8. Though the liqueur was disgusting to our taste buds, we did visit a local Calvados distillery.8

PPTs that are not derived from verbs may not yield such a neat pattern as 'boring' or 'disgusting', but they are still remarkably easier to be felicitously modified by a 'to' or a 'for' phrase than paradigmatic moral predicates. The naturalness of (9) constrasts with the infelicity of (10):

9. Fat white worms that grow in rotten logs are nourishing and tasty to many people. 9 10. # Female genital mutilation, a practice that harms thousands of girls and women, is wrong to many doctors.

Although the speaker may try to make it clear that the intended reading of ( 10) is that many doctors judge FGM to be wrong, the attempted modification of the adjective 'wrong' by the corresponding 'to' phrase sounds bad; or, at any rate, remarkably worse than (9); nor does it improve if we replace it with a 'for' phrase. 10 The second test that has been proposed to detect adjectives with experiencers is whether they can be used felicitously with the verb 'find' [START_REF] Saebø | Judgement ascriptions[END_REF][START_REF] Bylinina | The grammar of standards: judge-dependence, purposerelativity, and comparison classes in degree constructions[END_REF], Umbach 2016) as in:

11. Cold oatmeal? The idea sounds disgusting, but I can't stomach another bite of hot oatmeal, so I try it and find it delicious. 11

However, this test may be treacherous, because adjectives that arguably do not come with an experiencer argument can, in a suitable context, felicitously co-occur with 'find'. Thus presented with a 70€ bill for a one-person meal, you might say:

12. I find this meal expensive.

The fact that (12) sounds felicitous is not yet evidence that 'expensive' encodes an experiencer argument. Rather, as we argue in [START_REF] Mcnally | The semantics of aesthetic judgment[END_REF], the speaker of ( 12) is conveying that they are judging the meal to be expensive based on their previous experience with meal prices. Thus appearances notwithstanding, the 'find'-construction introduces an experiencer argument, even if the argument is not lexically associated with the adjective embedded under 'find'.

The fact that 'find' can easily force the introduction of an experiencer by the mechanism of coercion will not yield infelicity results that are as neat as one might wish when applied to moral adjectives. Nevertheless, it can be argued that there is a pronounced preference for a doxastic phrase, such as 'believe' or 'consider', over the experiential phrase 'find': 13. Many doctors consider/?find female genital mutilation (to be) wrong. Twenty-Third Publications 2003, p. 131. 10 This test must be applied with care, because 'to' and 'for' constructions can be used as appositives, with a reading equivalent to "in the opinion of". Thus, as pointed out by an anonymous referee, "For many doctors, female genital mutilation is wrong" sounds perfectly felicitous. However, the proposed sentence is not an instance of a modification by a 'for'-construction that tests for the presence of an experiencer argument. In order to be so, the 'for' phrase must occur as an argument rather than an adjunct (see [START_REF] Bylinina | The grammar of standards: judge-dependence, purposerelativity, and comparison classes in degree constructions[END_REF]. One must also keep in mind that some adjectives select a 'for' phrase while others select a 'to' phrase (see Stephenson 2007: 520). And yet another complication arises from the fact that 'to' and 'for' constructions (in their argument rather than adjunct use) can also serve to introduce not an experiencer argument, but a beneficiary argument, as in "Noriko is kind to everyone" (see Stojanovic 2016: 683) If we want (13) to be a claim about many doctors' moral judgment on FGM, then we will prefer to use 'consider' rather than 'find'. This is not to say that 'find' sounds impossible in such constructions, but, as repeatedly stressed, what we are seeking to establish is that experience-sensitive adjectives, such as 'disgusting' or 'tasty', occur with a much greater ease and propensity with experiential constructions ('for', 'to' and 'find') than moral predicates do.

Empirical Evidence for Experience-Sensitivity

My aim is to show that, from a linguistic point of view, moral predicates do not function in quite the same way as PPTs. So far, I have appealed to the theoretical evidence for the distinction, which appeals to two criteria for diagnosing the presence of an implicit experiencer argument, both of which rely on felicity judgments. However, one may be left with a slight feeling of frustration, because felicity itself is not an all-or-nothing property; it comes in degrees and is sensitive to context. And if a moral predicate can be made, with the help of the context, to sound felicitous in a construction that aims to detect experience-sensitivity, then one may wonder whether we have any solid evidence for the distinction. The issue is actually much broader than the case of PPTs vs. moral predicates and relates to questions about semantic methodology and metasemantics: how can we draw any conclusions from judgments of felicity, if these turn out to be a matter of degree and vary with the context? Due to similar concerns, researchers in semantics are growingly keen to consolidate their theory with empirical findings. The first kind of empirical evidence relevant to the present debate is quantitative evidence. In a nutshell, instead of picking out a predicate, putting it in a 'to' or 'for' or 'find' construction, and asking ourselves how felicitous the resulting sentence sounds to us, the idea is to see how likely that sort of construction is to occur in natural discourse and text. In different terms, the idea is to look for quantitative measures that reveal a difference in the behavior of PPTs, on the one hand, and moral predicates, on the other. Quantitative approaches of this sort are used in computational linguistics, although applications to truth-conditional semantics and, in particular, to the semantics of adjectives, have been scarce so far. A notable exception is [START_REF] Sassoon | A typology of multidimensional adjectives[END_REF], who uses quantitative data to motivate the division between unidimensional and multidimensional adjectives, as well as some subdivisions among the latter.

Although to my knowledge no extensive study has been conducted to compare the behavior of PPTs and moral predicates, some relevant preliminary evidence is reported in [START_REF] Mcnally | The semantics of aesthetic judgment[END_REF], who examined a representative sample from the British National Corpus involving the 'find' construction. Their findings reveal a sharp contrast between, on the one hand, all-purpose evaluatives ('good', 'bad', 'great', 'excellent', 'mediocre', 'awesome') and aesthetic evaluatives ('beautiful', 'pretty', 'ugly', 'gorgeous'), which hardly ever appear with the 'find'-construction, and, on the other, experiencer-sensitive adjectives ('difficult', 'hard', 'easy', 'interesting') which appear with a much greater frequency. To be sure, this pilot study does not yet provide conclusive evidence that would clearly demarcate PPTs and moral predicates. However, it does point to a promising direction of research that could corroborate the theoretical claims made in the last section.

