Philosopher George Berkeley radicalized this mode of inquiry: “When we do our utmost to conceive the existence of external bodies, we are all the while only contemplating our own ideas.” Berkeley’s argument was that we have no experience of material objects that exist outside the perceptual apparatus of our mind and senses. Both the primary and the secondary qualities of objects are perceived in this way. We don’t experience the ocean, we experience only our image and sound and feel of the ocean. Berkeley famously concluded that we have no warrant for believing in a material reality existing independent of our minds!

The great Samuel Johnson famously “refuted” Berkeley by kicking a rock. There! The rock exists! Alas, this is no refutation. Berkeley’s reply to Johnson would be that his entire experience, from perceiving the rock to the sharp pain he felt upon kicking it, occurred entirely within his mind. And Hume completed Berkeley’s skeptical argument by applying it to human beings themselves. We have no experience of ourselves other than our sensations and feelings and thoughts. While we know that sensations and feelings andthoughts exist, we have no basis for postulating some “I” behind them that is supposed to be having those reactions.

It was Hume, Kant wrote, who awakened him from his “dogmatic slumber.” Kant conceded Berkeley’s and Hume’s point that it is simply irrational to presume that our experience of reality corresponds to reality itself. There are things in themselves—what Kant called the noumenon —and of them we can know nothing. What we can know is our experience of those things, what Kant called the phenomenon. If you have a dog at home, you know what it is like to see, hear, smell, and pet it. This is your phenomenal experience of the dog. But what is it like to be a dog? We human beings will never know. The dog as a thing in itself is hermetically concealed from us. Thus from Kant we have the astounding realization that human knowledge is limited not merely by how much reality there is out there, but also by the limited sensory apparatus of perception we bring to that reality.

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. If a tape recorder apprehends reality in a single mode, human beings can perceive reality in five different modes: sight, hearing, smell, taste, and touch. There is no other way for us to experience reality. We cannot, for example, perceive reality through sonar in the way that a bat does. Our senses place absolute limits on what reality is available to us.

Moreover, the reality we apprehend is not reality in itself. It is merely our experience or “take” on reality. Kant’s point has been widely misunderstood. Many people think that Kant is making the pedestrian claim that our senses give us an imperfect facsimile or a rough approximation of reality. Philosophical novelist Ayn Rand once attacked Kant for saying that man has eyes but cannot see, and ears but cannot hear—in short, that man’s senses are fundamentally deluded. But Kant’s point is not that our senses are unreliable. True, our senses can fool us, as when we see a straight twig as bent because it is partly submerged in water. Human beings have found ways to correct these sensory distortions. Kant is quite aware of this, and it is not what he is after.