Stephen Hawes's Conversion

Marco Nievergelt

C S Lewis

AND PRINTING THE NAME OF JESUS

quarrels, as is suggested by a series of cryptic allusions in his late poetry. 5 Hawes makes much of his association with the ruling dynasty, most notably in the Example of Vertu, where Dame Sapience invites the titular hero, Vertu, 'of myn owne chaumbre [to] be grome ' (l. 403). But the nature of his intimacy with the royal person may have been far more symbolic and impersonal than his official title would tend to suggest. 6 There is little evidence that Hawes's ideological project, as articulated in the Pastime of Pleasure and the Example of Vertu, ever had any direct impact on royal policy, although there are interesting affinities between the two, and by 1510, a year after the coronation of the new King Henry VIII, he appears to have been out of a job. He bewails the loss of influence in the Conforte of Lovers, his last datable work (written 1510-11, published 1515). In this bitter, cryptic, and somewhat crazed and solipsistic meditation Hawes provides an oblique account of the reasons behind his exclusion from power while prophesying the future 'resynge of a knyght' (l. 547), who is none other than the poet himself. 7 In what follows, I turn to a work that is rarely discussed even by the small number of critics who have written on Stephen Hawes in recent years, and a work that is difficult to situate even within Hawes's already eclectic oeuvre. The Convercion of Swearers, first printed by Wynkyn de Worde in 1509, and reprinted on at least two, possibly three occasions, is at 5 See Edwards, Stephen Hawes,[78][79][80][81][82]Poets and Power, See Meyer-Lee, Poets and Power,179,188,and 190. 7 See Edwards, Stephen Hawes, pp. 77-87; Fox, 'Stephen Hawes and first sight a highly conventional diatribe against blasphemous speech. The topic is frequently attested in late medieval pastoral literature throughout Europe, and blasphemous speaking may indeed have been particularly widespread in the British Isles, if the slightly later statement by the well-travelled Andrew Boorde is to be believed: 'for in all the worlde ther is not suche odyble swearyng as is vsed in Englande, specyally amonge youth & chyldren'. 8 Hawes's poem, by contrast, is remarkable for its deliberate targeting of courtiers, and for its appeal to secular rulers to repress blasphemous speech in their dominions. 9 This combination makes the poem doubly conventional insofar as it deploys a wellestablished condemnation of blasphemy and participates in a popular early Tudor satirical tradition denouncing the evils of life at court, in the manner of Skelton and Barclay. 10 But as is often the case with Hawes, this unusual juxtaposition of otherwise traditional and predictable materials creates rather novel, perhaps even experimental results. Rather than merely reiterating a set of conventional beliefs about the evils of blasphemous speech, Hawes's short piece also functions as a sort of devotional meditation 'in reverse', containing some striking reflections on the affective, cognitive, and spiritual processes associated with the devotion to images of Christ's suffering body. 11 As I will argue, this meditation is 8 Boorde, The Fyrst Boke of the Introduction of Knowledge,p. 243. 9 See Convercion of Swearers ll. 64-77, 171-77, 353-9. I cite the edition in Stephen Hawes, The Minor Poems, ed. Gluck and Morgan. On the number of early editions, see further Morgan ''The Conuercyon of Swerers: Another Edition''.

10 See esp. Burrow, 'The Experience of Exclusion'.

11 On late medieval devotional practices and the focus on Christ's body in England, see Ross, The Grief of God. supplemented by an evocative reflection on the ability of print technology to serve the interests of both religious conformity and political control.

Figuring blasphemy and the sins of the tongue

As a 'lytell treatyse' (l. 39) running to 366 lines written in rhyme royal, the organisation of The Convercion of Swearers is simple enough, but the poem nonetheless manages to present an embedded voice within its primary narrative. It opens with a preamble praising the work of 'notable clerkes' in 'olde antyquyte' (ll. 2-3), drawing attention to the allegorical techniques of ancient 'poetes', who 'vnder cloudy fygure / Coueryd the trouthe of all theyr scryptures ' (ll. 13-14). This is characteristic of Hawes's fondness for allegorical expression elsewhere in his poetry, as is Hawes's celebration of his 'good mayster Lydgate' in this prologue (l. 22). However, there is little that is allegorical in the Convercion, and this is a poem that explores a rather different facet of the power of human speech: the 'treatyse' clearly stakes its claim to 'enlumyne' its readers (l. 54) but does so by analysing the destructive potential of blasphemous speech, and more specifically it does so by ventriloquising the voice of Christ himself, from line fifty-seven until the penultimate stanza.

The majority of the poem is framed as an address of Christ himself to the reader, in turn symbolically enclosed in a 'chartre' that is sealed by the images of Christ's 'blody woundes' (l. 346). The reader's attention is specifically drawn to the woodcut that adorns the central section of Wynkyn de Worde's printed edition, placed next to a set of indented verses that have sometimes been described as an early example of a pattern poem (see fig 8.1, below).

Hawes's meditation on the power of blasphemous speech thus establishes a close association between the effect of spoken, written, and printed word on one hand, and the power of visual and mental images on the other. 12 Ultimately the work suggests that for Hawes himself the ability to conjure up vivid mental images in the reader is the defining feature of poetic writing. This transforms the role of the poet, who is no longer just the author of a loosely didactic work aiming to facilitate moral regeneration. For Hawes, rather, poets can harness their craft and the new possibilities afforded by the mechanised reproduction of books to directly condition the mental, moral, and intellectual disposition of their readers, allowing them to participate in a much larger and more ambitious project of social, religious, and political engineering in the service of the king.

In choosing to approach the problem of blasphemous speech, Hawes would have been able to tap into a rich range of medieval discourses on the matter. The question of blasphemous speech was routinely discussed both in the rarefied realm of medieval law and theology, as well as in swathes of more accessible pastoral literature produced across Europe during the late medieval period; it would also have featured regularly in sermons. In the pastoral literature of the time such discussions would have been contextualised within a wider and more capacious discussion of 'Sins of the Tongue'. 13 As artisans of the tongue, poets too were in a particularly strong position to reflect on the theory and practice of transgressive speech in general, and blasphemous speech in particular. It is not surprising, then, that we have at our disposal a substantial body of discussions of blasphemy and/or 12 On the multimedial writing of Hawes's predecessor poet, John Lydgate, see the essay by Flannery in this volume.

13 See Owst, Literature and Pulpit in Medieval England, pp. 414-25; Craun '"Inordinata locutio"'; Casagrande and Vecchio, I peccati della lingua; Gambale, '"Par la langue les livre a martyre"; and Irène Rosier-Catach, 'Le blasphème.' instances of blasphemous speech in the work of medieval poets, including the work of Geoffrey Chaucer, which would have been familiar to Hawes. Both Chaucer's 'Parson's Tale' and the 'Pardoner's Tale' explore at some length the familiar and influential idea that blasphemous speech, and in particular swearing by the name or body parts of Christ (per membra Christi), amounts to a literal replaying of the passion, dismembering God's body anew:

In Flaundres whilom was a compaignye Of yonge folk that haunteden folye […] Hir othes been so grete and so dampnable That it is grisly for to heere hem swere.

Oure blissed Lordes body they totere --Hem thoughte that Jewes rente hym noght ynough --And ech of hem at otheres synne lough. ('Pardoner's Tale', CT vi. 463-76) For Cristes sake, ne swereth nat so synfully in dismembrynge of Crist by soule, herte, bones, and body. For certes, it semeth that ye thynke that the cursede Jewes ne dismembred nat ynough the preciouse persone of Crist, but ye dismembre hym more.

