 It is not the aim of this entry to decide questions of Aristotle interpretation. What is important is that the relationship of the question of “human nature” to biology is, from the beginning of the concept’s career, not as unequivocal as is often assumed (e.g., Hull 1986: 7; Richards 2010: 217f.). This is particularly true of the slogan according to which humans are rational animals. In the history of philosophy, this slogan has frequently been detached from any attempt to provide criteria for biological classification or characterisation. When Aquinas picks up the slogan, he is concerned to emphasise that human nature involves a material, corporeal aspect. This aspect is, however, not thought of in biological terms. Humans are decisively “rational substances”, i.e., persons. As such they also belong to a kind whose members also number angels and God (three times) (Eberl 2004). Similarly, Kant is primarily, indeed almost exclusively, interested in human beings as examples of “rational nature”, “human nature” being only one way in which rational nature can be instantiated (Kant 1785, 64, 76, 85). For this reason, Kant generally talks of “rational beings”, rather than of “rational animals” (1785, 45, 95).