Full Idea
We can significantly ask what properties it is necessary for something to possess in order to be a thing of such and such a kind, since that asks what properties enter into the definition of the kind. But there is no such definition of the individual.
Gist of Idea
We see properties necessary for a kind (in the definition), but not for an individual
Source
A.J. Ayer (The Central Questions of Philosophy [1973], 9.A.5)
Book Reference
Ayer,A.J.: 'The Central Questions of Philosophy' [Penguin 1976], p.197
A Reaction
[Quoted, not surprisingly, by Wiggins] Illuminating. If essence is just about necessary properties, I begin to see why the sortal might be favoured. I take it to concern explanatory mechanisms, and hence the individual.