('Parson's Tale', CT x. 590)

The framing voice of the Convercion provides a closely analogous account of the effects of blasphemy:

The cruell swerers whiche do god assayle On euery syde his swete body to tere With terryble othes as often as they swere (ll. 40-42) Christ himself reprises the idea within his own address to the reader, giving the highly traditional motif a new, embodied, and experiential urgency, and lending it a distinctly visual character that is reminiscent of the iconography of the woodcut at the centre of the booklet:

Behold my body all to torne and rente With your spytefull othes cruell and vyolente I loue you ye hate me ye are to harde herted I helpe you ye tere me lo how for you I smerted (ll. 321-24) I must take vengeaunce vpon you sykerly That by your swerynge/agayne me cucefye (ll. 330-31) The conventional nature of this motif should not distract from the powerful symbolic logic that underpins it. As an act of speech, the gratuitous naming of Christ symbolically amounts to a complex inversion of the salvific power of language in the Christian tradition. Swearing per membra Christi amounts to a simultaneous denial of the divine nature of Christ, the redemptive power of the Word as manifested in the Incarnation, and the generative power of the Word. This idea plays an important role in Hawes's poetics more broadly, as is suggested by his observation in the Pastime of Pleasure that 'By worde the worlde was made orygynally' (l. 603). 14 Blaspheming in this manner thus also appears as a perversion or inversion of the sacramental efficacy of human speech in a liturgical context, particularly the ritualised formulae associated with the consecration of the host. This was a prominent and delicate question frequently discussed in the writings of medieval scholastic theologians. 15 Given such rich and powerful symbolic associations resting on the central idea that blasphemous speech literally amounts to inflicting physical injuries on the body of God, it is perfectly appropriate that a wide range of medieval religious authors should dwell on the often spectacular divine punishments visited on the originators of blasphemy. The mouth of a blasphemer becomes horribly deformed in an exemplum from the collection 'De superbia et presumptione', while a blaspheming gambler swearing by the 'eyes of God' is punished by having his own ocular bulbs burst forth from his face, and a youth swearing by the blood of Christ is punished by bleeding to death himself, and he ends up with a blackened tongue to boot. 16 As well as providing an adequate punishment of the perpetrator, such exempla 14 Cited from Hawes, The Pastime of Pleasure, ed. Mead.

15 See Rosier-Catach, La Parole Efficace, The first two examples are quoted in Craun Lies, Slander, and Obscenity, pp. 99 and 147, and are taken respectively from the collections with the incipits 'de superbia et presumptione' (BL, MS Add. 33956, fols 2 r -90v and 'incipiunt narraciones' (BL, MS Harley 3244, fols 72 r -86 v . See also Craun, 'Inordinata Locutio', pp. 159-62. The third is related in Owst, Literature and the Pulpit, pp. 423-4, and is taken from Richard Whitford''s Werke for Householders, printed by Wynkyn de Worde in 1530, sig. C.iiii. reaffirm, a contrario, the sacramental power of the spoken word to effect physical, corporeal, and metaphysical change in the created world and in the speaker. 17

The idea of blasphemous speech as a degrading and gratuitous 'repetition' of Christ's passion is widely attested not only in sermons, pastoral literature, and the poetry of Chaucer, but also in other forms of cultural production that may have provided a more easily accessible source for Hawes's elaboration of the idea, and which seem to provide closer symbolic equivalents for the latter. In particular, a number of fifteenth-century church wall paintings and at least one stained glass window similarly choose to develop the idea of blasphemous speech as a 'dismembering' of God's body by pairing a visual image of Christ's tormented body with an inscription reporting the direct address of Christ, or in some cases the Virgin Mary, to the reader/observer. 18 Such visual renderings provide close analogues for the central part of Christ's address (ll. 113-56), which is metrically differentiated from the surrounding text by its use of tail-rhyme to create an effect that has sometimes been 17 The idea is captured in the recurrent formula Sacramentum id efficit quod figurat ('the sacrament is a sign that also causes what it represents'), bequeathed to the scholastics by Peter Lombard''s Sentences, iv. i. 6. See Peter Lombard, Sententiae in IV libris distinctae, ii. 746. See further Rosier-Catach, La Parole Efficace, pp. 74-78 and passim. 18 For further details on the materials presented here, see Rouse, 'Wall Paintings in the Church of St John the Evangelist, Corby Glen'; Woodforde, 'The "Blasphemy" Window at Haydon ', pp. 183-92; Gill, 'Preaching and Image', esp. pp. 169-71; and De Filippis, 'Exhibete Membra Vestra', esp. pp. 139-44. compared to that of later 'pattern poems'. 19 The address is placed immediately after the image of the wounded Christ (see fig. As will be evident from this short description, the combination of the visual presentation of the mutilated Christ with the inscribed lamentation provides a powerful analogue to the Convercion. The comparison with the window at Haydon suggests that four of the five figures in the upper panels above the image of Christ in the Convercion may denote the swearers themselves. Their style of dress appears to be distinctly aristocratic and suggests that they may have to be identified as the blasphemers dwelling 'in [...] regall courtes' (69) who are singled out for particular censure in Hawes's poem. Hawes, with East Anglian origins, may thus be developing the idea in response to the Window at Haydon very specifically, although it must be stressed that the association between blaspheming and extravagant clothing is part of the conventional set of associated motifs that also includes excessive food and drink, quarrelling, and gambling, all of which are combined in Chaucer's 'Pardoner's Tale'.

At Broughton Church, in Buckinghamshire, we find a wall painting with a similar configuration of swearers surrounding a pietà, this time without any accompanying text. The painting again visualises the notion that blaspheming amounts to a dismembering of Christ's body. At the centre, the body of Christ is horribly mutilated, missing a hand and leg, with bones protruding from the lower leg, and each of the swearers who surround the central pietà carrying a body part that corresponds, presumably, to their specific oath: hand, food, heart, bones, and a eucharistic wafer. The swearers can be seen quarrelling and playing a board game, activities widely associated with blasphemy in the pastoral literature as well as in Chaucer's 'Pardoner's Tale'. 21 The woodcut employed by Hawes similarly references gaming and gambling through the inclusion of a pair of dice in one of the small panels that 21 For a reproduction, see De Filippis, 'Exhibete Membra Vestra', p. 141. surround the figure of Christ, which also illustrate various body parts and instruments of the passion in the manner of the iconographic tradition of the arma christi. 22 A similar mise en scène could be found in the church of Walsham-le-Willows in Suffolk, only now preserved in a historical photograph. 23 Christ again appears badly mutilated and is accompanied by several figures engaged in the usual iconic activities: quarrelling/wrestling, eating, drinking, and gaming. In addition, Christ is surrounded by a variety of tools whose significance remains more difficult to interpret correctly, as they do not appear to depict the more usual arma christi flanking the figure of the injured Christ in the woodcut that adorns Hawes's short poem. The implements at Walsham-le-Willows may simply denote different trades and profession in an attempt to stress the ubiquity of blasphemous speech across all sections of society. 24 Scrolls are placed in proximity to these implements, and appear to record a variety of blasphemous utterances, although the text is largely illegible given the deterioration of the painting and the quality of the photographic reproduction.

A third wall painting on the same subject can be found at Corby Church in Lincolnshire. 25 Again we find a central pietà with the Virgin holding a mutilated Christ in her arms and surrounded by several fashionably dressed figures with scrolls issuing from their 22 On which, see Denny-Brown, and Cooper, eds. The Arma Christi in Medieval and Early Modern Material Culture. 23 For a reproduction, and a transcription of the legible portions of text, see Woodforde, 'The "Blasphemy" Window at Haydon', pp. 185-86 and facing page.

