 First, if Pellegrin and Balme are right that Aristotelian zoology is uninterested in classifying species, then ascribing the capacity for “rationality” cannot have the function of naming a biological trait that distinguishes humans from other animals. This is supported by two further sets of considerations. To begin with, Aristotle’s explicit assertion that a series of differentiae would be needed to “define” humans (Parts of Animals 644a) is cashed out in the long list of features he takes to be their distinguishing marks, such as speech, having hair on both eyelids, blinking, having hands, upright posture, breasts in front, the largest and moistest brain, fleshy legs and buttocks (Lloyd 1983: 29ff.). Furthermore, there is in Aristotle no capacity for reason that is both exclusive to, and universal among anthropoi. One part or kind of reason, “practical intelligence” (phronesis), is, Aristotle claims, found in both humans and other animals, being merely superior in the former (Parts of Animals, 687a). Now, there are other forms of reasoning of which this is not true, forms whose presence are sufficient for being human: humans are the only animals capable of deliberation (History of Animals 488b) and reasoning (to noein), in as far as this extends to mathematics and first philosophy. Nevertheless, these forms of reasoning are unnecessary: slaves, who Aristotle includes among humans (Politics 1255a), are said to have no deliberative faculty (to bouleutikon) at all (Politics 1260a; cf. Richter 2011: 42ff.). Presumably, they will also be without the capacities necessary for first philosophy.