(I love that song) There has been an interesting and fundamental discussion on the Anthro-L listserv email list recently. Again, it is epistemology. Do you "believe in" evolution? Do you "believe in" gravity? What it comes down to is the willingness to "act as if" [evolution|gravity|etc] is true, just as it seems necessary to act as if you have faith to have faith. A good illustration is the electron. No one has actually seen an electron, you know, with their own eyes except perhaps with the help of some psychoactive substance. Yet it is necessary to act as if our theories regarding electrons are true in order for us to enjoy everything we do on the grid. How many users of the internet can last more than a couple of minutes without throwing a switch?
Actually, both evolution and gravity are observable facts, it's only their explanations that are theoretical. That the earth is flat is also observable, that the sun rises in the morning and sets at night are also observable. It is only after one starts acting as if the theoretical explanations for these observations are true that any practical actions can be taken. A customer came into the store last night and asked me what I thought The Answer was: I said Fake It.