59G THE PKINCIPLES OF SCIENCE. [CHAP. 



respects are similars in other respects. Thus it comes to 

 pass that a large part of the reasoning processes in which 

 scientific men are engaged, consists in detecting similarities 

 between objects, and then rudely assuming that the like 

 similarities will be detected in other cases. 



Distinction of Generalisation and Analogy. 



There is no distinction but that of degree between what 

 is known as reasoning by generalisation and reasoning by 

 analogy. In both cases from certain observed resemblances 

 we infer, with more or less probability, the existence of 

 other resemblances. In generalisation the resemblances 

 have great extension and usually little intension, whereas 

 in analogy we rely upon the great intension, the ex- 

 tension beiug of small amount (p. 26). If we find that the 

 qualities A and B are associated together in a great 

 many instances, and have never been found separate, it is 

 highly probable that on the next occasion when we meet 

 with A, B will also be present, and vice versa. Thus 

 wherever we meet with an object possessing gravity, it i,-* 

 found to possess inertia also, nor have we met with any 

 material objects possessing inertia without discovering that 

 they also possess gravity. The probability .has therefore 

 become very great, as indicated by the rules founded on 

 the Inverse Method of Probabilities (p. 257), that whenever 

 in the future we meet an object possessing either of the 

 properties of gravity and inertia, it will be found on 

 examination to possess the other of these .properties. 

 This is a clear instance of the employment of generalisation. 



In analogy, on the other hand, we reason from likeness 

 in many points to likeness in other points. The qualities 

 or points of resemblance are now numerous, not the 

 objects. At the poles of Mars are two white spots which 

 resemble in many respects the white regions of ice and 

 snow at the poles of the earth. There probably exist no 

 other similar objects with which to compare these, yet the 

 exactness of the resemblance enables us to infer, with high 

 probability, that the spots on Mars consist of ice and snow. 

 In short, many points of resemblance imply many more. 

 From the appearance and behaviour of those white spots 

 we infer that they have all the chemical and physical 



