were, the inference would cease to be and fact would exist in its 

 stead. In the case of the ultimate inferences of science, we act upon 

 them with great confidence, not because they cannot be shown cer- 

 tainly to be true or false, but because experiment has so far strength- 

 ened the inference of their validity. Between the spheres of fact and 

 pure assumption or convention, lies the scale of inferences more and 

 less certain; and because a theory is not a certainty, we cannot con- 

 clude therefore that it is a mere convention: it may be an inference to 

 a greater or less extent verifiable. If I am justified in considering the 

 law of inertia as typical of ultimate scientific inferences, we can con- 

 clude that the so-called a priori truths or conventions are inferences 

 based upon and to a great extent verifiable in experience. 



We conclude then, from our examination of the verifiability of 

 theories, that theories are verifiable in so far as the grounds upon 

 which their inferences are based are subject to experimental investiga- 

 tion ; and it matters not what the problem involved in the theory may 

 be, or what its method of procedure in solution. 



From the fact that verification is defined to be the showing of the 

 truth of a theory, the relation between verification and believability of 

 theories is manifest. We have already discovered in our study that 

 those theories for which there is the most experimental evidence, which 

 are most verified; Darwin's theory, The Theory of Ions, and the 

 theory of Inorganic Evolution, are the most believable; while those 

 theories which were not founded upon experiment were not verified and 

 were difficult of verification, were the least believable theories. Be- 

 lieva'bility increases in proportion as verification approaches complete- 

 ness. 



MEANING OF TRUTH OF A THEORY. 



We defined verification to be the showing of the truth of a theory. 

 We shall consider now what is meant by the truth or falsity of a hypo- 

 thesis. It has been considered, on one side, that the propofitions 

 which a theory asserts are either true or false from the moment they 

 are asserted; while, on the other side, it is conceived that theories are 

 not, when proposed, true or false, but are made so by the success or 

 failure of verification. The former view is that verification shows the 

 truth of a theory; the latter that verification makes the truth of a 

 theory. According to one view, at the time Darwin first affirmed 

 that species originated by natural selection, that theory was either 

 true or false, even though entirely unconfirmed or unrefuted; that 

 the conditions of the origin of species existed at that time and either 

 did or did not correspond with Darwin's Statement. While, accord- 

 ing to the second conception of truth, Darwin's statement before it 



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