510 THE PRINCIPLES OF SCIENCE. [chap. 



employ analogy, as I shall point out, to guide him in the 

 choice of hypotheses. The manifold connections between 

 one science and another give him clues to the kind of laws 

 to be expected, and out of the infinite number of possible 

 hypotheses he selects those which are, as far as can be 

 foreseen at the moment, most probable. Each experiment, 

 therefore, which he performs is that most likely to throw 

 light upon his subject, and even if it frustrate his first 

 views, it tends to put him in possession of the correct 

 clue. 



Requisites of a good Hypothesis. 



There is little difficulty in pointing out to what condi- 

 tion an hypothesis must conform in order to be accepted 

 as probable and valid. That condition, as I conceive, is 

 the single one of enabling us to infer the existence of 

 phenomena which occur in our experience. Agreement 

 with fact is the sole and sufficient test of a true hypothesis. 



Hobbes has named two conditions which he considers 

 requisite in an hypothesis, namely (i) That it should be 

 conceivable and not absurd ; (2) That it should allow of 

 phenomena being necessarily inferred. Boyle, in noticing 

 Hobbes' views, proposed to add a third condition, to the 

 effect that the hypothesis should not be inconsistent with 

 any other truth on phenomenon of nature.^ I think that 

 of these three conditions, the first cannot be accepted, 

 unless by inconceivable and absurd we mean self-cuntra- 

 dictory or inconsistent with the laws of thought and 

 nature. I shall have to point out that some satisfactory 

 tlieories involve suppositions which are wholly inconceiv- 

 able in a certain sense of the word, because the mind can- 

 not sufficiently extend its ideas to frame a notion of the 

 actions supposed to take place. That the force of gravity 

 should act instantaneously between the most distant parts 

 of tlie planetary system, or that a ray of violet light 

 should consist of about 700 billions of vibrations in a 

 second, are statements of an inconceivable and absurd 

 character in one sense ; but they are so far from being 

 op])osed to fact that we cannot on any other suppositions 

 account for plienomena observed. But if an hypothesis 

 involve self-contradiction, or is inconsistent with known 



' Boyle's rhysical Examen, p. 84. 



