Yoann Verger
email: yoann.verger@grignon.inra.fr

comment on Martins' Ecosystems, strong sustainability and the classical circular economy

A basis for Sraffian ecological economics. A comment on

Martins' "Ecosystems, strong sustainability and the classical circular economy"

The standard commodity and the universal substance I will start with a note from Martins where there seems to be confusion between the role of the standard commodity in Sraa's theory and the search for a universal substance: [START_REF] Sraa | Production of Commodities by Means of Commodities[END_REF] circular conception can be more easily combined with a study of the various biophysical cycles of ecosystems when addressing supporting ecosystem services, since the cost of production of the commodities used in the economic cycle is measured in physical terms, that is, in terms of the physical inputs consumed in the process.

A crucial question concerns which is the unit used to measure those physical inputs. As Kurz and Salvadori (2010, pp. 203) note, Sraa mentions electricity in his unpublishedwritings. But Sraa was aware that it is dicult to nd one universal substance in terms of which everything else can be measured. For this reason, [START_REF] Sraa | Production of Commodities by Means of Commodities[END_REF] advances the idea of a standard commodity, which is a mixed 1 commodity, made up of the basic commodities necessary for the reproduction of the economy in a certain proportion [START_REF] Martins | Ecosystems, strong sustainability and the classical circular economy[END_REF].

My contention is that Sraa solves the problem about nding a universal substance, but not in the way that Martins suggests. My argument has two steps.

First, the standard commodity is just a numeraire that does not explain why commodities acquire exchange values; and second, the search for one universal substance was a search not for a numeraire but for an explanation of the commensurate measurement of commodities and of their exchange values and Sraa found an elegant solution to overcome this problem.

To begin with, the standard commodity was constructed by Sraa to answer a totally dierent question: can we nd a numeraire such that a change in the distribution would not change the value of this numeraire, so that impacts of alternative distributions of the surplus can be studied? Sraa proves that the standard commodity owns such property, and that use of this numeraire transforms the relation between the average rate of prots and the wage per unit of labor into a strictly decreasing line. This is an important result for Sraa, because he also shows that the maximum rate of prots (corresponding to a zero wage) can be calculated without knowing any prices, just by looking at the conditions of production. Connecting the two results, he can show that the problem of distribution of the surplus is independent of the problem of nding the prices. The levels of the wage and the rate of prots appear to be dened entirely by the social process of sharing the surplus, and not connected to any necessity from the conditions of production (the sharing of the surplus is truly a political choice).

On the other hand, the search for a universal substance is an old debate in the history of economic thought: utility plays that role in neoclassical theory, as human labor does in Marx's theory. In these theories, utility or labor are seen as an explanation of the commensurate measurement of commodities, as well as the explanation of the level of their exchange values. In ecological economics, energy was advocated as the universal substance, with the advantage that it is also used to produce non-manufactured resources such as natural resources.

The energy theory of value could then lead to the valuation of natural resources (Gilliland, 1976;[START_REF] Slesser | Energy analysis[END_REF][START_REF] Costanza | Embodied energy and economic valuation[END_REF][START_REF] Costanza | Embodied energy and economic valuation[END_REF]Neill, 1981, 1984;[START_REF] Hannon | How might nature value man?[END_REF]see also Georgescu-Roegen, 1975 for a strong criticism of such approach). Sraa was also interested in the search for one universal substance.

For instance, [START_REF] Kurz | Sraa and the labour theory of value[END_REF] show that Sraa closely investigated Marx's labor theory of value, where human labor is the universal substance, and rejected it:

There appears to be no objective dierence between the labour of a wage earner and that of a slave; of a slave and of a horse; of a horse and of a machine; of a machine and of an element of nature (?this does not eat). It is a purely mystical conception that attributes to human labour a special gift of determining value. Does the capitalist entrepreneur, who is the real subject of valuation and exchange, make a great dierence whether he employs men or animals? Does the slave-owner? (Piero Sraa Papers -Wren Library, Trinity College, Cambridge -le number D3/12/9: 89 -cited in [START_REF] Kurz | Sraa and the labour theory of value[END_REF]).

Sraa's theory of value

My contention is that Sraa's theory of value overcomes the problem of the search for a universal substance in quite a dierent way than Martins suggests. In Sraa's theory of value, exchange value appears once we set the constraint that all commodities produced must be re-used in production so that the amount of commodities produced during the present period of production can be reproduced during the following period of production. From this constraint, exchange between present owners of the dierent commodities (i.e. the industry owners who sell their production on the market) becomes necessary. The ratios of these necessary exchanges are the exchange values.

It is possible that, after the necessary exchanges, a surplus remains. Therefore, some commodities that are not needed for the reproduction of present production may not have yet have an exchange value. These commodities are called non-basic. Commodities that already have an exchange value are called basic. Non-basic commodities can only acquire an exchange value if their production directly or indirectly uses basic commodities. The value of non-basic commodities is deduced from the value of basic commodities. That is why Sraa sets the conditions that, in any system of production under study, all commodities of the system must directly or indirectly use for their production at least one common basic commodity (Sraa, 1960, 6).

The interesting thing to note is that values are inuenced by the way the surplus is distributed to the dierent classes of agent. The surplus is the set of commodities that are not needed to reproduce the present production (i.e. all commodities basic or non-basic that are over-produced in relation to the system's conditions of production). The dierent classes of agent are: the capitalists, who demand prots in return for the commodities advanced for production; the workers, who demand a wage in return for the labor advanced for production; and the rentiers, who ask for a rent in return for the use of the natural resources that they own. Depending on the distribution of the surplus to the dierent classes of agent, i.e. depending on the value of the rate of prots, wages and rents, the value of the commodities will change.

