 In the APo., Aristotle returns regularly to such standard examples of natural phenomena as thunder and eclipses—but he also, and importantly, provides an extended biological example, the seasonal loss of leaves in broad-leafed  plants.[2]  In ch. 16, Aristotle imagines an inquiry that begins with questions such as ‘Why do fig trees and grape vines lose their leaves?’. The model answer is ‘Because they are both broad-leafed.’ That is, the inquiry seeks some other feature, common to both kinds, related to the target of inquiry, the seasonal loss of leaves. The ‘because’ is, however, preliminary—it is best to see being broad-leafed as a step toward causal explanation. Chapter 17 picks up the example, in the context of arguing that basic scientific inquiry seeks, wherever possible, co-extensive predications, which those between leaf loss and fig trees, or leaf loss and grape vines, are not. The candidate major premise, however—‘Whatever is broad-leafed loses its leaves’—does identify a co-extensive relationship between subject and predicate. It may thus serve as a proper scientific explanandum, and ‘broad-leafed’ can serve to identify a kind, all and only the members of which lose their  leaves.[3]  Thus, the middle term of the preliminary explanation becomes the subject of a more basic, co-extensive predication. The cause of broad-leafed trees losing their leaves will, then, be something more fundamental about broad-leafed trees, here identified as the solidification of moisture at the leaf juncture, which can thus serve as the middle term in a causal explanation of this fact. But it will also serve as part of a definition of leaf loss.