On your words.

Does cognitive development really happen in stages? What happens in the brain when we have that "Oh, now I get it" moment? Or are we just putting an arbitrary line in the sand for something that happens gradually and continuously? When we look at a baby who giggles and smiles when we play Peek-a-boo, does the child really think that we have disappeared or, as Kagan would have us wonder, does it just think what you are doing and the faces you are making is funny? Are there any physiological correlates in the brain to support your position?