“But what makes the exchangeable value of a definite commodity? Here Ganilh does not get beyond the crudest superficiality. A is of greater exchange-value when it exchanges for more B, C, D, etc.” (p 205)
That leaves the door open to a theory of subjective value, whereby more of A exchanges for B, because B is in more demand, is considered more valuable than A. The later “Marxists” are in advance of this, in that they understand, following Marx, that the basis of this relation between commodities is that each has a value determined by the social labour-time required for its production. Where they are in error, as with Ganilh, is to believe that this value itself only comes into existence as a consequence of exchange, of commodity production and exchange, or worse only as a result of capitalism.
I can express the value of 10 metres of linen in its equivalent form, as 1 gram of gold. But, unless I know the value of 1 gram of gold, this in no sense tells me the value of 10 metres of linen. It only expresses the value in a different, equivalent, form. If I know that 1 gram of gold requires 10 hours of labour-time for its production, I then know that its value is equal to 10 hours of labour-time.
If this 1 gram of gold continues to exchange for 10 metres of linen, I thereby also know that this linen has a value of 10 hours of labour-time. Their equivalence is a function of their value. But, if I try then to express this equivalence in terms of value rather than exchange value, it becomes meaningless nonsense, because I would have to say 10 hours of labour-time is equal to 10 hours of labour-time.
Value can be expressed as a quantity of labour-time, whereas exchange value is expressed as the value of one commodity measured in terms of a quantity of some other use value. If the value of gold rises, the exchange value of linen, measured in gold, falls, and vice versa. If the value of linen and of gold rises or falls, in the same proportion, the exchange value of one measured by the other remains unaltered.
“This relativity, however, originates from the fact that they must present themselves as the form of existence of general labour, and can be reduced to it only as relative, merely quantitatively different expressions of social labour. But the exchange itself does not give them their magnitude of value. In exchange they appear as general social labour; and the extent to which they can appear as general social labour depends on the extent to which they can present themselves as social labour, that is, on the extent of the commodities for which they can be exchanged, and therefore on the expansion of the market, of trade; on the range of commodities in which they can be expressed as exchange-value.” (p 205)
In fact, its for this reason that Marx says, in the Grundrisse, that exchange value only takes on its mature form when wage-labour predominates, that is when producers must become wage labourers who do not produce for themselves, but who must satisfy their needs by buying in the market, and who likewise sell their labour-power in the market.
“For example, were there only four different branches of production in existence, each of the four producers would produce a great part of his product for himself. If there are thousands, then he can produce his total product as commodities. It can enter entirely into exchange. But Ganilh imagines, with the Mercantilists, that the magnitude of value is itself the product of exchange, whereas in fact it is only the form of value or the form of commodity which the product receives through exchange.” (p 205)