The second kind of empirical evidence comes from experimental research in semantics and philosophy. Experimental methodology is a recent trend in both areas, which explains why here, too, there are virtually no studies regarding the presence or absence of an experiencer argument. 12 Here, I will only draw attention to the recent study of [START_REF] Kaiser | Experience matters: a psycholinguistic investigation of predicates of personal taste[END_REF], who conducted an experiment to test the claim that PPTs involve experiencers while (other) multidimensional adjectives don't. The gist of their experiment was to compare contexts containing verbs that introduce experiencers vs. verbs that do not, under the assumption that a recently mentioned experiencer would be more easily perceived as the "judge" in the interpretation of a PPT (that is, as the person whose taste is relevant to deciding whether something is "tasty", "irritating" or "boring"). Kaiser and Lee presented their participants with pairs of sentences of the following form: 14. Lisa (a) nudged/(b) looked at Kate. She was (c) irritating/(d) smart.

The stimuli contrasted agent-patient verbs (condition a) with experiencer-theme verbs (condition b), and also contrasted PPTs (condition c) with other multidimensional predicates (condition d). The participants were then asked "In whose opinion is the other person irritating/smart?" and could select between Lisa, Kate and the narrator.

Kaiser and Lee's hypothesis was that when a PPT (condition c) is preceded by an experiencer-theme verb (condition b), the experiencer argument of this verb is more likely to be chosen as the "judge" for the PPT than is a non-experiencer argument of an agent-theme verb (condition a), while if the other multidimensional adjectives ('smart') are not experiencer-sensitive, then there should be no such an asymmetry for those. The results confirmed these predictions: "the patterns with Experiencer-Theme verbs show that in contexts where an experiencer is available, it is indeed a highly preferred choice as the judge of PPTs. Additional analyses show that in conditions with Experience-Theme verbs, the rate of subject-opinion responses is significantly lower with multidimensional adjectives than with PPTs, indicating that multidimensional adjectives do not 'seek out' experiencer judges as strongly as PPTs" (2017: 2). To be sure, this study says nothing about moral predicates. Nevertheless, the contrast that it reveals between PPTs and predicates such as 'smart' is a reason to expect a similar contrast to arise in a mirror study on PPTs and moral predicates.

5

Consequences for Disagreement

In this final section, I want to examine the consequences of the semantic differences between PPTs and moral predicates for the nature of disagreement in the two cases. A major consequence is that experiencer-sensitive predicates (including PPTs) are more likely to be interpretable in different ways than predicates that do not have an experiencer argument (such as moral predicates). Consider the predicate 'difficult', which clearly comes with an experiencer argument: 15. Skiing in bad weather conditions, with poor equipment and a heavy load is difficult. 16. Gosh, that last run was difficult! 17. I can't take you on that slope, it's a difficult one.

On its most natural interpretation, we understand (15) to mean that skiing in those conditions is difficult in general, for anyone (that is to say, for any potential skier in an event of that type). In linguists' jargon, the experiencer argument appears to be bound by a covert generic quantifier. While we easily interpret (15) without knowing who said it, ( 16) refers to a specific event; to understand (16), we need to know which run is being referred to and who was involved in the run. In a typical scenario, the speaker of ( 16) will be referring to a run that they performed, and we understand that it was difficult for them to perform this run. Now consider (17), and suppose that it is said by a skiing instructor to a novice. Then we understand that the slope in question is difficult for the novice, but presumably not for the instructor. In the three examples, the linguistic form interacts with various features of the context, including general background knowledge, to yield the most plausible interpretation.

The availability of multiple readings, by which I mean the fact that there is often more than one way of understanding who the relevant experiencers are, has a direct effect on disagreements that involve an experiencer-sensitive predicate. When the experiencer is left implicit, this provides a source for misinterpretations that can generate a form of disagreement. To make the point, consider a version of ( 16) in a disagreement-prone dialogue: 18. Noriko: Gosh, that run on Grand Col was difficult. 19. Suleïman: The one from Aiguille Rouge was even more difficult. 20. Noriko: What?!? Grand Col was more difficult than Aiguille Rouge! If we take (20) at face value, we will hear it as contradicting ( 19), and will deduce that Suleïman and Noriko disagree. However, I submit that merely by looking at ( 19) and ( 20) we cannot, and should not, draw any conclusions as to the putative disagreement betweeen them. The conversation in ( 18)-( 20) is underdeterminate with respect to the intended interpretation of the experiencer argument. One possibility is that Noriko is stating that for her the run on Grand Col was more difficult than Aiguille Rouge. Another possibility is that she takes Suleïman to be making a more general statement regarding the two runs, namely, that Aiguille Rouge was more difficult than Grand Col for anyone who participated in both events, and she is taking issue with that.

How can we tell which of those two possibilities is the case? Are Suleïman and Noriko merely stating their respective appreciations of the difficulty of each run? Are they disagreeing and contradicting each other? In line with the proposal defended in [START_REF] Stojanovic | Talking about taste: Disagreement, implicit arguments, and relative truth[END_REF][START_REF] Stojanovic | Emotional disagreement[END_REF], I suggest that in order to decide what is happening in dialogues such as ( 18)-( 20), one needs to look at the context more carefully and inquire how the conversation may further evolve. If the two parties aim to disagree, we expect them to support their respective stances by arguments. For instance, Noriko might continue: "I agree that Grand Col is normally easier than Aiguille Rouge, but on that run, it was all ice, that's what made it superdifficult!" On the other hand, if the two parties aim to relate their own experiences of the difficulty involved, we expect them to qualify their statements with first-personal devices. For example, Suleïman might reply to (20): "OK, no offense taken! Aiguille Rouge may have been more difficult for you, but personally, I just suffered so much more on Grand Col!" Additional evidence for the existence of multiple readings comes from the fact that the readings can be partly disambiguated in languages such as Japanese, which uses sentential markers akin to evidentials and offers several ways of expressing ( 16): 21. Ano kôka wa musukashikatta desu. 22. Ano kôka wa musukashikatta desu-yo. 23. Ano kôka wa musukashikatta desu-ne.

The difference between ( 22) and ( 23) is that the particle -ne is used to elicit approval from the interlocutor (as in "That last run was difficult, wasn't it?") whereas the particle -yo emphasizes that the speaker is reporting an experience of their own, which they normally don't expect to be shared by the interlocutor. ( 21) is neutral, but then, it is not the kind of sentence that a Japanese would normally use. The choice between ( 22) and ( 23) constrains the interpretation of the experiencer argument, so that it either includes the interlocutor (-ne) or doesn't (-yo).

