Peter Singer

It was wrong to capture wild animals and confine them in captivity for people to go and gawk at them. And that's basically how zoos got started. But once you do that, and once you have animals that have been bred in captivity, you're really stuck with them in some sense. You can't return them to the wild.

I think one of the most important directions to be pursued in the 'sciences of human action' is to develop a natural-law ethics based on nature rather than, or at least to supplement, ethics based on theological revelation.