[This excerpt is from Chapter 2 of Explaining Postmodernism: Skepticism and Socialism from Rousseau to Foucault]
Identifying Kant’s key assumptions
Kant’s strikingly skeptical conclusions depend upon philosophical assumptions that continue to inform contemporary debates between postmodernists and their foes. Most postmodernists take these assumptions to be solid, and many times their foes are at a loss to challenge them. Yet they are the assumptions that must be addressed if postmodernist conclusions are to be avoided. So it is worth highlighting them for future reference.
The first assumption is that the knowing subject’s having an identity is an obstacle to cognition. This assumption is implicit in many verbal formulations: the critics of objectivity will insist that the mind is not a diaphanous medium; nor is it a glossy mirror within which reality reflects itself; nor is it a passive tablet upon which reality writes. The assumption emerges when those facts are taken to disqualify the subject from awareness of reality. The assumption then is that for awareness of reality to occur, the mind would have to be a diaphanous medium, a glossy mirror, a passive tablet. In other words, the mind would have to have no identity of its own; it would have to be nothing itself, and cognition would have to involve no causal processes. The mind’s identity and its causal processes are thus taken to be the enemies of cognition.
The diaphanous assumption is implicit in the relativity and causality of perception arguments that were part of the background problematic to Kant’s philosophy.
In the relativity-of-senses argument, the diaphanous assumption plays out as follows. We notice that one person reports seeing an object as red while another reports seeing it as gray. This puzzles us because it draws our attention to the fact that our sense organs differ in how they respond to reality. This is an epistemological puzzle, however, only if we assume that our sense organs should have nothing to do with our awareness of reality—that somehow awareness should occur by a pure stamping of reality upon our transparent minds. That is, it is a problem only if we assume our senses should operate diaphanously.
In the case of the causality of perception argument, the diaphanous assumption is involved if we are puzzled by the fact that consciousness requires that one’s brain be in a certain state, and that between that brain state and the object in reality is a causal process involving sense organs. This is puzzling only if we have previously assumed that awareness should be an unmediated phenomenon, that one’s brain being in the appropriate state should just somehow happen. That is, the causal process of perception is a puzzle only on the assumption that our senses should have no identity of their own but rather be a diaphanous medium.
In the arguments based on the relativity and the causality of perception, the identity of our sense organs is taken to be the enemy of awareness of reality.
Kant generalized this point to all organs of consciousness. The subject’s mind is not diaphanous. It has identity: it has structures that limit what the subject can be aware of, and they are causally active. From this Kant inferred that the subject is prohibited from awareness of reality. Whatever we take our mind’s identity to be—in Kant’s case, the forms of sensibility and the categories—those causal processes block us. On the Kantian model, our minds’ structures are seen not as existing for the purpose of registering or responding to structures that exist in reality, but as existing for the purpose of imposing themselves upon a malleable reality.
The question to return to is: Is there not something perverse about making our organs of consciousness obstacles to consciousness?
The second key assumption of Kant’s argument is that abstractness, universality, and necessity have no legitimate basis in our experiences. This assumption was not original to Kant, but had a long history in the traditional problem of universals and the problem of induction. Kant, however, following Hume, declared the problems to be in principle unsolvable on the realist/objectivist approach, and he institutionalized that declaration in the subsequent history of philosophy. In the case of abstract, universal concepts, the argument was that there is no way to account for their abstractness and universality empirically: Since what is given empirically is concrete and particular, abstractness and universality must be added subjectively. The parallel argument in the case of general and necessary propositions was that there is no way to account for their generality and necessity empirically: Since what is given empirically is particular and contingent, generality and necessity must be subjectively added.
Institutionalizing this premise is crucial for postmodernism, since what has been added subjectively can be taken away subjectively. Postmodernists, struck by and favoring contingency and particularity for a host of reasons, accept the Humean/Kantian premise that neither abstractness nor generality can be derived legitimately from the empirical.
 This is exactly Rorty’s key conclusion in Philosophy and the Mirror of Nature (1979).
 The diaphanous assumption is sometimes but not necessarily assisted by a lingering mind/body dualism in two ways. In one way, dualism encourages us to conceive of the mind as a ghostly, pure substance that somehow magically confronts and comes to know physical reality. In another way, such dualism posits a non-physical mind that is distinct from the physical sense organs and brain, and so immediately leads us to conceive of the physical senses and the brain as obstacles standing in the way of contact between mind and reality.
 See Kelley 1986 for an extended analysis and response to the diaphanous and Kantian theses.
[This is an excerpt from Stephen Hicks’s Explaining Postmodernism: Skepticism and Socialism from Rousseau to Foucault (Scholargy Publishing, 2004, 2011). The full book is available in hardcover or e-book at Amazon.com. See also the Explaining Postmodernism page.]