Posterior Analytics (Bouchier)/Book II/Chapter XVI

Chapter XVI: On inferring the Cause from the Effect

 * When the Cause is known the effect also must be known, but in consequence of the Plurality of causes an effect may be known without the exact cause being known.

A difficulty might be raised in connection with the Cause and its result as to whether, when the effect is present, the cause must also be present; for instance whether, supposing leaves to fall from the tree or the moon to be eclipsed, the cause of the fall, or of the eclipse, is likewise present. It may be assumed that the cause of the former is the possession of broad leaves, that of the eclipse the interposition of the earth, for even if it be not so something else will be the cause of the phenomena. Now if the cause be present the effect must also be present; e.g. if the earth be interposing the moon is being eclipsed, if the tree have broad leaves it is deciduous. An objection might here be raised that if this were so, the cause and effect would be simultaneous, and each of them might be proved from the other. Thus: let A represent the quality of shedding leaves, B broad-leaved, C vine. Now, if A be true of B (since every broad-leaved tree is deciduous), and also B of C (since every vine has broad leaves), then A is true of C, and every vine is proved to be deciduous. The cause of this is B, which is here the middle term. It may also be demonstrated conversely that the vine has broad leaves from the fact of its being deciduous. Thus, let D be broad-leaved, E deciduous, F vine. Now E is true of F (for every vine is deciduous), and D of E (for every deciduous tree is broad-leaved). Therefore every vine is broad-leaved, and the cause of it is that it is deciduous. On the other hand these qualities cannot be the causes of one another, since the cause is prior to that of which it is the cause. Thus supposing the interposition of the earth to be the cause of an eclipse, then the eclipse cannot be the cause of the interposition of the earth. If then the demonstration of the cause give the reason of a thing, while the demonstration which does not explain the cause gives only the fact, this latter demonstration may suffice to inform us that the earth interposes between sun and moon, but not why it interposes. That the eclipse is not the cause of the interposition of the earth, but this latter of the eclipse, is obvious, for the interposition of the earth is an element in the definition of eclipse, and it is therefore clear that the latter is demonstrable by means of the former, not vice versa.

Can there be several causes of the same thing?

Since the same thing can be immediately predicated of several subjects, let A be immediately predicable of B, and also immediately predicable of another subject C, and again B and C of D and E. Here A will be predicable of D and E, and the cause of this will, in the case of D be B, in that of E it will be C. Hence when the cause is present the thing caused must also be present, but when the thing caused is present it is not necessary for every separate cause one may select to be present. Some cause must indeed be present, but not every cause. Again if the subject proposed be a universal one, not only must the cause be something universal but also that of which it is the cause. Thus ‘shedding leaves’ is a universal attribute of a whole genus, though this may include subordinate species, being applicable either to plants in general or to particular kinds of plants. Hence in these cases the middle term and the effect must be co-extensive and convertible. Take for instance the question ‘Why do trees lose their leaves?’ If the reason be that the sap is dried up, whenever the tree sheds its leaves this drying up must occur, and if it occur in tree and nothing but tree, that tree must shed its leaves.