Disagreements over moral issues are normally not subject to the same range of patterns as disagreements over matters of taste, because there is no experiencesensitivity involved. If two parties engage in (what looks like) a disagreement over the question of which of two courses of action was morally better, they are deprived of the possibility of saying "OK, no offense taken! I can see that you prefer this course of action, but my moral preference goes the other way". Of course, this is not to say that in the case of moral disagreement, there is an objective way of adjudicating the issue. The point is, rather, that whatever subjectivity there may be to moral questions, it works differently from the subjectivity that derives from experiencing a ski slope as difficult, or from taking pleasure in the taste of parsley. The latter corresponds, at the level of language, to experience-sensitivity. And because questions of taste depend in this way on our (gustatory or other) experience of an object, PPTs encode this dependence in their semantic structure. As a consequence, sentences involving PPTs are subject to a range of interpretations that are simply unavailable for moral predicates, which lack this kind of argument.

The fact that judgments of personal taste are sensitive to experience also has important implications for the debate between contextualist and relativist approaches to the semantics of PPTs. Over the last decade, the debate has been extensively shaped by arguments that appeal to (putative) disagreement data. However, I have argued that the implicit experiencer-sensitivity of PPTs is a reason not to take dialogues that look like disagreements at their face value. The availability of multiple readings for sentences involving PPTs predicts that some of the dialogues that look like disagreements are not genuine disagreements. In such cases, the impression of disagreement should go away once the two parties make it explicit whose experience they take to be relevant. Consequently, we ought give up the assumption that there are clear disagreement data to be relied upon in semantic theorizing -an assumption that has largely underlied the contextualism-relativism debate. Instead, a more cautious methodological attitude regarding such putative data is required. 13To conclude, my goal in this paper has been a modest one. I have defended the idea that disagreements about matters of personal taste and disagreements about moral issues behave differently in an important respect. This idea, strongly supported by common sense, has been resisted by a number of authors from the relativist and the contextualist camps alike. While those authors strive to provide uniform accounts to explain the perceived faultlessness of disagreements about taste and the resilience of moral disagreements, I have argued that there are good reasons to refrain from seeking such a uniform account. Judgments of personal taste are sensitive to experience in a way in which moral judgments are not; accounts that obliterate this difference are likely to misconstrue the complex nature of the disagreements in question.

  . 11 Kevin Runolfson, The Things You Find on the Appalachian Trail: A Memory of Discovery, Endurance, and a Lazy Dog, McFarland & Company, Inc. Publishers 2010, p. 54.

Isidora Stojanovic, "Disagreements about Taste vs. Disagreements about Moral Issues"

Brown Morton, Starting Out in the Evening, NY Crown Publishers, 1998; source: COCA.

Susan Laccetti Meyers, "Normandy beyond the beachheads", Atlanta Travel 2002; source: COCA.

Michael H. Barnes, In the Presence of Mystery: An Introduction to the Story of Human Religiousness, Isidora Stojanovic, "Disagreements about Taste vs. Disagreements about Moral Issues"

There are several experimental studies about argument structure in general, and experiencers in particular, conducted in the field of language acquisition; however, their relevance to the present discussion is not obvious.

[START_REF] Palmira | The semantic significance of faultless disagreement[END_REF] also questions the idea that considerations about disagreement (under one description of the phenomenon) are relevant to adjudicating between different semantic theories for taste discourse. Isidora Stojanovic, "Disagreements about Taste vs. Disagreements about Moral Issues"

Acknowledgments

I am very grateful to Michele Palmira and Sarah Stroud for their valuable support and suggestions, as well as to an anonymous referee, Ethan Jerzak and Andrés Soria Ruiz for comments on an earlier draft. At the institutional level, I have to mention the following grants: ANR-10-LABX-0087 IEC, ANR-10-IDEX-0001-02 PSL, MINECO FFI2016-80636-P and European Union's Horizon 2020 Marie Skłodowska-Curie grant agreement n. 675415.


Li Fu

Samy Merabia

Laurent Joly

What controls thermo-osmosis? Molecular simulations show the critical role of interfacial hydrodynamics

Keywords: numbers: 47.61.-k, 47.55.dm, 83.50.Rp, 47.11.Mn

Thermo-osmotic and related thermo-phoretic phenomena can be found in many situations from biology to colloid science, but the underlying molecular mechanisms remain largely unexplored. Using molecular dynamics simulations, we measured the thermo-osmosis coefficient by both mechanocaloric and thermo-osmotic routes, for different solid-liquid interfacial energies. The simulations reveal in particular the crucial role of nanoscale interfacial hydrodynamics. For non-wetting surfaces, thermo-osmotic transport is largely amplified by hydrodynamic slip at the interface. For wetting surfaces, the position of the hydrodynamic shear plane plays a key role in determining the amplitude and sign of the thermo-osmosis coefficient. Finally, we measure a giant thermo-osmotic response of the water-graphene interface, which we relate to the very low interfacial friction displayed by this system. These results open new perspectives for the design of efficient functional interfaces for, e.g., waste heat harvesting.

The fundamental coupling between thermal and hydrodynamic transport in the nanometric vicinity of liquidsolid interfaces has received scanty attention until recently. Thermophoretic phenomena, referring to the influence of temperature gradients on the flux of colloidal particles, were firstly studied for numerous applications such as optothermal DNA trapping or diseaserelated protein aggregates identification [1][2][3][4][5][6][7][8], and this interest for thermophoresis fostered work on its theoretical description [9][START_REF] Parola | The European physical journal[END_REF][11][START_REF] Dhont | The European physical journal[END_REF][13]. On the other hand, at variance with what has been done for electro-osmosis [14][15][16][17][18][19][20][21][22] and diffusio-osmosis [23][24][25][26], very limited theoretical work has been done so far on thermo-osmosis at solid-liquid interfaces.

Thermo-osmosis was first studied by Derjaguin and Sidorenkov through porous glass [START_REF] Derjaguin | On thermo-osmosis of liquid in porous glass[END_REF] and is usually interpreted as a thermal gradient-induced Marangoni flow [START_REF] Ruckenstein | [END_REF][START_REF] Anderson | Annual review of fluid mechanics[END_REF][START_REF] Derjaguin | Surface forces in transport phenomena[END_REF]. Advanced continuum descriptions have been developed recently for electrolytes [START_REF] Dietzel | [END_REF], but a molecular level understanding is still lacking. In that context, Ganti et al. [32] have explored three different methods to characterize thermo-osmosis using molecular simulations, and they found that all methods yield very similar results. Bregulla et al. [33] reported the first microscale observation of the velocity field imposed by a nonuniform temperature and deduced the thermo-osmosis coefficient for different surfaces. Nevertheless, the role of surface wettability and interfacial hydrodynamics have hardly been discussed so far.

