THE TESTING OF HYPOTHESES 41 



71. It is obligatory that we make this deductive appeal to reality. 

 When it is done, certain facts all we find it possible to employ in 

 this way are used as the basis of the hypothesis. Next other facts 

 all the rest which are relevant and known to us are used, in as 

 many ways as possible, to test the truth of the hypothesis. The 

 latter class of facts are usually much the more numerous. In brief, 

 we found the hypothesis on certain relevant facts and prove it by 

 means of the rest. In this way only, especially when we are 

 engaged in the elucidation of some obscure and difficult matter, is 

 it possible to use all the facts. If we do not proceed thus first 

 formulating our hypothesis, next making a rigorous deductive 

 inference of consequences, and lastly appealing to reality for con- 

 firmation our thinking is mere guessing. A hypothesis is quite 

 valueless to science unless it permits a deductive appeal to reality, 

 and quite unproved until it has been found to stand that test. 

 "The sole condition to which we need conform in framing any 

 hypothesis is that we both have and exercise the power of inferring 

 deductively from the hypothesis to the particular results, which 

 are to be compared to known facts." 1 No scientific worker, that 

 was not a mere observer, has ever done great work who did not so 

 test his hypotheses ; no hypothesis has ever been permanently 

 included in science but has been so tested. " If it be an advantage 

 for the discoverer of truth that he be ingenious and fertile in 

 inventing hypotheses which may connect the phenomena of nature, it 

 is indispensably requisite that he be diligent and careful in com- 

 paring his hypotheses with the facts, and ready to abandon his 

 invention as soon as it appears that it does not agree with the 

 course of actual occurrences. This constant comparison of his 

 own conceptions and supposition with observed facts under all 

 aspects forms the leading employment of the discoverer; this 

 candid and simple love of truth, which makes him willing to 

 suppress the most favourite production of his own ingenuity as soon 

 as it appears to be at variance with realities, constitutes the first 

 characteristic of his temper. He must have neither the blindness 

 which cannot, nor the obstinacy which will not, perceive the 

 discrepancy of his fancies and his facts. He must allow no indol- 

 ence, or partial views, or self-complacency, or delight in seeming 

 demonstration, to make him tenacious of the schemes which he 

 devises, any further than they are confirmed by their accordance 

 with nature. The framing of hypotheses is, for the inquirer after 

 truth, not the end, but the beginning of his work. Each of his 

 1 Jevons, Principles of Science, p. 265. 



