Full Idea
The category of person is a poor foundation for ethical thought, because it looks like a sortal or classificatory notion while in fact it signals characteristics that almost all come in degrees (responsibility, self-reflection etc).
Clarification
A 'sortal' is something with a separate identity, which can be counted
Gist of Idea
The category of person is a weak basis for ethics, because it is not fixed but comes in degrees
Source
Bernard Williams (Ethics and the Limits of Philosophy [1985], Ch. 6)
Book Reference
Williams,Bernard: 'Ethics and the Limits of Philosophy' [Fontana 1985], p.114
A Reaction
On the contrary, it must be the basis of moral theory, and its shifting character is strong support for Aristotle's approach to moral growth and responsibility.