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158 Piero SraffaThe latter thesis has been the object of long debate. Various economistsstress that as a matter of fact natural prices do not remain unchangedover the time span necessary for the completion of the gravitation processof market prices towards natural prices; the ‘natural position’ mayor may not be reached, depending on the assumptions adopted on thespeed of change of the elements determining the natural prices, on theone hand, and the time required for the adjustment of market to naturalprices, on the other. 34 Additional difficulties arise when it is recognisedthat the path followed by market prices may influence those very elements(technique in use, income distribution) determining naturalprices. Other economists stress that gravitation requires strict formalconditions, through analyses where market prices are treated as theoreticalvariables determined by supply and demand conditions, and wheresupply and/or demand respond to divergences between market andnatural prices. 35 Such a notion of market prices is necessary when interpretinggravitation as a theory concerning the level of market prices andtheir path over time. But such a notion can be attributed neither to theclassical economists nor to Sraffa. To them, market prices represent theexchange ratios actually observable in reality, influenced by a multiplicityof factors, both systematic and unsystematic; natural prices, instead,are the theoretical variables expressing the action of those factors alone,on which the economist chooses to focus attention. 36However, as already suggested, the thesis concerning gravitation of markettowards natural prices is not necessarily to be interpreted as a precisetheory of market prices. Analysis of the relationship between market andnatural prices may be pursued not by trying to theorise the path actuallyfollowed by market prices, but rather by pointing to the direction of theirmovement in each given situation, towards – or away from – naturalprices. When interpreted in this way, the thesis of gravitation emerges asnothing more or less than a different name for the classical (Smithian)theory of competition, according to which any deviation of market fromnatural prices provokes reactions on the part of economic agents whichtend to move the market towards natural prices.34Cf. in particular Parrinello (1977). We may think, for instance, of the extremelyrapid technological change in sectors such as that of personal computers, incounterposition to the near-staticity of other sectors; let us recall, in this context,that natural (or production) prices are relative prices and as such they depend onthe relative difficulty of production (and on income distribution).35Cf. for instance Arena (1981); Steedman (1989, Chapter 6); Boggio (1985,1990).36Cf. Roncaglia (1990b, 2009a).The Sraffa Legacy 159Garegnani, however, seems to add two other elements: (i) the idea,already mentioned, that the elements determining natural prices are‘persistent’, that is, relatively stable, so that the speed of movement ofnatural prices, due to exogenous changes in the factors determiningthem, would turn out to be significantly lower than the speed of movementof market prices in their process of competitive adjustment towardsnatural prices; (ii) the idea, which is a corollary of the first, that naturalprices, and hence their determinants, are (or can be considered) independentof short period movements in market prices. Both these ideas, asnoted above, have been disputed in the course of the debate concerninggravitation. (In that debate – as on so many other occasions – two aspectswere sometimes confused: first, whether these ideas represent more orless faithfully the classical economists’ views; secondly – and particularlyrelevant here – whether they are useful in representing the working ofcontemporary economic systems.)These critiques hit the central aspect of the thesis of gravitation,namely the strong characterisation of the idea of ‘persistence’. In fact,according to the thesis of gravitation, the forces regulating the process ofeconomic reproduction would be persistent, not only in the commonlyaccepted sense that their mode of action is stable and systematic, but inthe stricter sense of attributing persistence (stability) to the quantitativeexpression (the ‘levels’) of the factors determining the system of relativeprices. Specifically, persistence (stability) is thus attributed to technologyand the corresponding levels of production, which – together withthe system of natural prices they imply – constitute the ‘long period positions’towards which actual economic systems are said to gravitate.Together with this strong notion of gravitation, Garegnani’s lineof enquiry is characterised by the central role attributed to the ‘analyticalcore of the surplus theories’. As we saw above (§ 8.5), Garegnaniattributes logical priority to the ‘analytical core’, in the sense thatonly within it is it possible to identify ‘general quantitative relations’connecting economic variables. In some respects, this idea resembles –even if the boundaries of the analytical core differ – Pasinetti’s idea, discussedabove, concerning the two stages of analysis, of which priority isattributed to the one analysing the ‘natural’ properties of the economy. 3737When pushed to its extreme limits, this distinction between the ‘analyticalcore’ and the rest of economic analysis tends to coincide with the distinctionbetween economic theory and political economy as proposed by Lunghini (1975) inhis interpretation of Sraffa: a distinction with which Lunghini means to showhow limited the scope of constructive theoretical reasoning is in the economicfield.