The first difficulty in reading Aristotle's biological treatises, as often in reading Aristotle, may well be to decide exactly what their purpose is. They are so factual and so comprehensive that it is easy to mistake them for descriptive information, an animal encyclopaedia, as the ancients regarded them. Modern readers have tried to assimilate them to present-day categories, classifying the HA as natural history, the PA as comparative anatomy, the PN as physiology, the GA as embryology. But this assimilation does not fit. One can test it by looking for the facts about any given animal in HA. Without an index (which ancient readers did not have) the facts can only be found by reading through the whole treatise, for they are distributed all over it; and when found they may seem strangely inadequate. A striking example is the blind mole-rat, aspalax, which Aristotle quotes in the De Anima as an interesting case. He twice describes a dissection of its concealed eyes. But the only other fact that he reports is that it is viviparous – not what kind of animal it is, how many legs, what its coat or feet or tail are like, how it lives, nothing. His aim is clearly not to give a natural history of the mole, but to show how it differs from other animals: it is his only case of sightlessness combined with viviparousness.