 To this kind of contingency, however, another must be added. The productive relation can be contingent in another sense. It is contingent whether heat, e.g., produces homogeneity. Heat does not invariably nor necessarily produce homogeneity, although it does do so for the most part (Prior Analytics, 32b5–20). There are, then, two kinds of contingency possible in practicing medicine. The first kind has to do with whether the physician acts or not; the second kind has to do with whether the productive relation holds or not. On the one hand, if the first kind of contingency held but the second did not, the physician would have a nearly sovereign command of health. If the productive relations between heat and homogeneity and between homogeneity and health were invariable and necessary, then so long as the physician could produce heat, he could invariably produce health. On the other hand, if the productive connections were not invariable and necessary — but only held for the most part — then the physician could reliably but not invariably produce health (Metaphysics, 1026b30–1027a25). One could say of contingent productive connections that they hold unless something unusual happens, that, e.g., heat always causes homogeneity unless something unusual intervenes. Accidental factors — by definition factors which arise infrequently — can defeat the causal link between heat and homogeneity. But since accidental factors are infrequent, one can claim that for the most part heat causes homogeneity. In view of this kind of contingency, what the physician can claim to know is that heat produces homogeneity unless something unusual intervenes. In his deliberations, what he does not know is when something unusual will happen (Nicomachean Ethics, 1112a20–1112b10).