Here we explore the influence of solid-liquid interfacial energy on thermo-osmosis using molecular dynamics simulations. We show the crucial role of interfacial hydrodynamics, explaining in particular the giant thermo-osmosis coefficient observed on non-wetting surfaces and at the water-graphene interface, and controlling the change of sign of the coefficient observed on wetting surfaces.

Theory The thermo-osmotic response of a fluidic system can be described by the non-diagonal terms of its response matrix [START_REF] Anderson | Annual review of fluid mechanics[END_REF][START_REF] Derjaguin | Surface forces in transport phenomena[END_REF][START_REF] Kjelstrup | Non-equilibrium thermodynamics of heterogeneous systems[END_REF]:

v s j h = M 11 M 12 M 21 M 22 × -∇p -∇T /T , (1)
where v s is the hydrodynamic velocity, j h is the heat flux density, and M ij are phenomenological coefficients. M 12 is the so-called thermo-osmotic slip coefficient or thermoosmosis coefficient, which describes the surface-induced flow under a thermal gradient. M 21 is the so-called mechano-caloric coefficient, which describes the heat flux density generated by a pressure gradient. According to Onsager reciprocal relations, M 12 = M 21 [START_REF] De Groot | Non-equilibrium Thermodynamics[END_REF][START_REF] Brunet | [END_REF]. Derjaguin and Sidorenkov related the thermo-osmosis coefficient to the interfacial excess enthalpy using linear non-equilibrium thermodynamics [START_REF] Derjaguin | On thermo-osmosis of liquid in porous glass[END_REF][START_REF] Anderson | Annual review of fluid mechanics[END_REF][START_REF] Derjaguin | Surface forces in transport phenomena[END_REF]:

M 12 = M 21 = 1 η +∞ 0 z δh(z) dz, ( 2
)
where η is the liquid viscosity, z the distance to the surface, z = 0 the position of the interface, z = +∞ the bulk liquid region far from the interface, and δh(z) the excess of specific enthalpy as compared to the bulk. As detailed in the supplemental material (SM) [37], δh(z) can indeed be related to the thermodynamic force acting on the liquid under a thermal gradient, which drives thermoosmosis. This expression has also been obtained using local thermal equilibrium [32] or mechanical [38] routes. Note that Eq. ( 2) is based on a simple, macroscopic view of interfacial hydrodynamics, and it ignores the possible presence of a stagnant liquid layer, or of liquid-solid slip. However, for other surface-driven flows, the details of interfacial dynamics can play a key role [26]. In particular, hydrodynamic slip can amplify electro-osmotic flows 1. Illustration of the different configurations used to measure the thermo-osmosis coefficient with molecular dynamics simulations. (a) Mechano-caloric route, using a slit nano-channel without reservoirs: a body force per particle fi is applied to the liquid particles to model an external pressure gradient, and an excess heat flux is generated by the induced Poiseuille flow; (b) Thermo-osmotic route, where the nanochannel is connected to reservoirs at different temperatures and the thermo-osmotic velocity is measured. [14,[START_REF] Muller | Boundary effects in the theory of electrokinetic phenomena[END_REF][40][41][42] and diffusio-osmotic flows [23], but also thermophoresis of colloids [43]. Therefore, following previous work in the context of electro-osmosis [15,44,45], we rewrite Eq. ( 2), introducing the shear plane position z s in order to account for a possible stagnant liquid layer, and the slip length b to describe a possible liquid-solid slip:

M 12 = M 21 = 1 η +∞ zs (z -z s + b) δh(z) dz. (3)
Detailed derivations of the formula presented in this Letter are reported in the SM. Molecular dynamics simulations We tested the theoretical prediction with MD simulations, and explored in particular the effect of wetting. All simulations were performed with the LAMMPS package [46]. Technical details can be found in the SM, and here we report only the main features of the models. We considered a generic liquid made of particles interacting through a Lennard-Jones (LJ) pair potential, V (r) = 4ε[(σ/r) 12 -(σ/r) 6 ], with r the distance between the particles, and ε and σ the liquid-liquid interaction energy and distance, respectively. The liquid was confined in a slit configuration between two Einstein solids interacting with the liquid also through a LJ potential with the same σ. We varied the liquid-solid wetting properties by adjusting the liquid-solid interaction energy in the range ε ls = 0.1 ε (very hydrophobic) to 1.0ε (very hydrophilic); the corresponding contact angles displayed in Fig. 2 were estimated with sessile droplet simulations, as detailed in the SM.

We used both the mechano-caloric and the thermoosmotic routes to measure the thermo-osmosis coefficient, see Fig. 1. For the mechano-caloric route (Fig. 1a), we considered an infinite slit nanochannel (using periodic boundary conditions). We applied a body force per par-ticle to model a pressure gradient, and measured the resulting heat flux j h = δh(z)v x (z) dz, with v x (z) the measured velocity profile, and δh(z) the excess specific enthalpy profile. The local specific enthalpy was expressed as h(z) = (u i (z) + p i (z))ρ(z), (with u i (z) the energy per particle, p i (z) the atom-based virial expression for pressure, and ρ(z) the local density, see the SM for detail), and we obtained δh(z) by subtracting the bulk value in the middle of the channel. We then computed the mechano-caloric coefficient M 21 = j h /(-∇p). We also calculated a theoretical value for M 21 according to Eq. ( 3), where the hydrodynamic parameters η, z s and b were obtained by fitting the numerical velocity profiles. For the thermo-osmotic route, we connected the slit channel to reservoirs at the same pressure (imposed by two pistons) and different temperatures. We measured the thermo-osmotic velocity v s from the time evolution of the number of particles in the reservoirs, and the thermal gradient ∇T and the average temperature T from the linear temperature profile in the channel. We then computed the response coefficient M 12 = v s /(-∇T /T ).

Here again we calculated a theoretical value for M 12 according to Eq. ( 3), using the enthalpy profiles measured in the channel.

Results

We first focus on the measurements of M 21 using the mechano-caloric route. Figure 2 presents the evolution of measured and theoretical M 21 as a function of solid-liquid interaction energy ε ls (red open symbols). Within uncertainties, Eq. (3) predicts well the measured values of M 21 . Using typical molecular lengths (σ = 0.34 nm) and times (τ = 1 ps), the amplitude of the measured coefficients for large interaction energies are on the order of |M 21 | ∼ 0.1σ 2 /τ ∼ 10 -8 m 2 /s. This is comparable to the values reported in the recent experimental work of Bregulla et al. [33], on the order of 10 -10 to 10 -9 m 2 /s. Figure 2 then reveals two interesting features.

