So concepts aren't definitions, but do they have necessary or sufficient features?

Rips (1989) is one of the most widely cited demonstrations of the relevance of critical features in conceptual structure. A critical feature will impose constraints on how an object is classified, regardless of other factors, such as overall similarity. We review and clarify critical features proposals in terms of whether an assumption is being made about necessary features, sufficient features, or both (i.e. a definition). On the basis of this, we scrutinize Rips' investigation and present our own empirical re-evaluation of this work. The results reported here do not support any strong view of necessary or sufficient features.