If the subject interests you, I recommend reading Women, Fire, and Dangerous Things. It's somewhat slow going, but the author lays out a detailed story about the process of category formation in humans (that is, why we create the categories that we do) that does wonders for clarifying the issues involved.

I have no idea whether the specific story he tells is right or not, but sometimes it's useful to just have an example of what such a story might look like.