FIG. Measured and theoretical thermo-osmosis coefficient

for mechano-caloric and thermo-osmotic configurations, as a function of the solid-liquid interaction energy ε ls (the corresponding contact angles are indicated below). For ε ls ranging between 0.7 and 1.0ε, results are enlarged in the inset. A change of sign is observed for both configurations. A good agreement of predicted and measured coefficient is found for the mechano-caloric configuration, while a large disparity exists for the thermo-osmotic configuration, related to viscous entrance effects (see Fig. 4 and related text). Firstly, the sign of M 21 changes around ε ls = 0.8ε: at low ε ls , the heat flux along the flow is positive, and at high ε ls it is negative. Secondly, M 21 is strongly enhanced for the lowest liquid-solid interaction energies, reaching values up to ∼ 35σ 2 /τ ∼ 4 × 10 -6 m 2 /s.

In order to understand the change of sign of the thermo-osmosis coefficient, Fig. 3a shows the excess en-thalpy profiles for different solid-liquid interaction energies. We note that for all the profiles, the excess enthalpy vanishes at ∼ 5σ from the interface. At the vicinity of the surface, the excess enthalpy is positive everywhere for ε ls < 0.6ε, which explains the positive value of M 21 . For stronger interfacial interactions, the excess enthalpy (corresponding to the driving force for thermo-osmosis) shows large oscillations around zero, with a period on the order of the molecular size. This is in strong contrast with electro-and diffusio-osmotic flows where the driving force generally does not change sign in the interfacial region. According to Eq. 3, z s fixes the lower limit of the integration, so that the value and also the sign of the response coefficient M 21 depend strongly on z s , and cannot be predicted based only on equilibrium properties of the interface. Overall these results shed some light on the structural and dynamical molecular mechanisms underlying the response coefficients of different signs that have been reported in the literature [33,[47][48][49][50].

However, the excess enthalpy profiles can not fully explain the massive enhancement of M 21 at low ε ls , and hydrodynamic slip must be taken into account. Following a previous treatment of slippage in diffusio-osmosis [23], we introduce a characteristic length L representative of the thickness of the interfacial liquid layer where the enthalpy differs from the bulk,

L = +∞ zs (z -z s ) δh(z) dz +∞ zs δh(z) dz . (4)
One can then rewrite Eq. ( 3):

M 21 = M no-slip 21 (1 + b/L), (5)
where M no-slip 21 is the response that would be obtained in the absence of slip (b = 0), and the amplification factor 1 + b/L quantifies the contribution of hydrodynamic slip to the thermo-osmotic response. We plot in Fig. 3b the evolution of the amplification factor against ε ls . For ε ls > 0.6ε, the amplification factor converges to 1 since the slip length vanishes. On the other hand, the amplification factor grows rapidly when ε ls gets lower than 0.5ε, which shows that on hydrophobic surfaces the amplitude of the thermo-osmotic response is mostly controlled by slippage.

Such an amplification by hydrodynamic slip has already been reported for other osmotic flows [14,23,[START_REF] Muller | Boundary effects in the theory of electrokinetic phenomena[END_REF][40][41][42]. In particular, for electro-osmosis and diffusioosmosis with electrolytes as solute, the amplification factor is controlled by the ratio between the slip length and the Debye length, quantifying the thickness of the electrical double layer. In moderately concentrated aqueous electrolytes, the Debye length can reach tens of nanometers, while here the thickness L of the interfacial liquid layer is on the order of a few molecular sizes. Accordingly, the effect of slip is particularly large for thermo-osmosis.

One should note finally that the two lowest ε ls correspond to contact angles that cannot be achieved ex-perimentally on smooth surfaces. Nevertheless, these ε ls could be considered as an effective description of superhydrophobic (SH) surfaces, which display both very large contact angles and very high slip lengths [51,52]. The results obtained here therefore motivate further work on more realistic SH surfaces, which have been considered theoretically at the continuum level [53], but where molecular effects remain to be explored. We also emphasize that a significant amplification by slip is already observed for intermediate ε ls values corresponding to more realistic smooth hydrophobic surfaces. Finally, identifying the amplifying role of slip motivated us to consider a more realistic system exhibiting giant slippage, namely water/graphene, as detailed later.

We now turn to the measurements of M 12 in the thermo-osmotic configuration, represented with blue closed symbols in Fig. 2. The theoretical M 12 matches the numerical and theoretical M 21 as expected from Onsager reciprocal relations. Also, both the theoretical and measured M 12 display a change of sign, corresponding to a reversal of the flow for a given thermal gradient direction. However, in contrast to what we found with the mechano-caloric route, there is a large discrepancy between the measured and theoretical M 12 at low ε ls : the massive increase predicted by the theory is systematically attenuated. In order to understand this phenomenon, we plotted in Fig. 4a typical velocity profiles for low ε ls . Indeed, their parabolic shape reveals a Poiseuille backflow. We suggest this backflow is due to viscous entrance effects [54][55][56], whose key influence on nanoscale flows has been emphasized recently [57][58][59][60][61]. According to Poiseuille law, one can deduce the backflow velocity profile v bk (z) from the measured curvature of the velocity profile, using the hydrodynamic boundary condition parameters determined in the mechano-caloric configuration. We then obtain the true thermo-osmotic velocity profile v s (z) by correcting the measured velocity v m (z) with the backflow velocity v bk (z): v s = v m -v bk . Thus a corrected thermo-osmosis coefficient can be calculated from the definition, Eq. ( 1), and the results are plotted in Fig. 4b. We found that the corrected values match well the theoretical ones. Regarding the cases for which the backflow is hardly detectable on the velocity profile, the pressure gradient responsible for the backflow can be estimated from the correlation of measured and theoretical thermo-osmosis coefficients. For a given system, we computed a normalized pressure gradient ∇P/(η∇T /T avg ), which depends only on ε ls (see the SM). The tendency is shown in Fig. 4c with blue closed triangles. The lower is ε ls , the higher is the pressure gradient, in line with what we obtained from the velocity profiles. We also deduced the same term from the curvature of the velocity profiles for those with evident backflow (typically ε ls < 0.5), as shown with the black circles in Fig. 4c. Within uncertainties, the thermo-osmosis coefficient correlation successfully predicts the pressure gradient generating the

backflow.