24 See Woodforde, 'The "Blasphemy" Window at Haydon ', p. 191-92. 25 See Rouse, 'Wall Paintings, Corby Glen', plate VII, facing p. 162. mouths, representing the blasphemers. In addition, seven devils can be seen harrying the blasphemers, a possible reference to the Seven Deadly Sins. On top of these pictorial representations, it is also worth mentioning the existence of the Middle English 'Arma Christi', a devotional poem on the instruments of the passion from the late fourteenth century, often abundantly illustrated, and with a number of surviving copies preserved in the form of rolls, presumably produced for public display in a church setting. 26 While this latest item is not overtly concerned with blasphemy, it provides an important example for the combined use of text and image to support devotional engagement with the passion of Christ in a church setting, in ways that echo the pictorial representations just reviewed on the one hand, and anticipate Hawes's approach in the Conuercion on the other.

All of the compositions just mentioned provide striking evidence for the influence of stock themes and motifs found in sermons upon the visual arts. The theme of the 'Warning to Swearers' appears to have been particularly successful in this sense, and Miriam Gill has pointed out that this specific theme is one of only two such sermon themes to have inspired multiple pictorial representations in late medieval English churches, the other motif being, significantly, the 'Warning to Gossips', another 'Sin of the Tongue'. 27 Such visual renderings of the 'Warning to Swearers' would have provided an ideal backdrop for frequent sermons on the topic, and it is clear from sources such as Reginald Peacock's Repressor that the mnemonic function of such devotional images was widely understood and appreciated by the official clergy. 28 But the particular potency of these images also relies on the integration 26 See Robbins, 'The "Arma Christi" Rolls'. 27 Gill, 'Preaching and Image', pp. 155-56, 169, and passim. 28 The Repressor of Overmuch blaming the Clergy, ed. Babington, i, pp. 213-14.

of written text within the visual field. Churchgoers would thus have been exposed to a threefold sensory input, relying on the combination of visual, aural, and textual media-an experience whose very density and intensity would have helped to bring home the lesson about the conspicuously embodied, corporeal consequences of blasphemous speech.

Laura de Filippis has recently analysed the logical status and rhetorical mechanics of combined verbal-visual renderings of familiar themes from sermons or exempla, including variations on the theme of the 'Warning to Swearers', suggesting that these highly mediated visual renderings function in ways that are analogous to the enthymeme, or rhetorical syllogism, as defined by Aristotle. This is defined as a strategy deploying 'examples of verbal and visual expressions that are based on a rhetorical system that alludes to, but does not fully reveal the schemata it suggests'. 29 Crucially, then, such an allusive use of imagery presupposes a latent familiarity with the internal logic of existing iconographic and symbolic correspondences, and doubles the mnemonic function of such evocations with an invitation to complete the 'argumentative' operation that characterises this type of incomplete syllogism. As I will argue in what follows, it is precisely the ability of text-image compounds to trigger a distinctive sort of 'cognitive' response that appears to be Hawes's primary objective in committing the Convercion to the printing press.

Hawes's combined use of text and image to convey this traditional message clearly goes beyond the pictorial analogues just discussed, especially in its ability to deploy a more expansive set of inscriptions in what is after all an illustrated poem, rather than a visual representation comprising only short sections of inscribed text. Hawes is not entirely alone in developing the idea that blaspheming amounts to a real, physical dismembering of the 29 De Filippis, 'Exhibete Membra Vestra', p. 135. body of Christ in poetic form. There is at least one surviving fifteenth-century poem that provides an extended version of a Marian lamentation over the injuries inflicted upon Christ by the blasphemers, followed by multiple instances of blasphemous speech attributed to representatives of different trades or activities-including tennis players-and running to eighty-four lines in total. 30 But Hawes demonstrates a unique and sustained interest in the material form and documentary status of his own poem and in its specific ability, as an artifact incorporating both visual and textual representation, to 'give body' to the central idea that blaspheming amounts to a physical dismembering of Christ's body. Christ's own address to 'myghty princes of euery christen rygyon' (l. 57), which occupies the central section of the poem, is presented as being contained in a 'charter' in the closing stanzas of the address (l. 346). Christ himself reminds readers that his own spoken address is mediated by a 'letter', drawing attention to its own corporeal, material existence as a document, both at the beginning and at the end of his speech (l. 61, l. 350), indirectly reminding readers of the poem's own material and documentary status. This enhances the Conuersion's ability to perform a distinctive, efficacious operation on the mind of the reader/observer, but also lends it an official and legalistic status, suggesting the influence of a very different literary tradition otherwise entirely unrelated to the theme of blasphemy, that of the Middle English 'Charter of Christ'. 31 The Charter of Christ exists in two versions, short (thirty-two lines) and long (anywhere between 234 and 618 lines), and manuscript numbers (currently 42 in total)

30 For an edition, see Brotanek,Mittelenglische Dichtungen, See Emily Steiner, Documentary Culture.

suggest that it was widely known and appreciated. 32 The appeal of this tradition to Hawes is evident if one considers the strongly corporeal and incarnational nature of the basic premise of the 'Charter of Christ', resting on the implied analogy between the parchment of the charter itself and Christ's skin on the one hand, and between Christ's wounds and the traces of ink on the other. Its underlying sacramental logic is particularly relevant for Hawes, whose own approach to the 'efficacy' of blasphemous speech already presupposes a sacramental understanding of language, as I have suggested above. In the words of Miri Rubin, the Long Charter of Christ displays a 'sacramental symbolism carried to the limit; [...] signifier and signified dissolved into one. The Eucharist is God, and his offering, the Charter, is at once evidence of a sacrifice, and the sacrificed body itself '. 33 Hawes reactivates these latent associations in a single stanza placed towards the end of his poem, fittingly acting as a conclusive 'seal' appended to the poem so as to authenticate the preceding document, stamped with the authenticating mark of Christ's own 'blody woundes', his corporeal

18

Prynte it in your mynde and ye shal helthe recure (ll. 346-52) This desire to frame and enshrine Christ's 'portrayture' within the authenticating 'seale' further enhances the strongly imagistic devotional appeal of the poem as a whole. At the same time, it provides an apt illustration of the wider tendency of late medieval devotional culture to perform, in the words of Vincent Gillespie, 'a consciously willed alteration away from rumination on texts towards rumination on an image'. 34 In this sense, Hawes's poem fully embraces the expressive potential associated with the shorter Charter of Christ, which according to Laura Ashe 'combines the imagistic qualities of the legal document with those of the meditation on the Passion'. This idea is best rendered in the illustration found in London, British Library, MS Add. 37049, fol. 23 r . 35

