17 April 2008
The best quick way to state this is: an idealist thinks this keyboard I'm looking at right now is identical to my idea of the keyboard. The realist on the other hand, thinks the keyboard itself is a material fact outside of my skull, my idea of it is another sort of fact inside of my skull, so the two can't be the same thing.
Think of dreams to get a fix on the difference. If I'm dreaming right now, then it is fairly plausible to say that this keyboard is my idea of it -- since outside of my skull there will be a pillow and some blankets but probably no keyboard. Although idealists don't precisely believe that all of life is a dream, it is certainly more dream-like in their telling than it is in the telling of most realists.
Now, the realist/idealist dichotomy is one dispute. It's a matter of metaphysics. The rationalist/empiricist dichotomy is another thing. Its a matter of epistemology -- the philosopical study of knowledge, its components and conditions.
The history of philosophy has rationalist realists (like Leibniz). It also has rationalist idealists (like Josiah Royce). It has empiricist idealists (like George Berkeley). And it also has some fillers for the one remaining square in this grid, the empiricist realists (John Locke).
What about Immanuel Kant? His notorious example requires that we expand our grid a bit, because the best quick statement we can make about him is that he was clearly on the rationalist side of the epistemological split, but that he position was ambiguous on the metaphysical split.
He was a rationalist whose rationalism led him to a view of metaphysics that was in part realist and in part idealism.
Most of what we know about life is a dream, though we can also reasonably postulate what we can not prove -- a non-dream element.
Knowledge is warranted belief -- it is the body of belief that we build up because, while living in this world, we've developed good reasons for believing it. What we know, then, is what works -- and it is, necessarily, what has worked for us, each of us individually, as a first approximation. For my other blog, on the struggles for control in the corporate suites, see www.proxypartisans.blogspot.com.