Full Idea
Williams has expressed pessimism about the project of Aristotelian naturalism on the grounds that his conception of nature, and thereby of human nature, was normative, and that, in a scientific age, this is not a conception that we can take on board.
Clarification
'Normative' means that it prescribes rules for how things should be
Gist of Idea
We can't accept Aristotle's naturalism about persons, because it is normative and unscientific
Source
report of Bernard Williams (works [1971]) by Rosalind Hursthouse - On Virtue Ethics Ch.11
Book Reference
Hursthouse,Rosalind: 'On Virtue Ethics' [OUP 2001], p.256
A Reaction
I think there is a compromise here. The existentialist denial of intrinsic human nature seems daft, but Aristotelians must grasp the enormous flexibility that is possible to human behaviour because of the open nature of rationality.