prynte it in your mynde

One further area of cultural developments that shaped Hawes's handling of word and image-and his understanding of word-as-image-is the advent of the printing press. Seth Lerer in particular has proposed a detailed and nuanced argument to demonstrate that Hawes's 'concern with the seen as much as with the heard' has to be seen as a response to the new expressive possibilities of print, allowing him to 'present [...] literature as a seen thing rather than as a performed piece'. 36 The Conuersion appeals to readers/observers to 34 Gillespie, 'Strange images of death', p. 112. 35 Ashe, 'The "Short Charter of Christ", p. 36. See the online facsimile of the manuscript hosted by the British Library. 36 Lerer, 'Impressions of Identity ', in Chaucer and his Readers, pp. 181 and 183. 'print' the image of the suffering Christ in their mind, inviting them to abstract and preserve in their memory the combined verbal-visual 'impression' made by the document. This is most evident in the penultimate stanza of Christ's address, just quoted, but also comes to the fore in the opening stanza of Christ's address to the reader: Hawes is unique especially in paying such careful attention to the cognitive process associated with the 'imprinting' of images and ideas upon his readers, which he consistently expresses in terms of an analogy with the mechanics of the printing press. This process creates a linguistic, textual, and cognitive palimpsest, where the blasphemous speech of swearers is simultaneously effaced, exorcised, and overwritten by Christ's own reported words, visualised and reified in the form of an illustrated letter. Christ's address becomes an instrument of 'remembrance', recomposing a unified and orthodox sacramental body from the scattered and dismembered body parts produced by plural and reiterated blasphemous utterances. The poem thus plays a spiritually and socially unifying role, enabling the mutual integration of the sacramental body of Christ/Charter, and the mystical body of the Christian faithful in the form of its many readers, all exposed to the salvific effects of the Convercion's 'impression' on their senses and their mind. In many ways such a strategy manages not only to displace and overwrite the harmful effects of blasphemous speech, but also to point towards the possible role of the printing press as a new instrument of devotion in the age of mechanical reproduction. This tendency is evident also in Hawes's slightly earlier Pastime of Pleasure, written in 1505-06 and printed by de Worde in 1509, where the narrator invites the reader to 'prynte in his thought' the tower of Chivalry, and the four 'ymages' supported by it (ll. 3001-02). 37 As well as drawing attention to the phenomenon of 'imprinting' experienced by the poem's readers, the Pastime of Pleasure provides multiple descriptions of the sensory action of images and/or texts upon the poem's protagonist Grande Amoure. One example is in his encounter with a three-headed giant. This is preceded by the chance discovery of a mysterious shield, covered with the image of a lion rampant, a 'blasynge [...] grauen vpon the goodly shelde [...] And vnder the shelde there was this scrypture ' (ll. 4286-90). The image requires concerted contemplation-'Whan I the scryupture ones or twyes hadde redde|And knewe therof all the hole effecte' (ll. 4291-2)-and leads to an ulterior series of similar experiences, where the hero is required to decrypt the inscriptions on three flags, one for each of the giant's heads. An enlarged and programmatic version of this kind of experience is placed at the very beginning of the hero's quest, where Grande Amoure is confronted with an arras that already 'dyde make relacyon' (447) of his coming adventures. The effect of this 'famous storye [that] well pyctured was' (475) is not only proleptic and ekphrastic, but suggests that the hero's future adventures, and his very identity, are already scripted, and in some sense determined and even engendered through the 'imprinting' that occurs when the hero gazes upon this artifact: 'vpon the aras ybrobred all of blew / What was his name with lettres all of grewe' (ll. 433-4). In the words of Seth Lerer, 'Graunde Amoure's own reading of these texts and pictures is itself a kind of writing, now a printing in the thought, as if the mind and memory themselves were paper, plate, or wax on which the pen, engraver's tool, or student stylus could incise the truth'. 38 The learning experience of Hawes' chivalric heroes accordingly consists largely in an experience of embodied reading, absorbing reified and illustrated text embedded into the more experiential framework of traditional chivalric quest. 39 Significantly, Hawes considers that it is a similar imprinting of 'ymages Lerer is correct, I believe, in identifying Book v, metrum 4 of Boethius's Consolation of Philosophy as a crucial nexus for the transmission of a broadly 'empirical' theory of sense perception that envisages the perceiver as a passive surface, or tabula rasa, to be 'inscribed' by sensory experience. 40 Boethius briefly exposes, before rejecting it, the theory of the ancient Stoics, who wenden that ymages and sensibilities [...] weren enprientid into soules fro bodyes withoute-forth [...] ryght as we ben wont sometyme by a swift poyntel to fycchen lettres emprientid in the smothnesse or in the pleynesse of the table of wex or in parchemyn that ne hath no figure ne note in it. (v, m4, ll. 6-20) The general idea and its associated images are well attested in several later Middle English texts identified by Lerer, such as the Book of Courtesy and the conclusion of Chaucer's 'Clerk's Tale'. A particularly striking instance of such 'impression' can be found in the Book of the Duchess, where the Man in Black describes his own, largely passive experience of-or exposure to-the doctrines of fin amor, in the manner of a blank surface ready to be inscribed: I trowe it cam me kyndely.

Paraventure I was therto most able

As a whyt wal or a table, For it is redy to cacche and take 40 Lerer, 'Impressions of Identity ', p. 185. philosophical ones. 41 Rather than being obsolete and outlandish, then, the Stoic theory as characterised by Boethius in fact converges in several important and fundamental ways with the dominant late medieval theory of 'abstractive' perception derived from Aristotle's writings on the soul, which had emerged during the thirteenth century in the wake of the reception of the De anima and its commentaries by Arabic scholars. 42 While Aristotle himself was far from consistent in his discussion of this question, he too had used the influential image of the wax tablet in the De anima, and the image clearly implies a passive understanding of the role of the perceiver in the cognitive process:

What it [i.e. 'thought'] thinks, must be in it just as characters may be said to be on a writing-table on which as yet nothing actually stands written (De anima iii.5, 429 b 1-430 a 1) 43 Aristotle's and Boethius's discussions of mental 'imprinting' rely not only on the example of visual 'ymages', but include clear references to wax and parchment, suggesting that the same phenomenology of 'impression' applies to the letters of the alphabet and more extended inscriptions of text as well as images or other forms of sensory perception. This 46 Cited from Guillaume de Lorris and Jean de Meun, Le Roman de la Rose, ed. Lecoy.

Translation from The Romance of the Rose, trans. Frances Horgan (Oxford: Oxford World's Classics, 1994). The passage is a direct translation from Roger Bacon's Opus maius, as was first identified by Akbari, Seeing Through the Veil, pp. 92-93. The relevant section can be found in Roger Bacon and the origins of 'Perspectiva', ed. Lindberg, pp. 332-35. the principles and properties of mirrors: they have such marvellous powers that all tiny things, tiny letters far from the eyes and minute grains of sand, are perceived so large.

