>First off is the assertion that if x corn = y iron, then x corn and y iron must be mutually equal to some third thing. I almost get this but it still feels like a jump to me. Why *must* they be equal to some external third quantity?
Because iron and corn are two different things. They have little in common with each other, but they have some quantity by which you can compare them. The third thing isn't another commodity like >>383189
implies. The third thing is more conceptual. It is a factor that exists in common between the two things that allows you to compare them. The "third thing" is really just the comparator between the two.
>Secondly is the abstraction of use value in equivalence. I completely agree that in comparing corn to iron, their material uses are stripped away, leaving just different quantities to be balanced out. But what about 'usefulness' of a commodity? Marx seems to assert that as long as something *is* useful, it doesn't matter in what way, but can't we say that it might matter *how* useful something is, almost as if usefulness had a quantity of it's own which played into these weightings?
The best way to think about this is that there is a finite demand for things. Where you have less supply than demand you have scarcity. Where you have more supply than demand you have abundance. If the demand/need is fully supplied, there isn't use-value/utility in exceeding it. And in capitalism you can actually have a crisis of overproduction, where the supply is high enough it drives the price too low to be sold for a profit.
>I understand that the next step in the argument is that if usefulness is abstracted away, this necessarily only leaves the quantity of labour, or labour time, imbued in the commodity. Again, I agree with this for the nature of the usefulness, but I can't articulate as to why this also must abstract about 'quantity' of usefulness. What if iron is just more useful than corn as a general statement? Then surely this would factor into a difference in value, *as well as* the labour time required to produce a given quantity, no?
The factor in play is demand. There is more demand for iron. The reason everything gets stripped away but labor input is because the labor input is what you (the general, conceptual you) have to trade to get the product. You can think of it like the natural cost of the thing. Society or the economy has to pay nature the labor input to turn existing resources into the product, and then once that happens the product can be exchanged on the market according to that. The price fluctuates based on other factors, and even the value degrades with use or deterioration of the product.