Well, the brain, like other complex organs, owes its non-random organization to natural selection. That if there are circuits in the brain that accomplish improbable feats, then natural selection is the explanation for how they got wired up the way they are. And we're going to ask of various psychological features whether they are adaptations. That is, whether they increased the chances of reproduction in our ancestors. For a lot of psychological features that's pretty straightforward to do. It's no mystery why we see in stereo, because it's a ... for many reasons, highly adaptive