Chaucer too is an important conduit for this tradition, and his dream visions feature The Book of the Duchess is particularly clear in its allusions to contemporary theories of cognition and perception, notably the theory of the internal senses and the associated partitioning of mental operations in the human brain. 48 The theory was originally elaborated by the Latin scholastics on the basis of Galen and Avicenna, but soon became available in 47 See also Lerer, 'Impressions of Identity', pp. 184-85. The With typical Chaucerian meta-poetic flair, the Romance of the Rose contemplated in this mental 'chambre' is not only the object of the dreamer's meditation, but also the proximate inspiration for this kind of complex epistemological and phenomenological reflection on the combined operation of visual sense perception, textual interpretation, and poetic representation. Crucially, Chaucer's reflection on the operation of such 'colours fyne' upon the mind is not limited to the realm of visual perception, but concurrently examines the cognitive potential of verbal, poetic craft, alluding to the tradition of the colores rhetorici in medieval artes poetriae or treatises on grammar or rhetoric, just as had been the case in 49 On the Properties of Things: John Trevisa's Translation of Bartholomaeus Anglicus, De Proprietatibus Rerum, ed. Seymour, Ch. 16, I, 107-08. poets used fiction (fabula) as a vehicle of philosophy'. 53 What seems to me even more remarkable in Hawes' discussion of rhetoric is his emphasis on the cognitive and psychological potentialities of poetic expression. The defining feature of rhetorical eloquence is no longer simply verbal skill, but poetic inspiration, and it is thanks to its inspired, vatic, and prophetic nature that poetry develops its vivid imaginative potential and powerful sensory appeal. 54 The chamber that provides the setting for the hero's encounter with Lady Rhetoric is symptomatic of this new understanding of the power of rhetoric: the chamber is adorned with 'myrrours of speculacyon' (l. 660), and Lady Rhetoric herself is 'enspyred with the heuenly influence' (l. 669). The effect of rhetoric is to 'indoctryne' the hero, 'That my dulle mynde it shoulde enlumyne' (ll. 683-4). These descriptions provide an exalted characterisation of rhetoric, especially if compared with its generally peripheral place in medieval intellectual culture, and its instrumental role in support of the more prominent sciences such as logic and/or theology. Hawes's emphasis on the imaginative and cognitive remit of rhetoric is particularly evident in the description of 'Invention', one of the five 53 Copeland, p. 76. While the ultimate point of origin for the idea may arguably be found in Macrobius-inflected by twelfth-century traditions of philosophical allegory-the immediate source for this idea is almost certainly the well-known speech of Lady Raison in the Roman de la Rose, ed. Lecoy, See also Griffith, 'The Object of Allegory ', pp. 144-53. subdivisions of rhetoric (l. 701-819). 55 Invention, we are told, 'sourdeth [i.e. arises] of the most noble werke', and finds its origin in the five internal senses of Avicennan/Aristotelian faculty psychology, the '.v. inward wyttes' (ll. 702-03): 'comyn wytte' (l. 706, sensus communis); 'ymagynacyon' (l. 708, imaginatio); fantasy (l. 722, vis imaginativa); estymacyon (l. 736, vis estimativa); memory (l. 750, memoria). Rhetoric, rooted in invention, is above all concerned with the ability of 'poetes' to leverage the imagination to 'make vs lerne, to lyue dyrectly / Theyr good entent' (ll. The concern for social and political control that animates the Pastime of Pleasure also informs Hawes's Conuersion. At one level, of course, the poem is a strictly didactic work, 56 For a more extended discussion of Hawes's mechanistic understanding of didactic poetry as an instrument of 'social engineering', in the context of increasing centralisation of royal power during the early Tudor period, see Nievergelt, Allegorical Quests, p. 85-89. seeking to bring about the reformation or 'conversion' of individual swearers. But Hawes's poem is also characterised by its insistent appeal to the ruling classes to control the blasphemous tendencies in their subjects-a 'reiterated address that is too insistent to be ignored'. 57 Hawes is again developing an established tradition attested in the pastoral and moral literature, where the ruler of an aristocratic household is exhorted to restrain any blasphemous tendencies in his familia. 58 Hawes however transforms the idea into a more clearly political point about the role of the English king in ensuring the moral righteousness of his courtiers and subjects (ll. 66-69, 353).

Hawes's innovation implies a subtle transformation in the understanding of the nature of blasphemous speech, away from a strictly moral definition to a more deeply social and political conception, indissociable from a wider desire to regulate and maintain social order in an age of increasing centralisation of royal power. This is a development that in England is largely inaugurated by the first Tudor monarch, Henry VII. 59 The change also 57 Edwards,Stephen Hawes,p. 73. 58 Similar ideas are discernible in John Bromyard's influential Summa Predicantium, where 'princes and lords' are exhorted to impose fines and temporal punishments on blasphemers, stressing that the latter are far more effective than the threats of eternal damnation coming from the pulpit. See Owst, Literature and the Pulpit, p. 421, discussing Bromyard's entry for 'Juramentum'. A more 'domestic' version of the same argument, supplementing traditional pastoral concerns with the more social preoccupations of socalled 'conduct literature', can be found in Peter Idley's Instructions to His Son, ed. D'Evelyn, ll. 2.A.755-61, discussed in Strub, 'Oaths and Everyday Life in Peter Idley's Instructions'. 59 Watts, John L. '"A Newe Ffundacion of is Crowne"'

  the Political Allegory of The Comfort of Lovers'; Meyer-Lee, Poets and Power, pp. 179-90; Griffiths,'The Object of Allegory'; and Nievergelt, Allegorical Quests from Deguileville to Spenser, 91-95.

  8.1, below). [Insert fig. 8.1 here, approx. ½ page black and white]. The image as found in William Copeland's The Conuersyon of Swerers (London: 1551), which reproduces the woodcut as it appears in de Worde's 1509 edition of Hawes's text. The stained-glass window at Hayden Church in Norfolk, no longer extant but described in detail in two separate historical records, contained representations of multiple swearers, accompanied by a series of scrolls issuing from their mouths in the manner of speech bubbles, along with a representation of the suffering Christ and the transcription of an extended lamentation by his mother Mary, addressed to the blasphemers: Alas my chyld qwy haue ȝe þus dyht Ȝe cursyd swererys all bedene Hys lemys be rent asundyr ryth Alas my mone how may I mene Þe Jewys þat on þe cros hym pyte Þei wist nowt qwat þei ded inded 20 19 See the discussion in Edwards, StephenHawes, Cited from Woodforde, 'The "Blasphemy" Window at Haydon', p. 185-86.

'

  portrayture', and his implied voice, the instantiation of the Word made flesh: With my blody woundes I dyde your chartre seale Why do you tere at it / why do you breke it so Syth it to you is the eternall heale And the releace of euerlastynge wo Beholde this lettre with the prynte also Of myn owne seale by perfyte portrayture 32 See Spalding, The Middle English Charters of Christ.

  Ryght myghty prynces of euery crysten rygyon I sende you gretynge moche hertly & grace Right wel to gouerne vpryght your dominyon And all your lordes I greete in lyke cace By this my lettre your hertes to enbrace Besechynge you to prynte it in your mynde How for your sake I toke on me mankynde (ll. 57-63)

37

  See also PP 3407; and Example of Vertu 203. I refer to the edition of the Example of Vertu contained in Hawes, Minor Poems, ed. Gluck and Morgan.

  sygnyfycacyoun' upon the mind that allow the ideal Ciceronian orator to inwardly perform 'a recapytulacyon / Of eche ymage the moralyzacyon' (Pastime of Pleasure, ll. 1252-55): So is enprynted in his propre mynde Euery tale with hole resemblaunce By this ymage he dooth his mater fynde Eche after other withouten varyaunce. (Pastime of Pleasure, ll. 1261-64)The idea of a nearly physical imprinting of sensory 'ymages' upon the mind is of course a well-established idea with its roots in antiquity, and Hawes here draws on a number of closely related poetic traditions and contiguous scientific and philosophical discourses. Seth38 Lerer, 'Impressions of Identity', p. 185.39 Wakelin, 'Stephen Hawes and Courtly Education'; Nievergelt, Allegorical Quests,

41

  See esp. Tachau, 'Seeing as Action and Passion'; and, for a more general overview, Tachau, 'Approaching Medieval Scholars' Treatment of Cognition'. 42 The literature on the topic is vast. For overviews, see De Boer, The Science of the Soul; Mousavian and Fink, The Internal Senses in the Aristotelian Tradition; and Lagerlund, Forming the Mind. 43 Aristotle, The Complete Works of Aristotle, trans. Jonathan Barnes. idea is picked up and developed in the hugely influential Roman de la Rose, and goes on to influence the entire Anglo-French tradition of narrative allegory that is certainly a hugely important reference point for Stephen Hawes. 44 Not only does the Rose already present a series of inscribed objects, it also draws attention to the kinds of cognitive phenomena associated with the process of sensory 'abstraction' of both image and text in the mind, and does so with clear reference to recent scientific literature on the question, particularly the writings on optics and perspectiva by Roger Bacon. 45 Dame Nature thus presents les forces des mirouers [. . .] tant ont merveilleus pouers que toutes choses tres petites, letres grelles tres loing escrites et poudres de sablon menues, si granz, si grosses sunt veües (RR 18,015-20) 46 44 See Williams, The French Fetish from Chaucer to Shakespeare, pp. 114-80; and Nievergelt, Allegorical Quests, pp. 74-96. 45 See Pomel, 'Visual Experiences and Allegorical Fiction'; and Nichols, 'The Pupil of Your Eye'.

