On the Parts of Animals 639a16
"I mean, for example, should one take each being singly and clarify its nature independently, making individual studies of, say man or lion, or ox and so on, or should one first posit the attributes common to all in respect of something common? For many of the same attributes belong to many different kinds of animal, for example sleep, breathing, growth, wasting, death, and any other affections and conditions of this sort (for at present we are not in a position to speak of them with clarity and precision). If we do speak of the animals severally, it is plain that we shall often be saying the same things about many of them. For each of the above attributes belongs to both horses and dogs and men, so that if one refers to each of their attributes one will have to speak repeatedly about the same ones--all that are the same indifferent species of animal while having no differentia themselves. On the other hand there are no doubt others which, although they have the same designation, differ by the specific differentia. Animal locomotion, for example, is evidently not one in species, for there are differences between flying, swimming, walking, and creeping. Therefore we must not overlook the question how the examinations should be made, that is whether one should first survey common general attributes and then later the peculiar ones, or take them individually straight away. At present this has not been clarified."