Very small children understand "real" to be "what's inside" -- what's hidden, essential. Sometimes literally inside: ask toddlers "If you took a dog, and gave it the bones and insides of a cat, would it still be a dog?" they say "no," but "If you took a dog and made it look like a cat on the outside, would it still be a dog?" they say "yes." (I'm getting this from Paul Bloom's "How Pleasure Works.") Young children are essentialist about gender as well -- they assume more differences between the sexes than actually exist, not fewer.

What psychological evidence I've seen suggests that we're in some way wired to see categories as real. "Natural kinds." To think that there's a real difference "out there" between dog and not-dog, not just a useful bookkeeping convention. I'm inclined to believe that Anna's reasoning about "atoms are real" and Eliezer's reasoning about categories actually make more sense than essentialism -- but I suspect that this kind of question-dissolving is not the standard, evolution-provided brain pathway.