  extended meditations on the sensory stimulation provided by a combination of textual and visual input, often indistinguishable from each other, as in the liminal visionary scenes of the Book of the Duchess, the House of Fame, and the Parlement of Foules, These inaugural scenes demonstrably influenced Lydgate's Temple of Glas, a poem that Hawes himself knew, and mentions by name in the Pastime of Pleasure (l. 1365). 47

  Chaucerian motif of the pagan Temple replete with a combination of visual and textual fragments reminiscent of the classical past, and with particular attention to the matter of Troy (cf. Book of the Duchess ll. 321-43; House of Fame ll. 119-479; Parlament of Foules ll. 120-308; Lydgate, Temple of Glas ll. 15-142), is redeployed by Hawes in his description of the Temple of Mars in the Tower of Chivalry, in the Pastime of Pleasure, ll. 3018-38. 48 See especially the essays in Lagerlund, Forming the Mind; and Mousavian, and Fink, The Internal Senses in the Aristotelian Tradition. simplified form in encyclopaedic materials such as Bartholomaeus Anglicus's De Proprietatibus Rerum. Significantly, Hawes himself includes a relatively detailed overview of the theory of the internal senses in the more didactic sections of his own Pastime of Pleasure (2780-877). The Book of the Duchess situates the dreamer's liminal visionary experience of text and image, all but indissociable, within a 'chambre' (ll. 299 and 321) that ultimately denotes the dreamer's imaginative faculty-Chaucer's 'celle fantastik' (CT, 'KT' i.1376), identified as one of the brain's multiple 'chambres' in Trevisa's roughly contemporary Middle English translation of the De Proprietatibus. 49 In this chamber, [...] al the walles with colours fyne Were paynted, bothe text and glose, Of al the Romaunce of the Rose. (ll. 532-4)

  1114-16): And euermore they are ymagynatyfe Tales newe from daye to daye to fayne The errynge people that are retractyf As to the ryght waye to brynge them agayne (ll. 1121-24) Conclusion: The Politics of Print Hawes's reference to 'The errynge people' is symptomatic of the larger political and social ideals that frame his unique and distinctive interest in the cognitive possibilities of poetic expression. Instead of merely pursuing the moral reformation of an individual Christian viator-as was the case in the of the 'Pilgrimage of life' narratives that were among Hawes's sources-poets pursue the political reformation of those members of the commonwealth 55 See also the observations in McMillan, 'John Lydgate, Stephen Hawes, and the Making of Henry VIII', pp. 293-94; Edwards, Ratio and Invention, p. 86; and Reid, 'Through a Looking-Glass'. that are perceived to be 'retractyf'. Rhetoric becomes a precious political asset, an instrument of social control exploited by the sovereign with the aid of poets/rhetoricians: 56 The people [...] They hadde none ordre nor no steadfastnes Tyll rhetorycyans founde justice doubtles Ordenynge kynges of ryghte hye dygnyte Of all comyns to haue the souerainte The barge to stere with lawe and Iustyce Ouer the wawes of this lyfe transytorye [...] O what laude glory and grete honoure Vnto these poetes shall be notyfyed The whiche dystylled aromatyke lycour Clensynge our syght with ordre puryfyedWhose famous draughtes so exemplyfyed Sett vs in ordre grace and gouernance(ll. 877-84 and 890-95)

Al that men wil therin make, Whether so men wol portreye or peynte, Be the werkes never so queynte (ll. 778-84) Whether the Man in Black's exposition of this loosely Stoic theory allows us to infer that Chaucer too took this to be a viable account of cognition is more difficult to determine. Indeed, Boethius himself rejects this theory in the second part of Book v, m4, in line with his broadly Neoplatonic emphasis on the active powers of the mind: But yif the thryvynge soule ne unpliteth nothing (thet is to seyn, ne doth nothing) by his propre moevynges, but suffrith and lith subgit to the figures and to the notes of the bodies withoute-forth, and yeldith ymages ydel and vein in the manere of a mirour, whennes thryveth thanne or whennes comith thilke knowynge in our soule, that discernith and byholdith alle thinges? (V, m4, ll. 22-30) Chaucer was thus in a position to read these two accounts against each other, and would have been able to appreciate the weighty, and fundamentally divergent philosophical implications of both accounts. It is worth stressing that for an educated, and philosophically alert fourteenth-century reader like Chaucer, Boethius's reference to the Stoic theory of perception was far from being a purely academic and anecdotal detail. The question of the ultimate nature of perception, active or passive, was at the heart of heated scholastic debates on faculty psychology throughout the thirteenth and fourteenth centuries, as this raised some fundamental anthropological and theological questions, as well as more strictly 65, 705, 712, 796, 933, 942, 950, 1353, 1992, 2016, 2228, 2669, 2803, and 5113. On the distinctive allegorical aesthetics in Hawes's poetry, and on his deliberate obscurity and the use of 'cloudy' metaphors in his writing, see especially Griffiths, 'The Object of Allegory'; and Atkinson, '"Vnder Coloure I Dyuers Bokes Dyde Make"'. 52 Copeland, Rita, 'Lydgate, Hawes, and the Science of Rhetoric'. reflects a much broader shift in terms of the institutional means mobilised to repress blasphemy during this period, with a clear transition from ecclesiastical to secular authorities, suggesting that what is traditionally a 'sin' increasingly came to be perceived as a 'crime'. 60 Most importantly, perhaps, the shift in attitude suggested by Hawes's treatment provides an almost perfect illustration of David Lawton's contention that 'blasphemy is an exchange transaction, and what is at stake is community and identity formation'. 61 The transaction set in motion by Hawes's poem, then, is clearly one that simultaneously pursues the moral integration of the individual sinner, the unification or 'remembering' of the mystical body of Christ, and the imposition of uniformity within the body politic. Or, to cite Hawes's own words in his Ioyfull Medytacyon on the coronation of Henry VIII: 'Lordes I humbly you exhorte|Spyrutuall and temporall with the comyns vnyfyde' (ll. 54-55). 62 Hawes's fascination with the new possibilities afforded by the printing press needs to be seen in the context of this wider interest in fostering social uniformity. It is symptomatic that his description of the ideal Ciceronian orator/poet/rhetorician should dwell so insistently on the latter's infallibility, described as mechanised process of visual and cognitive 'imprinting':

Than eche ymage inwarde dyrectly

The oratoure doth take full properly So is enprynted in his propre mynde 60 Leveleux-Teixeira, La Parole Interdite. 61 Lawton, Blasphemy, p. 21.


M Khatib

B Bouilly

T Sim

R Chatila

Indoor Navigation with Uncertainty using Sensor-Based Motions

This paper presents an operational framework to bridge the gap between planning with uncertainty and real-time sensor-based motion control. The environment being known, a planner produces a plan composed of free space and sensor-based motion commands. The representations of uncertainty and its evolution, environment landmarks, and actions generated at the planning level are discussed. Sensor-based actions and command de nitions for a nonholonomic mobile robot based on a Task-Potential eld approach are developed. These various elements are integrated in a system that actually generates the motions of the Hilare2 mobile robot.

