Self, or essence, is not a natural kind, soul is the understanding of self as such a kind. There can be a class of all souls, but there is no class of selves, every self is thus the universe of all selves. Bodies and minds in their generations are permuted via the random shuffling of parental genes, this makes for a random traversal of the space of all bodies and minds, but apparently not an ergodic one. Souls which are said to evolve by way of affinities would have no chance of even that degree of diversity but would soon converge into uniformly mediocre subsets.