  In another type of demonstration, the middle term may be a definition of a concrete subject obliquely referring to its parts without any determination, namely in demonstrations concerning animals and human beings, in which the reality contains a plurality of forms. Thus a particular human being is able to learn, say, in virtue of the form which is its rational soul. If, as Thomas held, there is no plurality of forms here, but every natural substantial form, including biological forms, is simple, and its rational soul just is the form of the body, then no demonstration is possible at all, for the ability to learn is immediately in a human being through itself, so that to say that a human being is rational is to express a first principle. If there is a plurality of forms, this sort of demonstration nonetheless falls short of the highest sort, for it concerns only the concretum, the concrete instance that has several forms, not the substance considered in itself (simply as possessor of the form). (Note that here forms are the parts expressed in the definition obliquely, so the case differs from mathematical demonstrations.) Moreover, since the concretum receives the attribute, the attribute is either learning itself, which is clearly not demonstrable of every concrete instance of a human being, since some human beings may fail to realize their capacity to learn, or merely the ability to learn, which is not an actual occurrent constituting part of a real fact about the world as it is, but, as Albert insisted, merely a kind of possibility of an occurent thing. The notion that a kind of efficient causation produces the capacity for learning, which is to be taken as something really present in a human being even when the human being is not learning, is rejected by Ockham, of course, for he takes it that talk about the capacity is only oblique talk about real events, using modal propositions. All natural demonstration concerning efficient causes is reduced to this sort of thing by Ockham, and he never allows that, for instance, it can be shown that fire is capable of heating, unless it is taken that we are talking about concrete instances of fire, in which case the whole essence of a particular fire can be expressed in the middle term, thus: “Every hot thing is capable of heating, every concrete instance of fire is (essentially) a hot thing, therefore every concrete instance of fire is capable of heating.” This is as close as Ockham gets to Thomas's notion of the highest sort of demonstration.