Theorien über den Mehrwert
However, there may be several problems with this criticism. The starting point for Marx's argument was as such: "What is the common social substance of all commodities? It is labor." It is not possible to view grain, iron etc. as common to all commodities, whereas the production of commodities is impossible without labor (while it can also be said that other commodities such as tools are required, they cannot be properly aggregated by value because they are specialised and disparate in nature and their values, relative to each other and to labor, depend on prices that in turn depend on their values; Sraffa (1960), for instance, aggregates them according to the labor required for their production). Marx identifies the substance of value as labor, which in his view is "not" a commodity (though "labor power" is). This was a necessary aspect for the substance of value Marx elaborates upon in "Capital" and "Theories of Surplus Value".