Introduction

Motion planning and control are two important aspects required for mobile robot autonomous navigation. However, they are frequently considered as two separate phases. Many motion planning techniques have been proposed (see 7] for a survey) to produce a collision-free and feasible path avoiding the obstacles of the workspace. Motion control then consists in making the robot follow this path, possibly using a potential-eld based method 12,[START_REF] Khatib | Contrôle du mouvement d'un robot mobile par retour sensoriel[END_REF] for reacting to unexpected obstacles sensed during the execution. Such an approach su ers from a lack of robustness when experimented in real world settings since the execution of the planned trajectory is conditioned by several sources of uncertainties.

Control and sensing errors are particularly signicant in the context of mobile robot navigation. Indeed, mobile robots are not generally equipped with an absolute positioning system and are subject to cumulative dead-reckoning errors. Inaccuracies in localizing the robot with respect to its environment may lead to failures during the execution of the motions eventually preventing from reaching the goal.

To overcome the uncertainty accumulated during the motions, the robot has to be equipped with environment sensors that can provide additional information by identifying appropriate features of the workspace. Localization techniques (eg. [START_REF] Bouilly | A numerical technique for planning motion strategies of a mobile robot in presence of uncertainty[END_REF][START_REF] Lazanas | Motion planning with uncertainty: a landmark approach[END_REF]) can be applied to reduce the uncertainty of the estimated con guration. One may also use sensor-based motion commands which are more tolerant to errors than classical position-controlled primitives. Sensor-based control has been so far limited to speci c implementations -e.g., wall-following using ultrasonic sensors 11], vision 13]-, without connection with a planning component. It is however preferable to reason during the planning process on the uncertainty that will be accumulated during the motions and on the capacities of the available sensing functions in order to produce robust motions plans that will reliably guide the robot towards its goal. Among the contributions to motion planning under uncertainty 7] which addressed this issue, only a few described navigation experiments performed with real robots: the landmark-based planner of 8], applies the preimage-backchaining approach 10] for navigating a mobile robot subject to control errors through circular landmarks where it can exactly determine its position. The SUF-based planner of 15] allows to take into account more sophisticated models of sensing uncertainty for generating a trajectory that minimizes the expected localization errors.

In this paper, we consider the navigation problem for a mobile robot operating, in presence of control and sensing errors, in an a priori known o ce-like environment. We present the algorithms that have been developed and integrated onto our Hilare2 robot for achieving robust motion planning and control. The planner, based on the approach presented in 3], considers a set of motion commands which may accumulate uncertainty and plan the corrective sensor-based motions needed to reduce it. It is able, to combine several sensing modalities, and also to navigate, when-ever possible, without relocalizing the robot when the task does not impose it. The sensor-based motions are implemented based on a general Task-potential eld approach 11, 5] which allows to directly express the potential function from a space parametrized in terms of the sensor output variables. This system actually runs in our laboratory and enables Hilare2 to achieve motion plans robustly.

The motion primitives

We rst describe the available motion primitives dened both at planning and execution level. These primitives can be classi ed into two categories: Open-Loop Free Space Motions 1 and Sensor-Based Motions.

-a-

MOVE LANDMARK(

; L; r): has a similar de nition as the previous command. However this command uses the measure provided by an external sensor to perform an absolute relocalization of the robot during the motion.

SBC MOVE TO CONTACT(d c ; L max ): is a guarded motion command which executes a straight line motion stopped when an object is detected at a frontal distance d c (Fig. 2-a). A maximum motion length is de ned by the parameter L max .

SBC PARALLELIZE ( max ; side): is a sensor-based command executed to align the robot to a wall (Fig. 2-b). max is the maximum allowed rotation and side determines the direction where the wall has to be searched.

SBC WALL FOLLOW (walls list ): is a composed sensor-based command for following a succession of walls (Fig. 2-c). Several events (vertex, traveled distance, obstacles, ...) are checked during its execution. Each wall is de ned by a structure where: 1 ie. using no exteroceptive feedback -a- side 2 fleft; rightg is the wall relative location, d w is the distance to the wall, L 1 is the length of the motion executed without activating the vertex detection system , L 2 is the maximum motion length allowed for detecting and matching a vertex de ned by its type (type 2 fConvex; Concaveg) and by the relative orientation of the next wall in walls list .

3 Planning robust strategies

Problem statement

The planner 3, 2] considers a polygonal description of the obstacles and also a set of landmark regions where the robot can be relocalized in the environment. It maintains a model of the uncertainty in order to produce robust motion strategies composed of a sequence of the primitives de ned in section 2. The strategy guarantees that the robot, starting from an uncertain initial position, can reliably reach its goal with an error lower than a pre-speci ed value, despite the errors on its position accumulated during its motion. The uncertainty can be reduced either by entering in the landmark regions (absolute relocalization) or by detecting some edges or vertices of the known polygonal obstacles.

Dealing with uncertainty

For the planner, motion primitives are de ned by the initial situation where they can be applied, by the MOVE primitives induce cumulative errors due to odometry. The position uncertainty is then dened as a function linearly dependent on the distance covered by the robot. After a SBC MOVE TO CONTACT, uncertainty is set to the proximity sensor accurary along the direction orthogonal to the edge. For the SBC WALL FOLLOW command, the error linearly grows only in the direction of the followed edge because of the use of the odometric sensor to determine the covered distance. MOVE LANDMARK uses an absolute localization of the robot which resets the position uncertainty to the sensor accuracy modelled by a disc of xed radius.

Figure 3 shows the uncertainty evolution for di erent strategies that could be produced by the planner for going from \Init" to \Goal" position. Free space motions only are not possible since the accumulated error would be too important to safely pass the doorway. Strategy 1 solves the problem by traversing a landmark region (within which the uncertainty remains constant) whereas strategy 2 illustrates the evolution of uncertainty using wall-following motions.

Planning approach

The algorithm is based on the propagation of a numerical potential modeling the uncertainty over a grid superposed on the workspace.

During the propagation of the potential, collisions are checked between the obstacles and the robot shape grown by the current uncertainty. The robot is approximated by its enclosing disk during the free-space motions. The exact shape is only considered during the wall-following commands for which the robot has to be aligned with the wall. This allows to solve constrained problems (eg. crossing of narrow doorways) where the circular approximation would be too conservative, while only considering a two-dimensional conguration space.

When the propagation reaches a position which may collide with a given edge because of a too large uncertainty, a geometric visibility test is performed to check if a SBC MOVE TO CONTACT is guaranteed to end onto this edge. When this test succeeds, the algorithm starts from this position a propagation adapted to the uncertainty model of the SBC WALL FOLLLOW command. Similarly, a propagation adapted to the MOVE LANDMARK command is started at positions where the robot grown by its current uncertainty is included in a landmark region.

Finally, once the goal position is reached with an admissible uncertainty, the motion plan is deduced by backchaining con gurations from the goal to the initial position. The strategies returned by the planner either minimize the distance traveled by the robot (ignoring in this case relocalizations when the task does not impose it) or the nal uncertainty.

Sensor-based actions

The sensor-based motion primitives are de ned at the execution level by speci c functions called Task-Potential functions. The arti cial potential eld method 12] is the basis of a Task-potential concept developed in [START_REF] Lozano-Perez | Automatic synthesis of ne motion strategies for robots[END_REF][START_REF] Crowley | World modeling and position estimation for a mobile robot using ultrasonic ranging[END_REF]. This approach can be viewed as increasing the low level control capabilities. Here, we present the basic idea of this concept and describe how to implement low-level sensor-based actions.

