"All science would be superfluous if the outward appearance and the essence of things directly coincided", Capital, vol III, p 817 Lawrence and Wishart ed.
The differentiation between science and ideology is as old as Plato's simile of the cave. It is the difference between higher thought, that penetrates beneath the surface appearance of things to get to their essence. As such it takes as its starting point, as Marx implies, upon the proposition that appearance and essence do not coincide.
More recently this proposition has come under attack, on the grounds that there cannot be an essence (see Richard Rorty, Philosophy and the Mirror of Nature, the later Wittgenstein etc etc.).
To some extent the *anti-essentialist* critics are caricaturing the argument. They would say that it is conservative to argue that there could be a stable and fixed essence. Jindrich Zeleny's Logic of Marx demonstrates that one can still distinguish between appearance and essence without arguing an ahistorical essence.
Other criticisms challenge particular findings of scientific thought to show that the passage from appearance to essence is suspect, and what was thought to be essential has proved to be a chimera. This is easily done. You can show that scientific racists deluded themselves in identifying genetically warranted distinctions within the human race. You can show that Newtonian Physicists arrived at a conclusion only to have it overthrown by Einstein. And so on and so on.
But none of these criticisms demonstrate that it is impossible to make the passage from appearance to essence. They only show that in specific instances we deluded ourselves that we had made it. Scientific method does not imply any given set of results. Indeed it suggests the opposite that all results are contingent, ie contingent upon proofs, that is upon objective verification. And because scientific theories are consistently overthrown, it might seem that they are each delusions. But it would be more appropriate to say that they are ever more refined approximations of the essence of the matter.
Even if the truth is never finally reached (in fact it is implied that it never could be finally reached) the passage towards the truth is a valid one. So the question, show me an absolute truth is a good one, because only metaphysical propositions are absolute. Scientific propositions are always relative (relative to the object of which they are theories) and subject to being overturned. But that does not mean that the approach towards the truth is invalid. On the contrary, it is the only basis upon which reason - science - could operate.
-- Jim heartfield