Full Idea
The special effectiveness of the 'what is it?' question is that, in the case of continuants, it refers us back to our constantly exercised idea of the persistence and life-span of an entity.
Gist of Idea
Asking 'what is it?' nicely points us to the persistence of a continuing entity
Source
David Wiggins (Sameness and Substance Renewed [2001], 2.2)
Book Reference
Wiggins,David: 'Sameness and Substance Renewed' [CUP 2001], p.61
A Reaction
Compare 'this is a human' with 'this is a member of a family noted for its longevity'. We can't simply answer 'what is it?' by tossing it into the nearest category. I say we need an individual essence for explanation, not just a sortal.