 When one creates a nominal essence, one is cobbling together a collection of particular qualities—from among these three kinds of qualities—that one observes together in a substance and making that collection definitive of a species or genus. Having said that, I should point out that when Locke thinks of a species or genus, he is not thinking of these terms as applying exclusively in the biological realm, but rather as applying generally to any possible classification scheme we create when we organize our world through naming. So, for example, gold is as much a species of the genus metal as human is a species of the genus animal.