Aristotle claims that just as a builder uses ‘tools’ to build a house, so too the soul ‘use[s] heat and coldness as tools’ to build an animal (Generation of Animals 740b25–34). I consider two questions about this claim: (1) what sorts of things does the soul use, and what is it for things like them to be organized? and (2) what philosophical work does this sort of organization do in Aristotle’s account of animal generation? I argue that the soul needs (...) flexible intermediate agents, that it organizes them in a way that exploits their flexibility, and that this is due to the demands of a complex, multi-stage process directed at a single end. (shrink)