 Although Roschian basic level objects are good candidates for informative answers to the question “What is it?” they do not provide traditional essences. Traditional essences as conceptualized by philosophers and most psychologists are necessary and sufficient conditions for being a member of that kind. On that conception, learning a new sortal requires knowing the elements and how they combine to define the new kind. On Rosch’s conception, sortal words are learned in families and the primary learning experience is becoming familiar with the prototypical members of the kind. Prototypes are the best examples, produce the quickest responses when subjects are queried about kinds, and are the most frequently cited when subjects are asked to give examples. In the category of birds, robins are one of the most prototypical, swans are intermediate and penguins are very unprototypical. Evidence for Rosch’s view of categories, which has clear roots in the views of Wittgenstein, consists of the failure to produce adequate necessary and sufficient conditions for most common sortals, as well as the positive experimental evidence just cited.