Dr. WHEWELL'S CRITICISM OP ARISTOTLE'S ACCOUNT OF INDUCTION. 65 



how many such species there are, yet we wish to be able to assert that all acholous animals 

 are long-lived. t In the proof of such a proposition, put in a syllogistic form, there must 

 necessarily be a logical defect ; and the above discussion shews that this defect is the substi- 

 tution of the proposition, " All acholous animals are as elephant, &c," for the converse of the 

 experimentally proved proposition, " elephant, &c., are acholous." 



In instances in which the number of particular cases is limited, the necessary existence of 

 a logical flaw in the syllogistic translation of the process is not so evident. But in truth, 

 such a flaw exists in all cases of Induction proper : (for Induction by mere enumeration can 

 hardly be called Induction.) I will, however, consider for a moment the instance of a cele- 

 brated proposition which has often been taken as an example of Induction, and in which the 

 number of particular cases is, or at least is at present supposed to be, limited. Kepler's 

 laws, for instance the law that the planets describe ellipses, may be regarded as examples of 

 Induction. The law was inferred, we will suppose, from an examination of the orbits of 

 Mars, Earth, Venus. And the syllogistic illustration which Aristotle gives, will, with the 

 necessary addition to it, stand thus, 



Mars, Earth, Venus describe ellipses. 



Mars, Earth, Venus are planets. 

 Assuming the convertibility of this last proposition, and its universality, (which is the necessary 

 addition in order to make Aristotle's syllogism valid) we say 



All the planets are as Mars, Earth, Venus. 

 Whence it follows that all the planets describe ellipses. 



If, instead of this assumed universality, the astronomer had made a real enumeration, and 

 had established the fact of each particular, he would be able to say 



Saturn, Jupiter, Mars, Earth, Venus, Mercury, describe ellipses. 



Saturn, Jupiter, Mars, Earth, Venus, Mercury are all the planets. 

 And he would obviously be entitled to convert the second proposition, and then to conclude 

 that 



All the planets describe ellipses. 

 But then, if this were given as an illustration of Induction by means of syllogism, we should 

 have to remark, in the first place, that the conclusion that "all the planets describe ellipses," 

 adds nothing to the major proposition, that "S., J., M., E., V., m , do so." It is merely the 

 same proposition expressed in other words, so long as S., J., M., E., V., m., are supposed to be 

 all the planets. And in the next place we have to make a remark which is more important ; 

 that the minor, in such an example, must generally be either a very precarious truth, or, as 

 appears in this case, a transitory error. For that the planets known at any time are all the 

 planets, must always be a doubtful assertion, liable to be overthrown to-night by an astronomical 

 observation. And the assertion, as received in Kepler's time, has been overthrown. For Saturn, 

 Jupiter, Mars, Earth, Venus, Mercury, are not all the planets. Not only have several new 

 ones been discovered at intervals, as Uranus, Ceres, Juno, Pallas, Vesta, but we have new ones 

 discovered every day; and any conclusion depending upon this premiss that A,B,C,D,E,F,G,H, 

 to Zare all the planets, is likely to be falsified in a few years by the discovery of J', B',C', &c. 

 If therefore, this were the syllogistic analysis of Induction, Kepler's discovery rested upon a 

 Vol. IX. Pakt I. 9 



