> First, technically returning 9 for a test of add(4,5) is correct. Your tests define what the function does, not the name. If you are thinking "a correct add adds all numbers", then that isn't wrong, but it is bringing your tacit world knowledge into the code without that knowledge being expressed in the code. And that's Not a Good Thing™.

How do you feel about just starting with tests that are actually more of a specification? For example, I'm starting to feel that unit tests aren't something we should be writing, but are something we should be generating. We should be writing things like QuickCheck specifications. So the first test I'd probably write is:

given x. y. add(x, y) == x + y

This is a specification that is essentially tested against a "model" (native addition, in this case).