Task-potential Formulation

Let q denotes the robot con guration. If F i (t); t 2 0; T ]; i 2 0; : : : ; n] are positive and monotonous scalar functions of time, and P i (q) are positive, di erentiable and convex potential functions, the potential function P(q; t) obtained from a linear application P(q; t) = X i F i (t) P i (q) ; 8t 2 0; T ]; (1) is a convex potential function where @P @q j qm;t=T = 0; and has q m as a unique solution. F i (t) is called a control function. Proposition 4.1 Executing and controlling a robot sensor-based task can be expressed by the gradient descent of a Task-Potential function P(q; t); t 2 0; T ], within a linear composition of several convex potentials.

Proof and details are developed in 5]. Based on this proposition, we give, in the following subsections, the implementation ideas for sensor-based actions needed by our system.

Move To Contact

The contact is de ned by a distance d c between the robot and the object (Fig. 2). If d is the current distance to the wall, the robot has to generate a motion which minimizes the term kd d c k. The robot displacement is limited by the length L max (see denition in section 2). Using the Task-potential formulation (eq. 1), this can be achieved by the following convex potential P c (d; t) = f (t) P c (d); where f (t) 2 0; 1]; t 2 0; T ] is an onto monotonous decreasing function of the current displacement value l(t) 2 0; L max ]. In fact, f (t) insures a null potential value for l(t) L max . P c (d) is a user-de ned convex potential having its global minimum at d = d c (an example of such potential and control functions are proposed in 5]). Here, the distance measures and the force are totally projected on the robot x-axis thus generating, a pure linear acceleration. The asymptotic stability, at the global minimum, is insured by adding a dumping term to the total force giving F c = rP c k v _ q;

(2) where k v is a positive gain and _ q denotes the robot velocity.

Parallelize

The aim of this command is to position the robot in a valid con guration to start a wall following command. More precisely, the goal is to minimize the angle between the wall segment and the robot x-axis (Fig. 2.b). This action formulation is completely similar to the previous one. But here, the force derived from the potential represents a pure torque around the robot rotation center. This is why the dumping term depends on the angular velocity (eq. 2).

Wall Following

We assume here that the robot is initially placed at a position such that the wall is detectable by the sensors. We desire it to follow the wall keeping a given nominal distance d w from it. This task is maintained until a certain condition is disabled (e.g. end of the wall reached, traveled distance value, etc.). The wall is represented by a segment S lying on the x-axis of a Fresnel frame R w associated to it (Fig. 4).

The robot con guration q = (x; y; ) T is expressed in this frame. In order to achieve this action, we imagined an attractive potential P x along the wall x-axis that attracts the robot to the end of the wall at the position x p . This potential will guide the robot along this direction until the global minimum at x = x p . Similarly, to maintain a d w distance, we attribute to the wall y-axis a potential P y , function of the robot y position whose minimum is at y = d w . In addition, in order to keep the wall visible by the sensors, we have to achieve an incidence angle max to the wall. Thus, we introduce a rotational potential P , function of the robot orientation whose global minimum is at = 0. P will aim to maintain the parallelism with the wall.

The task potential is thus: P wf = P x + P y + P :

In order to insure a collision-free motion, a repulsive potential, function of the shortest distance to the robot, can also be added to keep it away from unexpected obstacles occurring within a certain distance.

All force components derived from task potentials are applied to a control point P c de ned on the robot

x-axis at a certain non-null distance from its center. P c is considered as a holonomic point 14, 1] for it can move in all directions whereas the robot is nonholonome. This is a key issue of these actions stability proof developed in 5]. Execution control, however, can be insured using some suitable control functions (e.g. if l(t) represents the current displacement, the potential value has to be null for l(t) L 1 + L 2 , see section 2).

The angular variation and the type of vertex, allow to achieve the wall switching using the following potential function of vertex position (x v ; y v ) P sw = ((x x v ) 2 + (y y v ) 2 r 2 ) 2 (x x v ) 2 + (y y v ) 2 r = d w + E 2 , where E is the robot width. Transition between the wall task potential P wf and the switch wall potential P sw can be done by a pure convex composition such as

Wall switch

Wall follow P = (1 f wf (t)) P wf + f wf (t) P sw where f wf (t) 2 0; 1] is a monotonous increasing control function, depending on the robot abscissa in the frame R w . Figure 5 shows the distribution of the potential P. Switching to the next wall task potential can also be achieved by such a control function which depends, in this case, on the executed angular variation with respect to the given value describing this vertex.

Experimental work

The Hilare2 robot is equipped with a belt of 32 Figure 6 shows one of the experimentations. Hi-lare2 has to move from Init to Goal position in a known environment. Given the important uncertainty value of the initial position, the goal is not reachable using a direct open-loop free-space motion. That is why, the planner strategy was to generate several sensor-based actions (move to contact, follow wall until vertex) to reduce the uncertainty, then the goal could be reached with open-loop actions. The experiment of gure 7 corresponds to a case where the wall cannot be reliably reached with openloop actions, because of their uncertainty evolution. The motion plan rst reduces the position error by using the landmark zone before seeking the contact with the wall. Dynamic robot relocalization in the landmark zones has not been implemented yet; several MOVE LANDMARK actions are sent to the execution level to achieve this task. A more signi cant task is shown in gure 8. Indeed, this mission is not feasible using open-loop actions in such a contrained space. We can note here the important traveled distance by the sensor-based commands. This task was successful almost at every execution, the robot runs more than (60m) at a rate of (.4m/sec) while following the walls. The robustness of the system lies not only in uncertainty reduction, but also in the ability of coping with environment variations (e.g., unexpected obstacles) thanks to the sensor-based motions. This is exampli ed in the experiment shown in gure 8 where the doorway almost in the middle of the corridor was not modelled, and the wall following action can comply with this during execution.

Conclusion

Robot displacement uncertainties make its motion by open-loop actions unreliable, preventing it from reaching an assigned goal. We have presented here an operational framework linking planning with uncertainty and sensor-based motion, thus providing more robust execution. Hence, the planner generates an execution plan composed of both open-loop and sensorbased motions. At the planner level, robot sensing (for localization) and motion constraints are taken into account to produce a plan in terms of sensor-based actions. These actions are de ned as gradient descents in adequate potential elds called Task-Potentials. Several experimentations with real data in an o ce environment show the robustness of the system. On-going developments consider more realistic sensor properties in terms of uncertainties and eld of view.

Figure 1 :

 1 Figure 1: Two exemples of the MOVE command: with parameters ( ; L; 0) (-a-) and ( ; 0; r) (-b-) MOVE( ; L; r): is a composed free-space motion command controlled by odometry. It corresponds to a straight-line motion of length L executed after a circular arc of angle and radius r (Fig. 1.a).

Figure 2 :

 2 Figure 2: Sensor-based motions commands.

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  Free space" strategyMotion envelopeOuter circle of robot shape

Figure 3 :

 3 Figure 3: Motion strategies possibly generated by the planner set of nal situations they can reach, and by the trajectory envelopes they induce. A distinction is made between free-space motions controlled by odometry only and sensor-based motions controlled by both odometry and proximity sensors, or by the external sensor used in the localization areas.

Figure 4 :

 4 Figure 4: Wall Following idea.

Figure 5 :

 5 Figure 5: Potential distribution for switching walls.

Figure 6 :

 6 Figure 6: Experimentation 1 on the robot running under the VxWorks real-time system.

Figure 7 :

 7 Figure 7: Experimentation 2

Figure 8 :

 8 Figure 8: Experimentation 3