In TDD, how do you write tests first when the functions to test are undefined?

If you have nothing, you cannot write a test because there is nothing to test. This seems pretty obvious to me but never seems to be addressed by proponents of TDD. To write a test, you have to first decide what the method or function looks like that you're going to test. You have to know what parameters to pass to it and what you expect to get back. THAT is what comes first, NOT the test. Tests can NEVER come first. The thing that comes first is the design which specifies what classes and methods are going to exist.