While logic, mathematics and rhetoric were all in use thousands of years ago, in pre-Classical Greece, they would not bear their sweetest fruit until the Enlightenment in the 18th century C.E. Inspired by the writings of Bacon, Locke, and others, reason came to be seen as the source of authority. This movement was called “modernism.”
The innate human capacity for reason meant that every human being had the capacity to self-govern, and to deny that right, as in traditionally statist European systems, was to deny the fulfillment of the human race. Under the guidance of reason, modernism established free markets and scientific method; abolished slavery and affirmed tolerance.
Post-modernism is a reaction to that.
The most essential argument of post-modernism is that, because human beings are not perfect and infinite, our perceptions of the world around us are imperfect. It then postulates that, because our perceptions are imperfect, our logical inferences based upon those perceptions are also imperfect. Thus far, these assumptions are true, and the modernist would agree.
The divergence occurs here: the post-modern asserts that, because our logic is incapable of addressing an absolute truth, of being without flaw, that it is valueless. Any attempt at reason or argumentation is no more true or valuable than any other blank assertion, and exists only to be “deconstructed” into a chiaroscurio of the author’s various biases and ignorances.
Further: since no position can have any inherently superior value to another (such as by virtue of being logical) all human interaction is viewed as nothing more than the oppression of one group by another. The freedom-affirmation of the modernist Civil Rights and Women’s Liberation movements became, under post-modern influence, vindictive race activism and feminism. The Enlightenment was viewed, not as the promotion of discovery and liberty throughout the world, but as rich white men forcing their culture on everybody else.
Post-modernism is worse than a useless philosophy; it is an inherently destructive philosophy–which, after all, is exactly what “deconstruction” is just a spineless way to say.