Water-graphene interface Finally, we estimated the thermo-osmosis coefficient of a water-graphene system using the mechano-caloric route, with a pressure of 1 atm and a temperature of 323 K (see the SM for details). We used the TIP4P/2005 force field for water [62], the LCBOP one for graphene [63] and a recently proposed force field for water-carbon interactions [64], which has been shown to reproduce accurately quantum chemistry calculations of interaction energies between water and carbon nanostructures [65]. With these force fields, we measured a contact angle of ca. 85 • and a slip length of ca. 32 nm, within the range of values reported in the literature [66,67] (see details in the SM). We obtained M 21 = (2.5 ± 0.3) × 10 -6 m 2 /s. This value substantially exceeds those documented in other studies [START_REF] Derjaguin | Surface forces in transport phenomena[END_REF]33] on the order of 10 -10 to 10 -9 m 2 /s. It is well known that the water-graphene interface presents a very low friction, and accordingly a very large slip length [67], largely exceeding the spatial range over which the specific enthalpy differs from its bulk value. In that limit, and using the relation between the slip length b and the interfacial friction coefficient λ: b = η/λ [68], Eq. ( 3) can be simplified as M 21 ≈ 1 λ δh(z) dz, where the response coefficient only depends on the total enthalpy excess and on the interfacial friction coefficient. Indeed we were able to reproduce the measured M 21 using this formula. Note that the excess enthalpy profile of this system (see the SM) oscillates around zero and resembles a surface with intermediate wettability. Therefore, it is the very low friction coefficient which explains primarily the giant thermo-osmotic response. With an enhancement of about 3 orders of magnitude compared to existing experimental data, this water-graphene system shows a great potential for thermal energy harvesting, although the very large thermal conductivity of graphene [69] could reduce the efficiency of such systems, an effect which we plan to investigate in the future.

Summary We measured the thermo-osmosis coefficient using MD simulations via mechano-caloric and thermo-osmotic routes for different solid-liquid interfacial energies. A good agreement is obtained using these two methods, in line with Onsager reciprocal relations. We showed that the standard picture of thermo-osmosis as a thermal gradient-induced Marangoni flow can only give a qualitative description of the phenomenon, because it does not account for the pivotal role of the hydrodynamic boundary condition. For high interfacial energies, due to the oscillations of the excess enthalpy profiles in the vicinity of the solid-liquid interface, the thermo-osmosis coefficient depends closely on the thickness of the stagnant liquid layer, and a change of sign is clearly observed. For low interfacial energies, hydrodynamic slip largely amplifies the thermo-osmosis coefficient, which has also been confirmed by simulations of the water-graphene interface. Finally we showed that viscous entrance effects reduce significantly the amplitude of the thermo-osmotic flow in the considered nanopore geometries. We hope these results will motivate future experimental development and characterization of new functional interfaces for efficient harvesting of thermal energy.

FIG. 3 .

 3 FIG. 3. (a)Excess specific enthalpy profiles for different solidliquid interaction energies ε ls ; Inset: excess enthalpy density profile for ε ls = 1.0; zs is the lower limit of the integration in Eq. 3 and has a large influence on the amplitude and sign of the thermo-osmosis coefficient. (b) Amplification factors for different solid-liquid interaction energies ε ls . All these data are measured using the mechano-caloric configuration.

FIG. 4 .

 4 FIG. 4. (a)Measured velocity profiles for the cases where a large disparity is observed between the theoretical and measured M12 coefficient through the thermo-osmotic route. The lower the ε ls , the larger the curvature of the parabolic backflow. (b) Comparison between 3 estimates of the thermoosmosis coefficient M12: theoretical prediction, value deduced from raw velocity measurement, and value deduced from velocity corrected with the backflow velocity (see text for details). (c) Evolution of the normalized pressure gradient responsible for the backflow as a function of the interaction energy. Blue triangle: derived from the correlation of theoretical and measured M12; black circle: calculated from the curvature of velocity profiles according to Poiseuille law.

The authors thank A.-L. Biance, O. Bonhomme, C. Ybert and C. Cottin-Bizonne for fruitful discussions. This work is supported by the ANR, project ANR-16-CE06-0004-01 NECtAR.


Morgan Meyer

Placing and tracing absence: A material culture of the immaterial

This essay engages with recent work on an unusual, yet fascinating theme: absence.

Although work on absence is rather rare and dispersed, there have been some recent texts and developments in this area that call for a more programmatic and focused engagement. This review essay considers two recent books that have contributed to these developments and are concerned with absence. It further considers them in relation to other relevant texts, such as the work of Kevin Hetherington (2003,2004). Two strands of arguments are pursued: first, ideas around the materiality and agency of absence; and, second, the related argument that absence is something that is 'placed' and that needs to be 'traced' and leaves 'traces'.

The materiality and agency of absence

Absences matter. The way in which they do is one of the key questions explored in the book An Anthropology of Absence: Materializations of Transcendence and Loss (Bille et al., 2010, referenced as AA hereafter). The introduction sets the scene: 'what may be materially absent still influences people's experience of the material world' (p. 4) The editors' starting assumption is that 'absence … may have just as much effect as material presence … We thus take absence to be as much of an occurrence in real life as presence ' (p. 10). Drawing mainly on the disciplines of anthropology and archaeology, the various chapters of the book tell stories of present absences in various forms: pictures of deceased people, inscriptions on graves, absence of knowledge and materializations of a tsunami disaster. We learn that the absent is made present through talk and texts, through thoughts and things. One of the key messages to take from the book is that it is via various kind of places, objects and practices that the absent can have an important effect on the social world -in other words, that absence has agency. This argument resonates with a key text by Kevin Hetherington (2004: 159), in which he argues that:

The absent can have just as much of an effect upon relations as recognisable forms of presence can have. Social relations are performed not only around what is there but sometimes also around the presence of what is not … Indeed, the category of absence can have a significant presence in social relations and in material culture. (emphasis in original)

In other words, absence does things, it is performative. Fowles (2010: 28, in AA) puts it this way: 'Absences push back and resist. They prompt us into action. And like present things, absences also have their distinctive affordances and material consequences.' Yet absence is not only something that does. Absence is also something we engage with, something we do something to. In comparing cemeteries and museums, Meyer and Woodthorpe (2008) note:

In a museum and a cemetery we can 'feel', 'see ', and 'hear' absence. In cemeteries, we are confronted with absence in the loss of people … In museums, we are confronted with the absence of the 'world out there' and/or the 'world that once was'. Both sites, hence, do something to and something with the absenttransforming, freezing, materialising, evoking, delineating, enacting, performing, and remembering the absent.

