Kant begins with a simple premise: all human knowledge is based on experience. We gain access to reality through our five senses. This sensory input is then processed through our brains and central nervous systems. Think about it: every thought, even the wildest products of our imagination, are exclusively based on things that we have seen, heard, touched, smelled, or tasted. If we imagine and draw creatures from outer space, we can give them four eyes and ten legs, but ultimately we have no way to conceive or portray them except in terms of our human experience. It is an empirical fact that our fives senses are our only lenses for perceiving reality.
D'Souza is writing a popular book and some license must be given for loose expression. But the first sentence is not loosely phrased, it is simply false. Kant does not just hold that we have knowledge that is not based on experience; the point is essentially what differentiates him from prior philosophers like Hume. Kant's problem with Hume was that Hume's pure empiricism undermined human knowledge, specifically knowledge that could be acquired through the then newly developed empirical sciences. Kant's project in the Critique of Pure Reason was to explore what reason could know independent of experience and therefore prior to it. This a priori use of reason results in genuine knowledge; among the things it can come to know are the conditions of empirical experience, and from that what distinguishes legitimate empirical knowledge (for Kant, basically modern science) from pseudo-knowledge (for Kant, most of what constitutes classical metaphysics.) This was Kant's plan to rescue science from destruction by Humean empiricism.
But this misunderstanding, although present, isn't the principal one I have in mind in this post. That misunderstanding is found in the fact that, although D'Souza emphasizes (in other passages) just how revolutionary and deep are the implications of Kant, D'Souza himself fails to take Kant fully seriously. D'Souza is right when he points out that the implication of Kant is the distinction between the noumenal (reality in itself) and the phenomenal (reality as it comes to us through the senses). We have no way of "stepping outside" our senses and finding a place from which to compare our perception of reality with reality itself; all we know (empirically) is reality as it is conditioned and filtered by our senses. D'Souza's example of a tape recorder is apt:
Consider a tape recorder. A tape recorder, being the kind of instrument it is, can capture only one mode or aspect of reality: sound. Tape recorders, in this sense, can "hear" but they cannot see or touch or smell. Thus all aspects of reality that cannot be captured in sound are beyond the reach of a tape recorder. The same, Kant says, is true of human beings. We can apprehend reality only through our five senses.
Now consider what a tape recorder could empirically discover in trying to understand its own nature. It's only empirical sense is sound. So the the only thing the tape recorder can do is listen to itself. If it could, it might put itself in a soundproof room and turn itself on (how I don't know, but that's immaterial). It might record a few button clicks, and then the soft whirring of the electric motor that rotates the reels. That's it, and it's not much. It might attempt an empirical theory of its own nature based purely on sound, but we can see that any such theory would be hopelessly inadequate to the nature of the tape recorder that can be known through all five of our senses. So it's not just knowledge of the outside world that is subject to the empirical conditioning of the recorder's single sense; it is empirical knowledge of its own nature as well. The tape recorder, if it managed to find an audio version of the Critique of Pure Reason, might understand all this through an application of pure reason.
The same line of thinking, of course, applies to us. Empirical knowledge of our own human nature is just as subject to Kantian limitations as knowledge of anything else. So when D'Souza writes sentences like "We gain access to reality through our five senses. This sensory input is then processed through our brains and central nervous systems" he is not taking Kant seriously enough. "Brains" and "central nervous systems" are empirical constructs; they are phenomenal, not noumenal. Taking Kant seriously means acknowledging that our phenomenally known human nature may be inconceivably different from human nature as it is in itself; just as the tape recorder's understanding of its own nature purely on the basis of sound can't compare with the nature of the tape recorder as it actually is. Kant is careful not to talk about brains, nervous systems or other organs in the Critque of Pure Reason. Human nature in that work, and for Kant in general, is an unknowable X. We can know that human nature, whatever it is, must involve a rational principle (since the fact that we are intelligent beings is known simply from the act of thought itself), but anything beyond that is a matter of phenomenal empiricism.