  Aquinas, it is to be noted, holds that we know all the principles of a demonstration of the highest sort through an intellectual vision of the substantial nature of the subject, for this tells us the real definition, and so reveals the minor premise, per se in the first way, and it also enables us to see, through the intellect, what primary attribute arises through emanatio from such a thing. Like Grosseteste, Aquinas thinks that the causal capacities of things (their functions) must be understood to know what they are, but he sees no problem in working out how these forms do what they do within the actual world. In the case of the highest form of demonstration, they simply do it. It is as though we were to say that an adding machine adds, without concerning ourselves how it must be designed to do so. Of course, real natural substances are simple, and artificial things like adding machines are not. An adding machine adds because of the way it is put together, but it is absurd to say that heat heats or a person knows, because of the way it is put together, for it is not put together. There must be something for which simply to exist is to exercise its function, or else we have an infinite regress, and must explain every functioning as though it were the functioning of a complex machine, without ever settling on simple parts for the machine which simply function as they do because of what they are. It is Aquinas who lies behind Descartes's view that first principles in the sciences are clearly and distinctly perceived by reason.