I realised that the "What exists, and in what ways do these things exist?" question is a bad one. We have the word "exists", we don't really know what it means, we want to. I was just writing to an email list about how doing the same thing with "love" is a terrible idea. So here's a solution to the question of existance: regard things to exist, when it is epistemically necessary to explaining reality.
on March 17, 2003