March Against Marx - Part 1
David Ricardo gave us the labour theory of value we all know and hate. Marx actually deviated from it and took up some more complex thing of his own. What kind of thing is what we'll be getting at in future. For now let's just sum up the two possible ontological approaches to objective value. They are not epistemologically distinct because both are ideas that value is objected and gives rise to (and/or is measured using) prices.
We all know what prices are, right? Well, they're just exchange rates/ratios. On to the porn!
~~~ SUBSTANCE THEORY ~~~
Substance theory is thee ontological position that substances are separate from their components. For enquiry into economics this means that labour-induced value resides in things. This literal interpretation underlies the labour theory of value as posited by Ricardo. 
~~~ FIELD THEORY ~~~
Field theories of value hold that value is not a literal quantity out there in the world but that its presence can be inferred from prior axioms or by empirical observation of economic agents' behaviour. This is the category where Marxian economics belongs. 
Any objective value theory opens up the possibility of building a theory of economics that basically consists of a kind of metaphysical accountancy as we shall see soon enough. What I mean by metaphysical is that it's all based on epistemological and ontological claims that can't be verified at source and are taken on faith. What I mean by accountancy is... well, Marx actually thinks economics has units, an amazing innovation on his part and something that was actually ingenious. His unit is socially necessary labour time.
It was also utterly ridiculous. But more on that later.
The embarrassing mistake made by modern critics of Marx is that we so often attack a substance value theory of labour, which is not what Marx advocated, thus building a straw man that Marxians can gaily ignore.
Time to up one's game. A serious enquiry into the value form and socially necessary labour time will be needed, but care must be taken lest the pupil of economics start talking about ideas like surplus value, dialectical materialism, or abstract labour that Marx himself never mentioned.
 Substance Theory article on Wikipedia
 Field Theory article on Wikipedia