I never actually interpreted baby Spock as a person in the sense that he would develop into an actual thinking being like Spock. Instead he was more along the lines of a mindless drone who's actions are driven purely by instinct and natural urges. And as this was brought up earlier, McCOY: Rapid ageing, ...all genetic functions highly accelerated. KIRK: What about his mind? McCOY: His mind's a void. It seems, Admiral, that I've got all his marbles. And that's what I think it all boils down to. You have a working body but nothing that is capable of forming it's own personality. There was no one there to replace.