There are various public and private practices concerned with 'presencing absence'. One important group of professionals whose activities are directly concerned with making the absent present are archaeologists. Buchli and Lucas (2001: 173) write: 'The presencing of absence, that is, its materialisation through the archaeological act, makes things "matter".' This brings us to the second book, The Matter of Death: Space, Place and Materiality (Hockey et al., 2010;referenced as MD hereafter), which looks at the material culture of death, that is: 'the stuff, the spaces, the practices which surround the ending of embodied life, the disposal of the body, the mourning and memorialisation which then ensue ' (p. 16). In so doing, the book takes us to a variety of spaces where we encounter death: hospitals, residential homes, cemeteries, burial grounds, roadside memorials, sites where ashes are scattered. One chapter of the book even takes us to the imagined terrains of deities and spirits, or 'spaces of spirits': séances with mediums, temples, churches, shrines, pyramids, and sky burials. As the author of the chapter tells us 'spirits would not be the spirits they are without the places devised for their "presence"' (Davies, 2010: 212, in MD), arguing that these spaces have economic and political dimensions.

All in all, it is the materially grounded nature -the spaces, places, bodies, objects, architectures, equipments -of the human experiences and practices related to death that are explored in great detail in MD. For example, we learn about the many 'things' we find associated with death and on people's graves: flowers and plants, wind chimes, gnomes, glass ornaments, letters and cards, toys, mementos, ceramic mushrooms, solarpowered frogs (see the chapters by Bleyen and Woodthorpe in MD). One object, for instance, that powerfully evokes death is the 'spirit mask' of dead people used by the Melanesian Rauto during memorial dances. This spirit mask '"brings back to life" a representation of the dead, with personal memories evoked once more' (Davies, 2010: 212, in MD). Similarly, in AA, several chapters describe the various objects put on, or next to, graves, whether they are plants, candles, or stones. Two things are important to stress here: first, that people take 'care' of graves and objects; and, second, that the immateriality of the deceased may be 're-formulated and achieve serious immediacy and presence' by being subjectified and materialized through such objects (Sørensen, 2010: 118, 128, in AA; see also Hetherington, 2004: 171 on the notion of care for the absent). In her captivating chapter about the Asian tsunami disaster in 2004, Hastrup (2010: 100, in AA) examines a memorial monument and various 'scarred objects'; she argues that 'missing, broken or unfinished artefacts and their ambiguous nature as both present and absent, served as common materializations of loss and seemed to function as a means for the survivors to reclaim their everyday life in the aftermath of the disaster.' However, we must be careful not to conflate absence and materiality. For absences are 'part of the material world', but also 'foreign to that world' (Fowles, 2010: 39, in AA). Absence turned into matter is necessarily uncertain, unstable matter. It is difficult, perhaps impossible, to stabilize, formalize, or define absence. For absence is, by its nature, unattainable, unstable, not present -hence the paradoxical nature of any discussion about the materiality of absence, an issue raised in several chapters in both books. Perhaps one way to overcome this paradox and the 'ambiguous materiality' of absence is to think of absence as an 'immaterial thing' or 'more than immaterial and less than material' (Bille, 2010: 179, in AA). What is more, the focus on materiality in recent academic debates might have given us a 'crude notion of presence linked to physicality and tangibility' (Fowles, 2010: 25, in AA) and the risk that we too quickly equate materiality with physicality (Meskell, 2010: 207, in AA). But, conversely, we should not conflate absence with immateriality either (Buchli, 2010, in AA).

Places of absence

There are some edifices that are famous for having been partially or entirely destroyed, yet still have an enduring presence: the Berlin Wall, the Bastille in Paris, the Buddhas of Bamiyan, the city of Hiroshima, the Twin Towers in New York. Yet if there is one place that is probably most commonly associated with absence, it is the cemetery. The cemetery is an institutionalized place for memorializing absence. It is a space where we literally find the remains of those who once lived, usually underneath the ground and hidden from view. It is also an institution in the sense that we encounter professionals, explicit and implicit rules and codes of behaviour that manage and order death in a certain way within this space. (On cemeteries, see especially Woodthorpe's chapter in MD and the chapters by Renshaw and Sørensen in AA.)

This does not mean, however, that death and absence can be entirely limited to, and contained in, places and institutions such as cemeteries. As Hockey et al. (2010: 18) point out, 'death … cannot be contained within spaces specifically and conveniently "set aside".' We are always dealing with partial containment of death (p. 233). The trajectories of ashes in the UK -from the crematorium to the diverse sites where they are finally disposed -are a telling example of the multiple destinies of human remains. Apart from the cemetery, ashes are disposed in the landscape, in water, in pubs, on football grounds; they are kept at home, sent into space, turned into diamonds … Ashes have no simple, predetermined destinations, but their trajectories are rather open, creative, indeterminate, participative processes that take time and are fraught with hesitations. In fact, the matter and the location of ashes is important to people (Kellaher et al., 2010: 137, in MD). Another example are burials in people's own private gardens, which trigger varied reactions, some people welcoming them, others seeing them as inappropriate, tasteless, or disgusting (Walter and Gittings, 2010, in MD). What is at stake here is that the often unmarked and unbounded grave threatens the boundaries between the living and the dead, between death space and domestic space. Roadside memorials are a further example: their 'location, outside of the conventional and acceptable contexts such as cemeteries … disrupts attempts to contain these reminders of mortality' (MacConville and McQuillan, 2010: 207, in MD). But, it must be noted, even within the cemetery the 'location' of the dead can be ambiguous and multifaceted. A study of the visitors to the City of London Cemetery shows that people refer to 'three different "spirits" of the deceased, or possibly one "soul" with different "spirit" manifestations of "presence"': one separated from the body and gone to heaven; one that is at the gravesite (their 'new home'); and one present 'all the time' or at people's homes (Francis et al., 2005: 123). As we see, the location of deceased people relates not only to (partial containment in) real spaces, but also to imagined ones. Metaphors and images can play an important role, for instance, in the case of stillborn babies, as the following quote suggests: 'Just like the child, gnomes and stars are ambiguous. They are absent and present at the same time. They are close and far away. They are here and there. They are part of our time and space and they are not part of it' (Bleyen, 2010: 79, in MD).

Traces of absence

One argument that is only rather implicitly made in the two books reviewed here is what we could call the relational ontology of absence. We find some discussion that points in this direction, for instance the argument that 'personhood and identity emerge as relational, negotiated concepts that refer to spatially located practices' (Hockey et al., 2010: 227) or that we should think about materiality and immateriality as 'a spectrum of engagements and entrapments' (Meskell, 2010: 212, in AA). In other chapters we read about the 'connections', 'dialogues' or 'bonds' between the living and the dead. And most chapters contend that, in one way or another, absence goes with presence and that both stand in a mutual relationship. We might, therefore, talk about the relational ontology of absence. This is how John Law (2010: 272) defines relational ontology: 'what there is in the world, social and natural, is an effect of uncertain and provisional relations of representation, political and scientific.' All material entities are produced in relations; what defines and makes them is their relational materiality. Bones, for instance, are such relational entities, the forms and material properties of which are emergent through the interactions which take place with them … they come to be defined and shaped within relations of various sorts -relations which can be at once material and social, emotional and political, and which develop and change over time. (Hallam, 2010: 467, 468).

