system
Given the following post, respond as if you're commenting on Reddit:
human
In Marxism, why can only humans create value?

I understand that capital is supposedly stored labor, and this is the explanation, but in Marx's mechanistic, materialist metaphysics, what separates a robot (no matter how human-like in its productive capacity) from a human? Why, then, is a human not also simply stored labor, if a robot is? What separates a man from a machine? It's not a soul, so I have absolutely no idea what it could be.
gpt
I apologize if this isn't sufficiently clear but, in brief: 

Value for Marx has to boil down to human labor because for capitalism, value has to have a *measure*, and that measure is quantified labor time. You *could* propose to quantify other things relative to each other as an alternative form of measure (e.g. *use-value* might itself refer to certain social necessities), but one of the central questions of *Capital* is how surplus value is possible; how it is that someone who, say, owns a factory is able to get value out of a commodity *above* what it costs to produce--the whole M-C-M' (money-->commodity-->money prime, or money *above* what you put in) issue. Another way to put this to make it relevant to your question: what determines the value of a commodity such that it's possible for the commodity to cost more than the cost to buy and maintain the collection of robots used to produce it? Somehow value is added in the process of production, and that's evidenced by the fact that the capitalist is able to skim some of it as surplus value. So the question is where that's coming from: it can't be the value of machinery or other stuff goods tied up in the production process, because otherwise you'd never leave the C-M-C cycle (where equivalences are traded for equivalences, without value added). There's be no reason the commodity cost more than the cumulative cost of robots+materials+maintenance. The only thing that's truly variable, i.e., exploitable, in the M-C-M' circuit is the labour that's put in, measured as units of time. Obviously not all time is equally valued, to say the least (hence the social necessity) but it's only in terms of relative valuations of labor time that Marx thinks you can explain price variability and the possibility of exploitation and surplus value.