Prove It! How We Know What We Know, Part 2

In my last post, we talked about the theory that we can only know for sure that which we experience with our own senses — and how such a viewpoint is untenable.

We looked to David Hume to start setting this variety of empiricist straight, and in this post, we’ll see how Immanuel Kant builds on Hume to dismantle this theory even further.

Immanuel Kant

Kant noticed that Hume is right about the existence and importance of X beliefs. Kant considers several X beliefs, including the belief that we exist in three-dimensional space, the belief that we are in time, the belief that there is such a thing as causality, and the belief in substances existing independently of the mind.

Kant does a bit of philosophical psychology and finds that these beliefs don’t come from sensory experience. The mind applies them to sensory experience in order to make sense of sensory experience.  For example, consider the picture below of Victoria Falls from the Falls’ tourist website.

Think about what you’re looking at when you look at that picture. What you’re looking at is actually just a two-dimensional image. However, your mind knows how to apply the idea of three dimensions to it, so you can perceive that the the trees are behind the falls.

But all you ever really see with the eyes (unaided by the mind) is, like this image, no more than an arrangement of shades and colors, which the eyes by themselves can’t tell you involve any three-dimensional objects.  In short, depth-perception is not the gift of the eyes to the mind, but the mind’s gift to what the eyes see in order to make sense of it.

Now Kant has the same standards for knowledge of the world outside the mind as Hume: It has to come from experience. So an X belief about the world outside the mind is unknowable. But Kant cleverly treats X beliefs as a priori ideas (ideas not derived from experience) about the world as we perceive it—not about the world itself. And as a priori notions, they can be known.

If you’re following all this, you might have some questions; and you might be thinking you just read something ridiculous. Let’s put some likely concerns in the form of a little dialogue between Kant and a critic:

Critic: Wait just a minute, Kant! What about the requirements for knowledge of the world outside the mind? You said that knowledge has to come from experience, and now you’re saying it doesn’t.

Kant: Well, those requirements are still in place: The X beliefs we know are about the world inside the mind.

Critic: But that makes you a skeptic, Kant! You think there’s no world outside the mind.

Kant: No, obviously the world outside the mind exists. We just can’t know it directly.

Critic: But that still makes you a skeptic! You don’t think we can know anything about the world.

Kant: Well, we know all about the world inside the mind.

Critic: But you still think we can’t know about the world outside the mind! So you are still a skeptic after all.

Kant: Thinking we can know the world outside the mind is what leads to skepticism! Anyway, I can prove that we have to believe that X beliefs pertain to the world outside the mind; X beliefs shape our perceptions; we cannot perceive without them.

And then Kant goes on to prove just that! Like we saw with the waterfall picture above, perception of the world in three dimensions depends on belief in space, belief in time, and other X beliefs.

So far we’ve looked at Hume and Kant; in our next post, we’ll top it all off with Thomas Reid’s common-sense approach. Stay tuned!