In two places in his extant works, in Physics 11.9 and three connected passages of the Parts of Animals 1.1 (639b21–640a10,642a1–13,31–b4) Aristotle introduces and explains his notion of ‘hypothetical necessity’ (anangkē ex hupotheseōs). Judging from this terminology one might think a hypothetical necessity would be anything that is necessary given something else, or something else being assumed to be so (cf. APr. 1.10 30b32–40) – in effect, anything that follows necessarily from something else's being so, but that may not, taken in itself, be necessary at all. But this is not so: the necessity of New York's being north of Princeton given that it is north of New Brunswick and New Brunswick is north of Princeton, is not an example of what Aristotle calls a hypothetical necessity. This is because the hypothesis relatively to which a hypothetical necessity, in Aristotle's usage, is necessary is always a goal posited or set up (hupotethen) as something to be achieved. Hence the necessity in question is always that of a means to some end. A hypothetical necessity, as Aristotle intends the term, is something necessary if some goal is to be attained. Thus, he says, a saw or an axe has to be hard in order to do its job of cutting or splitting, and in order to be hard it has to be made of bronze or iron: hence it is necessary for a thing to be made of bronze or iron if it is to be a saw or an axe, and there must be bronze or iron to hand if one is to carry out the intention to make a saw or an axe.