If the surplus is entirely given to the workers, all exchange values become proportional to the labor used in production, while the value of the surplus, i.e. the added value created by the system, becomes equal to the sum of the wages. Likewise, if the surplus is entirely given to the owner of the natural energy resource, all exchange values become proportional to the use of energy in production, while the value of the surplus is equal to the sum of the rent given to energy owners. We can thus create a labor or an energy theory of value, just by virtue of a political decision on the sharing of the surplus. But it must be clear that it would be a reversal of the logic to explain that labor or energy explain the exchange values of the commodities. This is why Sraa's theory overcomes the sterile debate about the ultimate source of value, which is behind the search for one universal substance in the exchanges:

The signicance of the equations is simply this: that if a man fell from the moon on the earth, and noted the amount of things consumed in each factory [. . .] during a year he could deduce at which values the commodities must be sold, if the rate of interest must be uniform and the process of production repeated. In short, the equations show that the conditions of exchange are entirely determined by the conditions of production (D3/12/7: A7.29.iii -cited in [START_REF] Garegnani | On a turning point in sraa's theoretical and interpretative position in the late 1920s[END_REF].

Distinction between physical world and commodities world

This description of Sraa's theory of value appears to me important because it helps us to understand how this theory can be used to construct sustainability economics. In this respect, another confusion may arise if Sraa's prices are presented as sustainability prices. In this way, the following assertion from Martins is misleading:

The prices of the basic commodities that constitute the standard commodity are those which ensure the sustainable reproduction of the system, within a standard system dened in terms of inputoutput analysis [START_REF] Sraa | Production of Commodities by Means of Commodities[END_REF] [START_REF] Martins | Ecosystems, strong sustainability and the classical circular economy[END_REF].

One important distinction must be made clear: physical exchanges that support the production of commodities are not all represented in the system of value equations. In this system, only commodities, i.e. goods or services that are exchanged on the market, and the natural resources that yield a rent, are represented. Hence, many physical relations are not represented: the extraction of raw material from nature, the disposal of waste and pollution into the environment, and non-marketed exchanges of goods and services between industries and between agents are not represented. Furthermore, the various stocks of natural resources are not accounted for.

The latter point explains why, even if Sraa's prices represent the ratios of exchanges that ensure the reproduction of the system, they cannot ensure the physical possibility of such exchanges. It may well be that a natural resource has been depleted during the present period of production, and that its use during the next period of production is no longer possible (see also [START_REF] Chiodi | On non-self-replacing states[END_REF] for a distinction between a viable system and a self-replacing system; Sraa considers only viable systems, but not all viable systems are self-replacing).

Furthermore, to call these prices sustainable is misleading, because even if reproduction is possible in the next period of production, hidden non-marketed pollution ows could alter the possibilities to continue the production process in the future. Sraa's prices do not forecast the future, they just characterize present conditions of production (and thus present needs for reproduction).

Distinction between produced and non-produced commodities (natural resources)

Still in relation to the construction of sustainability economics, a really interesting point made by Martins is that classical economists and Sraa have underlined the distinction between natural resources and manufactured commodities. This distinction is crucial and genuinely calls for an implementation of strong sustainability measures in order to ensure that the production process can be made sustainable. It also appears in the value system, because natural resources use no commodities or human labor as input, and can therefore acquire value only if a rent is demanded in return for their use in production:

The distinction between rent and prots, an expression of the dierence between natural resources and manufactured capital, provides a theory of value where scarcity is seen as a specic property of natural resources and studied in connection to strong sustainability [START_REF] Martins | Ecosystems, strong sustainability and the classical circular economy[END_REF]. I may add, however, in order to avoid any confusion, that scarcity of one natural resource does not imply the appropriation of a rent. Only the conjunction between scarcity and the appropriation of natural resources, such as more fertile lands, enable the appropriation of rent [START_REF] Martins | Ecosystems, strong sustainability and the classical circular economy[END_REF], and there are cases where privately owned scarce natural resources do not yield any rent. Furthermore, the fact that there is a rent or not on a resource does not mean that the use of the resource is sustainably managed. It is important to focus the analysis of the sustainable management of scarcity on resources that are truly scarce, i.e. natural resources. But I do believe that a solution for their sustainable management can be found without giving an exchange value to nature, hence without using rents as a concept for addressing the valuation of critical natural capital [START_REF] Martins | Ecosystems, strong sustainability and the classical circular economy[END_REF]. For instance, the natural resource may be managed under the regime of common property, with rules of sustainable use that do not imply exchange of money between the users.

Conclusion

In order to avoid confusion between commodities and non-commodities, and between the management of natural resources and their valuation, double accounting is required. Both the value system and the physical system related to the production of commodities must be made explicit, side by side. Both systems are of course mutually dependent, but it is important to keep them separate, as they both have specic characteristics. The value system, for instance, can produce services that will not be represented in the physical system, whereas waste, for instance, can be represented in the physical system but not in the value system. The physical conditions for sustainability can then emerge clearly, while valuation of the natural resources just appears as one of several possibilities for their sustainable management. Do my comments contradict Martins conclusion? I think his main point was to provide a direction for research more than a precise statement on how to conduct this research. My comments on the goal and the method of Sraa's theory of value, on the relation between sustainability and prices in this theory, and on the distinction between valuation of natural capital and sustainable management of this capital, aim at rening the basis of Sraan's ecological economics. To rephrase Sraa, if the foundation holds, the research may be attempted latter, by the writer or by someone better equipped for the task (Sraa, 1960, Introduction).