“Lenin” argues that the truth of concepts such as atoms is unknowable, then all truth is unknowable, and all we have is language...he rejects the concept of a knowable independently existing reality...
Here are some comments I made in reply to Andy:
I agree that the view that objective reality is completely unknowable because language gets in the way is one that Marxists should reject, but do we have to go to the opposite extreme and argue for the 'naive realist' view that the world can be experienced 'directly', without being mediated by the concepts that the mind provides?
I know that there are some in the Marxist tradition who have done this - Lenin with his 'camera theory of perception' in Materialism and Empiriocriticism is one - but there are also instances of a more subtle approach being taken (Lenin himself seems to take a more subtle approach in his later Philosophical Notebooks).
Language does not make reality unknowable in the way that Derrida and other post-structuralists have claimed it does, but equally we cannot experience reality without language, without a set of concepts. So both the extreme postmodern view you attribute to Richard of Lenin's Tomb and the naive realist view that the real Lenin fell for in his early work on philosophy are mistaken.
As I understand him, Marx argues that the key to thinking is 'abstraction' - that we grasp reality by 'abstracting' it through applying concepts to it. Marx tries to create abstractions that are dialectical - that contain contradiction and change - in contrast to 'static' bourgeois abstractions and the concepts that are based on them. But what he is abstracting doesn't pre-exist in concepts before he comes to it - he is imposing order upon a mass of data and sensations, and he can only ever 'capture' so much of that mass in any one abstraction. Reality is infinitely complex and cannot be grasped in toto, just as it cannot be grasped without language.
In Capital, of course, Marx goes back again and again to his massively complex subject, abstracting it in different ways to bring out its different aspects. That's why he 'contradicts' himself, in the eyes of bourgeois social scientists, by talking at one time about a capitalist class, at another time about capitalist classes, at another time about bankers as a distinct class, and so on.
Marx, then, has a radically different method than bourgeois realists, who think that reality comes in pre-cut categories and treat their concepts like pigeonholes. And underlying this different method is a different ontology - an ontology which denies that the world exists nicely and tidily 'out there', and merely has to be 'photographed' by us.
There are important epistemological consequences of Marx's ontology and his dialectical method. When making projections about the future or generalisations about the past, Marx often talks of 'tendencies', rather than laws, and he makes it clear that there are a multiple 'countervailing tendencies' which can cancel out these tendencies, at least for periods of time.
Bourgeois social scientists don't understand this subtlety, and delight in announcing that Marxism has been 'refuted' because, say, the tendency of the rate of profit to fall was largely absent in Britain in the first half of the twentieth century. They have a crude positivist method based on the belief that human life can be understood with the same sort of rigorous rules that are used in physics (or were used in physics - in recent decades the rise of chaos theory, which has more than a little affinity to the Marxist method, as Alan Woods has pointed out, has led to significant changes to methodology in physics and and a number of other natural sciences).
More on all this later. I bet you can't wait.