 In this connection it is important to notice that Aristotle recognizes the existence of idia, that is of properties that apply to all and only instances of a given species, which an instance of that species has necessarily, but which are not part of its essence: e.g., all and only human beings are capable of laughter (cf. Categories 5, 3a21, 4a10; Topics i 5, 102a18–30, and v 5, 134a5–135b6). The essence of a human being is rationality, and the fact that we all (apparently) have a sense of humour follows from the essence together with how the world is. Many characteristic changes of organisms may be best explained in a similar way: all ducks waddle, but waddling is not part of their function. Rather anything that fulfils the functional requirements of a duck must (in a world like ours) walk inelegantly.