In the first few pages, Jurgen Habermas delivers a helpful picture of the historical emergence of rational thinking from preceding mythical worldviews and the debate which then ensued between Platonism (rational, ideal forms of thought) and anti-Platonism (relative, contingent skepticism) that continues to this day. He demonstrates how a flux between the two sort of generates a need for the other, but also why anti-Platonism (like Rorty's relativism and Derrida's deconstruction of today) can never deliver a legitimate, conclusive argument against the necessity of Platonic idealism.

Habermas' argument in a nutshell, quoted here from page 4:

"The practice of criticizing Platonist pseudo-objects moves within a conceptual frame and employs conceptual means which in turn cannot be deconstructed without depriving anti-Platonism of it's own critical sting. The radical attempt to do away with any abstraction, idealization, or concept of truth, knowledge, and reality that transcends the local 'hic et nunc' would run into performative contradictions."

In other words, deconstructive critique operates from a rational, metaphysical premise of thought, even if that premise is obscured by labyrinthine linguistics (the critique requires a rational frame of mind to recognize the critique).

The point is not that relativism is always a worthless concept, but that idealized forms of thought can never be done away with because they are in fact a necessary premise of human communication. Therefore we ought to work toward mutual understanding and definition of ideas than destroying them. This is the basis of Habermas' Theory of Communicative Action, which tries to direct philosophy away from divisive moral relativism and toward enlightened intersubjective responsibility.