# Posterior Analytics (Bouchier)/Book II/Chapter XVII
### Chapter XVII: Whether there can be several causes of the same thing

    In a demonstration of the cause, no effect can be regarded as the outcome of several causes, for in such a demonstration things are regarded in the light of essentials and not of accidents. Here the middle term is the definition of the major.
Is it possible that it should not be the same cause which produces a like effect in all the subjects included in the minor term, but that another cause should exist, or is that impossible? If the cause has been demonstrated to be essential and not a mere external sign or accidental cause it is impossible that there should be more than one cause, for the middle term is the cause of the minor. Otherwise it is possible. One may indeed consider the effect and the subject in which it is produced from the point of view of their accidental attributes, but these cases are not to be looked on as scientific problems. If the cause be not made to depend on accidental attributes the middle term will be of like kind with the subjects themselves, but if the latter are one only from an applied use of the common name the middle term can only be employed with like restrictions, while if the subjects be all included under the same genus the middle will be so also. Thus, suppose that one had to state the cause of reciprocal proportion. The cause of this differs in the case of lines and in that of numbers, and yet is in both connections the same in so far as it depends on a certain law of increase. The same argument applies to all such instances. There is however a difference between the cause which makes one colour resemble another and that which makes one figure resemble another, for in these two cases ‘resemblance’ bears a two-fold meaning. In connection with figures it consists in having an equal number of sides and equal angles, in the case of colours that they convey a similar sensation to the senses or something of the kind. Things which are analogously identical will also have an analogous middle term.
The reason of this is that the cause, the effect and the subject in which the effect is produced correspond to one another. If one take particular species as subjects the term in which the cause produces the effect will be more comprehensive than any one of the subjects. Thus the quality of having the external angles equal to four right angles is more comprehensive than triangle or square, but the quality is co-extensive with all angular figures taken together. So too all figures which have their external angles equal to four right angles are included under the same middle term.
As the middle term contains the definition of the major, all scientific knowledge is based on definition. For instance, shedding leaves is a quality both of vines and figs, but is more comprehensive than either of them. Yet it is not more comprehensive than all deciduous species of trees but co-extensive with them. If one take the primary middle term (broad-leaved) it will contain the definition of this quality of shedding leaves. This term will serve as middle term both in questions concerning vines and figs and will shew that all vines and figs possess the quality of having broad leaves. The middle term or cause through which broad-leaved trees are deciduous is that the sap dries up, or something of the kind. In what then does this quality of shedding leaves consist? In having the sap dried up at the junction of stalk and stem.
We will answer enquiries as to the mode in which cause and effect can follow each other by this example. Let A be true of all B, and B be true of all D, but be more comprehensive. Then B will be universally predicable of D. By ‘universal,’ I here denote a predicate which is not convertible with its subject, while ‘primary universal’ is a predicate with which the separate individual subjects are not convertible, though the whole subject is convertible and co-extensive with that predicate.
In the above instance B is the cause why A is predicable of the various subjects D; A therefore must be more comprehensive than B; otherwise why should B be the cause rather than A? If A be predicable of all the subjects E, these latter taken together form a single concept distinct from B. Otherwise how could one say that everything which is E is also A, but that not everything which is A is also E? For why should not the cause be, for instance, that every D is A? Then the various subjects E will form a single concept which must also be considered, and may be denoted by C.
Thus it is possible for several causes to produce the same effect; but not when the subjects in which the effect is produced are specifically the same. For instance, the cause of long life in quadrupeds may be the absence of gall, in birds dryness of constitution, or something else. If however we do not at once attain some ultimate proposition, and if the middle term be found to be not one but many, then the causes also must be many.