for those of you who haven't heard of this: Collective unconscious - Wikipedia, the free encyclopedia
For everyone else, first of all, I find it strange that an INTJ (Jung) came up with something like this, though upon thinking about this further, at the time psychology would have been much farther ahead than neurology, so theory was about all he would have to work with.
I think the collective unconscious could be attributed to instincts, a sort of genetic command code. But, then if this were the case Jung's idea that the c.u. is the same in all humans couldn't be completely accurate providing the subtle differences in our DNA. I don't think this is what He had in mind anyway, but a unconscious psychic link. Still, my genetic hypothesis seems to be a more reasonable explanation of Jung's observations.
Anyway, I'd like to hear your thoughts on the subject of the C.U. (hear your thoughts... there's a joke there. XD)