Aristotelian science conveys understanding by showing the necessary relationship between immediately evident first principles and conclusions about the natural world. To take a trivial example: induction from repeated experience teaches us that all broad-leaved plants are deciduous. We discover that grapevines have broad leaves. We infer that grapevines are deciduous, and thereby we also learn that they lose their leaves in winter because they are broad-leaved (see Aristotle, Post. An. II.16–17).

Aristotelian science explains a subject’s possession of an attribute (the explanandum) by identifying the possession of that attribute with membership in a species, and then citing as explanans the inclusion of that species within a prior genus. In the present example, the property is “losing its leaves in winter,” the species is “grapevines,” and the genus is “broad-leaved plants.” The explanation is then presented in the form of a syllogism. In the first premise an attribute (being deciduous) is predicated of a subject (broad-leaved plant). The second premise introduces a new subject that belongs to the class described by the subject of the first premise, allowing us to conclude that the second subject shares an attribute of the first.

Aristotle describes this demonstration as propter quid science because it explains why grape leaves fall. If the deduction were valid, but its premises were not explanatory, it would count as quia science: knowing a fact without understanding why it obtains (ibid., I.13, 78a22–b3) (see also Chapter 26).