ii6 THE POPULAR SCIENCE MONTHLY 



portant to regard each invented hypothesis as an elastic conception, 

 whose form may be changed as necessity demands. The investigator 

 must even return to his field of observation and reexamine the facts, 

 particularly such as do not match with the well defined consequences of 

 a partly successful hypothesis; and he must search his field with the 

 sharpest scrutiny to see if any facts, previously unnoticed, really do 

 occur in the manner indicated by unmatched consequences. In every 

 way, the utmost care must be taken not to allow oneself to be satisfied 

 with imperfect or incomplete agreements. 



If these various recommendations are carefully carried out, the 

 danger, often feared, that an investigator may, Procrustes-like, force 

 the facts to fit the needs of a favorite hypothesis, is practically ruled 

 out; for if the investigator has several unlike hypotheses in mind, and 

 has deduced several unlike series of consequences from them, it will 

 evidently be impossible for him to force his facts to agree with all of 

 them, however much the facts may be trimmed or stretched. 



Irregular Order of Procedure. — In practise the several processes 

 that have been necessarily considered in systematic succession, are car- 

 ried on in a much more irregular fashion. It has already been pointed 

 out that invention may advisedly go hand in hand with observation. 

 It is evident that, after a hypothesis has been invented, any time 

 spared from observation may be devoted to deduction; and it often 

 happens that the consequences of the hypothesis may grow to a greater 

 number than that of the classes of observed facts then accumulated. 

 Confrontation and comparison may be made repeatedly as observation 

 advances, and revision is always in order the moment there seems to be 

 occasion for it. The active-minded investigator, thus continually re- 

 viewing the different aspects of his problem, may gradually come to 

 feel that one hypothesis, modified as far as needs be from its original 

 form, appears to deserve greater acceptance than any of its rivals ; then 

 arises the great question: Is this hypothesis really true? Is it surely 

 a correct counterpart of the invisible facts of the past? Clearly it is 

 essential that an investigator, on reaching this stage in his work, 

 should fully understand the nature of scientific proof. 



Final Judgment. — It is at this advanced stage of an investigation 

 that the exercise of a sound judgment is needed, in order to estimate 

 the measure of confidence that may be given to an apparently success- 

 ful hypothesis. The most important point to emphasize now is that, in 

 such problems as we are here dealing with, the only available method 

 of testing the truth of any hypothesis is to measure the agreement of its 

 deduced consequences with the appropriate facts of observation. In this 

 respect scientific proof is altogether unlike geometrical proof, in which 

 the correctness of a theorem is never tested by its agreement with ob- 

 servable facts, but only by the continuity with which successively de- 



