proprium

noun

1. A property that applies to all members of a species and only to them, serving to distinguish the species from other species within the same genus, yet is not part of the true definition or the essence of the species.

   “(So you can use a proprium to pick out a species—for example, you could say: “a human is a risible mortal animal”—but, in that case, you aren’t picking out the species by its true definition.)”

2. Selfhood.

   “Man of himself, so far as he is under the influence of his proprium, is worse than the brutes. If man should be led by his own proprium, he could not possibly be saved.”