15* THE SCIENCE OF LOGIC 



the supposition that it is so is an hypothesis for verification. 

 Or we may put the matter in this way : Ordinary observation 

 discloses relations of uniform concomitance or sequence between 

 phenomena. In these uniformities science endeavours to detect 

 reciprocal causal relations (221). We know a phenomenon or 

 fact scientifically only when we know its connexion with, and its 

 dependence on, all that constitutes its one sufficient and indispens 

 able cause ; and when we place this cause and this fact in the 

 relation of antecedent and consequent, or of subject and predicate, 

 we know that the relation is not only universal but reciprocal ; 

 for example, not only that &quot; all living organisms are mortal &quot; 

 but that &quot; all mortal things are living organisms &quot;. But in order 

 to establish such a reciprocal relation we must have made explicit 

 the one essential ground of the consequent in question ; we must, 

 in other words, be certain that it can follow from this antecedent 

 and from no other. &quot; If man is a living organism he is mortal ; 

 and if man is mortal he is a living organism &quot; : because mortality 

 is a proprium of organic life, a property in the strict sense of 

 the word, &quot; quod convenit omni, soli, semper, et ubique &quot;. &quot; If a 

 triangle is right-angled the middle point of the hypotenuse is 

 equidistant from the three vertices, and vice versa &quot; : because 

 right-angled triangles alone are inscribable in semicircles. From 

 all this we see that the very observations which give rise to the 

 enunciation of universal relations whether categorically, All S s 

 are P, or hypothetically, If S is M it is P suggest the hypothesis 

 that these judgments may, perhaps, be in reality simply convertible, 

 although formally they can be converted only per accidens : into 

 Some Ps are S (which is equivalent to All Ps may be 5), and 

 IfS is P it may be M. 



(2) We have already seen (217) that hypotheses may be sug 

 gested by Enumerative Induction. Even a single observed in 

 stance of a phenomenon may set one speculating or guessing as 

 to its cause. But the suggestion comes more easily when we 

 have observed a number of instances of the same coexistence or 

 sequence of two phenomena, particularly if this persists through 

 varying circumstances. 1 The sole scientific value of enumerative 

 induction lies in its suggestion of an hypothesis as to the content 

 or nature of the instances examined. For example, the obser 

 vation of the facts that I + 3 = 2 2 , I + 3 + 5 = 3 2 , I + 3 + 5 

 7 =4 2 , and so on, suggests the hypothesis of some necessary 

 1 As in the &quot; method of agreement &quot; (241). 



