You're still a Marxist, aren't you? If so, your perspective relies on a whole number of imagined devices (i.e. fictions), both to explain the world and to have any hope of changing it. These things are not 'real' in any material sense, but this has not prevented them from having influence over the course of events, for better or worse.
Ah, I see. Well, I'd argue that there's a distinction to be made between abstractions and fictions: that an abstraction is something that does not exist
in itself, but describes something which does (or, at least, is purported to), while a fiction is something that simply does not correspondence to reality. To take your example of Marxism, I would certainly agree that "class struggle" has no independent existence, that it is an abstraction drawn from numerous historically specific relationships, while the National Socialist concept of "racial struggle" is simply a fiction, because it poses entirely made-up groups doing entirely made-up things, and doesn't describe our reality any more than an episode of Scobby Doo.
When you say you don't believe in human rights, the implication is two-fold. On the one hand, you are pointing out that they are fictions; that they themselves have no material existence to speak of. On the other hand, you are suggesting that these fictions have no utility; that we would be better off if we stopped thinking and talking and acting as if they were real. The latter does not flow inevitably from the former here. Regardless of their metaphysical status, I would judge that these particular fictions have been a reasonably (and, on occasion, remarkably) effective catalyst for positive change, which makes me wonder why you're so ready to cast them aside.
Well, as I said above, I wouldn't reject the claim that abstractions are useful- necessary, in fact- I'd simply regard this particular abstraction as a fiction, as deriving from false premises. I don't think that we have any reason to believe that "natural rights" exist, and that, so far as I have encountered them, theories purported to demonstrating the existence of natural rights are flawed. I could well be wrong, but it seems to me reasonable to decline affirmation of their existence until it is demonstrate to me, rather than affirming it on the assumption that somebody, somewhere, is capable of demonstrating it should the need arise.
Of course, that raises the further question of the relationship between validity and usefulness, and I'll readily admit that it's far from apparent that this is the case; there are such as thing as useful fictions. I wouldn't criticise the rallying cry of "the rights of man" in the Early Modern ear, for instance, because I think that the political philosophy it expressed contained a genuinely emancipatory content, and that it articulated in an authentic manner the social and political experience of those who took it up as their own. I'm not as a matter of universal principle opposed to the concept of rights, but rather that I view their emancipatory potential as being ultimately limited, that at some point a new ethical and thus political paradigm is necessary to continue the project of human emancipation.
So at the risk of invalidating the pose of empiricism that I adopt above, I'd hazard that "natural rights" isn't simply an idea that is simply wrong, that needs to be abandoned as an error, but rather something that needs to be surpassed, that has functioned as the historically necessary condition of such a paradigm. You could debate when that becomes the case, and in all honesty I'm not entirely sure myself, but I think that it's a point in time that we have either arrived at, or that we will arrive at in the not too distant near future. My rejection of natural rights theory may seem nihilistic, and in a certain sense it is, but it's not an ahistorical nihilism, a claim that "this is objectively bollocks", but an historical one, a claim that the theory is no longer sufficient as the articulation of our emancipatory impulse.
(Also, it occurs that some of you will think me a hopelessly rash anarchist, while other will think me another tedious post-68er. The wonders of perspective!
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