DEDUCTION 53 



mental evolution, to the use of which we are continually driven by 

 the most imperious of our instincts. 1 Lacking it we should be 

 imbeciles of that not very uncommon kind that has a considerable 

 memory for facts but cannot learn to link them together except in 

 a very rudimentary way. The only people by whom it is not 

 habitually used are actual idiots and imbeciles of a low type. I 

 use the words advisedly and literally in reference to phenomena 

 which may easily be observed in reference to idiots and imbeciles 

 who are medically certified and placed under restraint as such. 2 

 One of the main causes of the mental inferiority of the lower 

 animals is that, like imbeciles, they are unable to employ it. 

 Scientific deduction, like scientific induction, differs from our 

 common use of the process only in the greater precautions we 

 take to make it accurate. It would be strange if a process, so 

 useful and necessary in the ordinary affairs of life, should become 

 valueless and dangerous the moment we endeavoured to use it with 

 care, and if men, to become scientific, must cease to be rational. 



83. All the foregoing is, I take it, quite indisputably true. At 

 any rate, all that I have said about the right method of scientific 

 inquiry is indisputably true. No one will maintain that facts 

 gathered in one way are on that account more valuable than 

 equally authenticated facts gathered in another way, or that it is a 

 rational proceeding to found untested judgments on fragmentary 

 evidence when completer evidence, by which the judgments may 

 be tested, is available to use, for example, only data gathered by 

 one laboratory method (e.g. experiment) when relevant data 

 gathered by another laboratory method (e.g. biometry) or by 

 simple observation is at hand. It may be said that no one has 

 ever maintained that authentic and relevant facts should be 

 ignored or that hypotheses should be left untested. No one, 

 indeed, would venture to do so openly and formally. But theory 

 and practice are not identical, and such astonishing dicta as " The 

 day of vague and inaccurate thinking is past; we must, like physicists 

 and chemists, rely on experiment," " The problems of heredity 

 can only be solved in the laboratory and breeding-pen," are not un- 

 common, and have meant in practice, as we shall see in numerous 

 instances, nothing other than an ignoring of relevant data combined 

 with a neglect to test thinking. Untested hypotheses hypotheses 

 which are merely founded on laboratory work, but are taken as 

 proved by it are, in fact, exceedingly common in biology. 3 At 



1 See 666. 2 See 762 et seq. 



3 See, for example, chapters iv. to x. 



