Posterior Analytics (Bouchier)/Book II/Chapter VI
Chapter VI: The Essence cannot be proved by the Definition of the thing itself or by that of its opposite
- Division supplies no common attribute by means of which the various attributes may be bound together, so as to produce one predication instead of several.
Is it possible to demonstrate the real essence of a subject when one has assumed hypothetically that the essential nature of that subject is one of the properties which constitute its formal cause, and that only those particular qualities, all being peculiar to the subject, are so included? (That is the meaning of the essence of a subject).
Has one not however really used the same term, the notion of formal cause, twice over? for one must effect the proof by means of the middle term which ought itself to be proved. Further, just as in a syllogism there is no assumption as to the nature of inference, for the premises on which the syllogism is based always bear the relation of whole or of part to each other, so the essential form of syllogism ought not to be included in the syllogism, but to remain outside the particular premises.
One should meet an objector who questions whether an inference is syllogistic or not, by saying, ‘that at least is the process which we meant by syllogism,’ and to one who asserts that we have not the essential form of syllogism, we should answer with a denial, saying that this was what we meant by the essential form of syllogism. Thus some conclusion may be arrived at without any definition of its essence or of its essential form being given.
Neither can a definition be proved by means of a hypothesis, as illustrated by the following example. Assuming that Evil consists in the quality of ‘the manifold,’ and, in the case of subjects which have an opposite, the opposite of evil, is the opposite of manifold; it might be inferred that good, as being the opposite of evil will possess the quality which is the opposite of manifold, and the essence of good will be proved to be the same as the essence of indivisible. Here too, however, the proof is effected by assuming the essential form of the subject and then proving it, and this assumption is made for the express purpose of effecting the proof. It may be objected that the same term is not really used both in the definition of evil and in the proof, and that there is difference. This may be admitted; for in demonstrations also it is assumed that one thing is predicable of another; but it is not, as in this case, the thing itself which has to be proved, nor yet that which has the same definition or is convertible with it.
The following difficulty applies both to a proof proceeding by division and to a syllogism based on definition. Why are the predicates of a definition not taken separately, as, in the sentence above ‘man is a biped animal,’ why should one not say ‘an animal’ and ‘biped’? The assumptions underlying the definition in no way demand that the attributes predicated should form a single expression; they might be stated separately, as one might call man both ‘musical’ and ‘capable of writing,’ not a ‘musical writer.’