206 THE PRINCIPLES OF SCIENCE 



tial and provisional tiTiths which are necessary to us, as steps on which 

 we rest, so as to go on with investigation; . . . 



Given faith in the possibility of progress toward superior knowledge, 

 we can aflFord to indulge that scepticism of present knowledge which 

 holds even our best established theories potentially open to review. 

 This is the sense of Thomson's dictum that a theory is "a policy rather 

 than a creed," and the substance of a delightfully matter-of-fact 

 statement by Lewis. 



The scientist is a practical man and his are practical [i.e., practi- 

 cally attainable] aims. He does not seek the ultimate but the proxi- 

 mate. He does not speak of the last analysis but rather of the next 

 approximation. . . . On the whole, he is satisfied with his work, for 

 while science may never be wholly right it certainly is never wholly 

 wrong; and it seems to be improving from decade to decade. 



THE MUTUAL CONTROL OF FACTS AND THEORIES 



Science is basically empirical, and all scientists share Newton's opin- 

 ion that: 



We are certainly not to relinquish the evidence of experiments for the 

 sake of dreams and vain fictions of our own devising; . . . 



But in the face of what evidence are "true propositions" to be relin- 

 quished as "dreams and vain fictions"? Generally the evidence must 

 be overwhelming. For always it is we who must interpret "the evi- 

 dence of experiments," and always we do so in the uncertain light 

 shed by those "older truths" of which James says: 



Their influence is absolutely controlling. Loyalty to them is the first 

 principle— in most cases it is the only principle; for by far the most 

 usual way of handling phenomena so novel that they would make 

 for a serious rearrangement of our preconception is to ignore them 

 altogether, or to abuse those who bear witness for them. 



Just so Galileo was abused for his telescopic observations. Just so 

 we ignore the repeated appearances of microorganisms in media 

 earlier supposed to have been sterile, and thus maintain the nonoc- 

 currence of spontaneous generation. For us these appearances are 

 not the "evidence of experiments" but only the evidence of experi- 

 mental errors. However temptingly simple may be the identification 

 of science with the policy of systematic doubt, we must recognize 



