system
Given the following post, respond as if you're commenting on Reddit:
human
How is the idea of reason rejected by postmodernism?

Somebody argued that Kant's philosophy is outdated because postmodern philosophy sort of shows the failure of the idea of reason. I'm not familiar with postmodernism and I'd like to understand the argument.
gpt
The first thing to note is that postmodernism and post-structuralism, generally speaking, critique reason with a capital-R, not reason the general faculty of cognition. This is an important distinction as it completely changes the type of conflict which characterises the "Modern"/Enlightenment vs. Postmodern debate. 

So what is Kant's idea of Reason? In a sentence, Kant attempted to find a "Third Way" which captures elements of both immanent (a priori) and experiential (a posteriori) reason. In other words, he wanted to find a base of knowledge and Reason which was empirically founded, but still relied on some kind of immanent or non-experiential human faculty. His metaphysics depends on a distinction between phenomena (a basic working definition can be "the world as it appears") and Noumena (an exact definition of this is hard as it is debated amongst Kantians, but Noumena are variously described as a 'suchness', or 'things as they are in themselves'). While the realm of phenomena is accessible to human perception, the realm of Noumena is inaccessible by definition, as noumenal qualities are those which exist prior to and/or apart from human perception and representation. This is important, and why Kant's metaphysics is called a *critique* of pure Reason. His whole setting up of the question in this way is intended to show that there are some metaphysical questions that we simply will never know the answer to via metaphysics--such as the existence of God or the immortal soul. These "Noumenal" questions are answerable only through Practical Reason--IE Reason which uses value systems and norms, rather than Theoretical Reason which uses observation, logic and metaphysics.  Back to the a priori/ a posteriori distinction: Kant's system of Reason therefore establishes a way of knowing the world which is primarily built on experience (phenomena), but which also accounts for some knowledge which is based on a conceptualisation of or relation to immanent logic (noumena).

In one sense, the postmodernist critique (especially by Lacan) is impossible without Kant. Kant, as well as being the Godfather of many modernist philosophies, is also the Godfather of Phenomenology, which forms the epistemological backbone of postmodernists like Derrida, Irigaray. Without Kant creating a metaphysics which is removed from transcendental and spiritual questions like the existence of God, the discipline of phenomenology--which is about the boundaries of human experience and perception--would not be possible. Kant is thus by extension one of the Godfathers of postmodernism, and his critique of Reason can be placed on a continuum with postmodern epistemological critique. As for how postmodernists critique Kant's version of reason, one of the main things that is common is to basically cut out the Noumena, and disavow the existence of an immanent grounding of Reason. For once you remove a presupposed noumenal reality, then you also remove a stable foundation on which to base knowledge claims which relate to any kind of Truth-with-a-capital T. So the pomo critique of Kant's Reason -- especially by ppl like Derrida -- is essentially an extension and warping of Kant's original critique such that it has no implicitly transcendent element. Because while Kant thought the Noumenal realm was inaccesible, he still thought of it as an important structurating factor which organises human Reason, even through its perceptual absence. It should be noted that there are non-postmodern Kantians who have done similar things with his philosophy, without opposing his notion of Reason wholesale. So part of what distinguishes the postmodern critique is what it does *after* it removes the existence / importance of a noumenal realm. In other words, the postmodern critique of Kantian Reason is not just arguing that there is no Noumenal realm and therefore no foundation or centre which grounds Reason. It is also arguing that there are specific consequences of this ungrounding, which (for Derrida at least) call into question the presumed legitimacy of the entire Western epistemological canon. 

One thing final thing to note is that anyone saying postmodernism shows the failure of reason with a little "r" likely doesn't know what they're talking about. If you want to get at the postmodern critique of Reason then you have to start talking about *Reason***s** as well. Each post-modern philosopher addresses these questions differently, too--IE Foucault's critique of knowledge does very different things to Derrida's critique of Truth. And the most important thing to remember is that, while postmodernism did challenge a lot of the core assumptions of the Western canon, it is also a philosophy firmly located within that canon that borrows as much as it opposes many of the core ideas and methods used.

[Edit: Cleared up some language/Imprecision.]