Kant's refutation of Idealism

I've decided to open my blog with a discussion of Kant's "Refutation of Idealism", which appears in the revised edition of his First Critique, the Critique of Pure Reason (B274-B279). It's a strong argument but it ultimately is undermined by unwarranted assumptions.

Kant means by idealism, the belief that there is no external world or at least the external world is not indubitable. Kant proves that there is an external world that we can know exists indubitably with our awareness of time. Time is motion and we can only know of motion in comparison to something stationary. Thus, we need to be able to perceive something unmoving to see that something is moving. This is the crux of the argument, since that which is unmoving must be the external world. The nature of consciousness is that it is always in motion, thus it can't be the source of that which is unmoving. Thus we can't have consciousness of motion without consciousness of an external world. Our awareness of time and even of inner consciousness, which is time bound, depends on perception of an external world.

The idea that we need something not in motion to perceive something in motion, I think is right. Einstein uses a thought experiment in his book, Relativity to demonstrate the relativity of motion, which is useful here. Imagine you're an astronaut floating in empty space. If there is no reference point around you, you might perceive yourself as motionless. Let's say another astronaut starts floating towards you: now it looks like he is in motion and you are stationary. But from his perspective you are in motion and he is stationary. In order to perceive motion we need a reference point to compare, since motion is relative.

But I think this thought experiment is in fact doubly useful because it can point us to the unwarranted assumption in Kant's argument. Both astronauts are only in motion relative to something else. There is no absolute motion. In fact, something can appear to be in motion if is in motion relative to something else in motion, just as the earth appears to be spinning around a fixed sun, though the sun is itself flying at great speed through a giant, rotating galaxy. The fact is, we perceive motion in our dreams because we can perceive something in motion relative to something that only appears to be motionless. Kant assumes that we need something that is really in motion to perceive motion and thus to perceive time. But obviously we can perceive motion without anything really in motion. Thus, we can perceive time without an external world.

I admit, of course, that we have no good reason to doubt the external world, but we must admit that Kant has not proven that it is completely indubitable.