The challenge is, then, to use such an approach to explore the immaterial world as well. Essentially, this means to see absence not as an existing 'thing' in itself but as something that is made to exist through relations that give absence matter. It means seeing absence as something performed, textured and materialized through relations and processes. Thus, a second point to make is that, rather than talking about 'connections' between the absent and the present, perhaps a more fertile and less dualistic way of thinking is to conceive absence as a trace and as something that needs to be traced. Hetherington (2004: 162) reminds us that 'the absent is only ever moved along and is never fully gotten rid of' (emphasis in the original). So the question, both methodological and theoretical, becomes: how can we, as social scientists, trace absence? How do we follow and describe the movements, the attachments, the translations and representations through which absence becomes matter and through which absence comes to matter? The polysemic nature of the word 'trace' is appealing for our purpose here. For it denotes, first of all, an active and spatial act of drawing or mapping out something. Second, it means to find, locate, follow, trail, etc. Third, the noun 'trace' stands for a sign, a residue, a vestige. Tracing absence is thus at once a spatial and performative act; a movement that is always following and 'behind' its object and therefore unable to capture it fully; and, finally, a trace is something that points to, something that is incomplete, something that once was. These traces always lead us into other places, other directions, other times; they are always incomplete, elusive, slippery and awkward.

Conceiving absence as a trace and as something that needs to be traced raises further thought about the relation between absence and geography. Hetherington (2003Hetherington ( : 1941) ) has argued that 'the absent has a geography -a surrounding that implies both presence and present' (emphasis in original). The present discussion shows that we not only need to explore the multiple geographies of absence but also to examine the spaces that contain absence and how absence defies, escapes and moves within these spaces. In fact, we could summarize both AA and MD as books concerned with people's efforts to 'place absence' and the ambiguities and difficulties thereof. It is in his discussion about disposal that Hetherington (2004: 159) used the term 'placing', further noting that this term refers to both a spatial and a temporal category. He writes: 'Terms like "waste disposal" and "waste management" are misnomers. Rather, disposal is about placing absences and this has consequences for how we think about "social relations'" (emphasis in the original). Both books, in this sense, are concerned with the material culture and the productivity and consequences of 'placing absence'.

Final remarks

Throughout this essay, I have provided one possible reading of the two books (in relation to recent debates about absence) and highlighted two related sets of arguments: the materiality and agency of absence and the argument that absence is (to be) 'placed' and 'traced'. Of course, both books offer many more insights, debates and case studies than this essay was able to consider. They provide rich, detailed and engaging accounts for scholars interested in issues around materiality, death, absence, the body, bereavement, etc., whether they have a disciplinary background in archaeology, anthropology, history, sociology, or theology. Furthermore, both books show that the study of absence, loss and death can offer very stimulating insights, sometimes counterintuitive, at other times heart-breaking (and we cannot but imagine the painful research process behind most studies presented here). Rather than being a dry, uninteresting, immaterial object, absences are 'cultural, physical and social phenomena' (Bille et al., 2010: 4) and thus deserve academic scrutiny. It is now common to argue that we should not consider absence and presence as a dualism and that we should not strictly oppose the two. Both have agency; both are performative and have effects. But perhaps the time has come to move beyond such arguments. Rather than talking about presence and absent presence, we might for instance use terms such as 'propinquity' in order to talk about 'degrees of nearness in different registers, rather than presence … Propinquity rather facilitates presence in terms of relation, analogy, nearness in time, or nearness of place' (Buchli, 2010: 186, in AA). In other words, we need to theorize about death, absence, presence, etc. through concepts and approaches that relate, that hold together the absent and the present. Such relational approaches invite us to embrace absence and presence, materiality and immateriality, and to be symmetric: the effects and presences of both present and absent things can be similar. Yet this should not prevent us from recognizing that sometimes absence and presence can be very different things. A 'thing' like a chair or a human body can be very different from an 'immaterial thing' like the present absence of a dead person. An immaterial thing is less solid, less bounded, less complete, less 'presentable' than a thing. A 'thing' like a chair or a human body can more easily be 'held together' in one place, whereas an immaterial thing is, simply put, less coherent. Also, it is important to note that, when presence is turned into absence (a building destroyed, a person dying), we are faced with cuts, ruptures and irreversibilities. Even though the dead might have effects on and leave traces in people's social worlds, they cannot be brought back to life. A dead person, after all, 'does not talk back' (Hockey et al., 2010: 11). There are some things that dead bodies cannot perform.

Terms like 'dialogue with the dead' (MacConville and McQuillan, 2010: 207, in MD) have therefore to be used with caution. First, there is a risk in stressing all too quickly that there are 'bonds', 'connections', 'communications' between the dead and the living, or between presences and absences. While, of course, such connections do exist, such a stance might blind us to see the very partial, distributed and heterogeneous nature of these bonds. These connections might only be present in certain places, at certain times, but not at others. Also, these connections are very much defined by separations and ruptures, that is, by disconnections. Perhaps our current concepts need to be enriched to better capture such 'connections with the disconnected' or 'dis/connections' and the processes through which absence is at once 'placed ' and 'displaced'. (Hallam's, 2010, article about the relationality of bones also makes the point that both joinings and separations come into play to articulate bones.) Second, if we use a notion like 'dialogue with the dead' we need to be clear about what our notion of dialogue entails. For a 'dialogue with the dead' is most likely to be less verbal, less articulate, less 'in-between' (but rather 'through') entities, and less symmetrical than a dialogue between living persons. Yet, in fact, both books convincingly show that absence is far more than a discursive 'thing' in that it is performed through a material, technical and discursive assemblage. Thus, rather than seeking to reveal a dialogue 'with' or 'between' entities, the way forward, as both books demonstrate, is to map out all the places, metaphors and objects that do the 'talking'.

Recasting Mary Douglas's (1984: 35) famous definition of dirt as 'matter out of place', we might argue that, while absence is matter out of place, it is still placed through matter. Although, strictly speaking, absence is a thing without matter, absence is ordered, remembered, evoked and made discussable and sufferable through materialities. And even though absence escapes -and can only ever be partially and temporarily contained in -certain places, it is within these places and through leaving various kinds of traces that absence comes to matter.

Acknowledgements

The author would like to thank Paul Basu, Catherine Rémy and Martin Tironi for their comments and suggestions.