Wittgenstein's admonition “don't think, but look” has had the important effect of stimulating psychologists to reconsider their common practice of equating concept formation with the learning of simple definitional rules. In the early 1970s, psychologists like Eleanor Rosch (e.g., Rosch, 1973), responding to the difficulty of identifying necessary and sufficient conditions for membership of all kinds of categories, proposed alternative models of category representation based on clusters of correlated features related to the categories only probabilistically. Without denying the importance and impact of this changed view of concepts (reviewed, e.g., by Smith & Medin, 1981), we think that in certain respects the “don't think, but look” advice may have been taken too literally. There are problems with equating concepts with undifferentiated clusters of properties and with abandoning the idea that category membership may depend on intrinsically important, even if relatively inaccessible, features. For example, on the basis of readily accessible properties that can be seen, people presumably will not judge whales to be very similar to other mammals. However, if they think about the fact that whales are mammals not fish, they will probably acknowledge that with respect to some important, although less accessible property or properties whales are similar to other mammals.