~ 3~ THE SCIENCE OF LOGIC. 



&quot; some colour other than blue &quot;), unless on the tacit assumption that &quot; Noble 

 blood is either blue or of some other colour&quot;. Here, then, we have not 

 obversion at all, but a real inference which is rather mediate than immedi 

 ate, being in reality a mixed disjunctive or alternative syllogism (181) an 

 inference by which we &quot; pass from a determinate positive predicate to another 

 predicate less determinate but still positive &quot;- 1 



Similarly, if, in passing from &quot; S is P &quot; to &quot; S is not not-P &quot; we were to 

 understand not-P as giving us something more than the mere denial of /*, as 

 giving us some positive alternative to /*, we must know that this not-P is 

 incompatible with /*, before we may, by affirming one of the alternatives (P), 

 infer the denial of the other (not-P}. It is only when alternatives are mutu 

 ally exclusive that we can, by positing one, sublate the others (182) : &quot;be 

 cause we can predicate of a goose that it hisses, we are not precluded from 

 applying any predicate but hissing &quot;. 2 And when we are precluded our reason 

 ing is not obversion, but disjunctive inference based upon a suppressed alter 

 native premiss. &quot; This ink is black ; therefore it is not not-black, i.e., any colour 

 other than black, e.g. red.&quot; This is a mediate inference, implying the pre 

 miss &quot;The same thing cannot be both black and red &quot;. 



We have already referred (97, 98) to the inconvenience of recognizing 

 these obverse forms with &quot; indefinite &quot; or &quot; infinite &quot; terms for predicates, as 

 independent types of proposition. But they are not devoid of meaning ; and 

 they are valuable as steps towards more important inferences. 



Obverse forms are not alone among the forms arrived at by the processes 

 investigated in this chapter in being unusual, strained, and violent modes of 

 expressing truths that admit of straight and simple statement. But even such 

 forms, though not in common use, are valuable as aids to understanding the 

 full import and implications of the ordinary proposition. 



The process of Obversion has been called by many other names : 

 Permutation (Fowler, Ray, Stock, Joseph) ; dLquipollence (Ueberweg, 

 Bowen, Ray, the Scholastics) ; Infinitation (Bowen) ; Immediate Inference 

 by Privative Conception (Jevons) ; Contraversion (De Morgan) ; Con 

 traposition (Spalding). 



Professor Bain, who gave currency to the term obversion, distinguished 

 between formal obversion (described above) and a process which he called 

 material obversion. He gives as instances of the latter : &quot; Warmth is agree 

 able, therefore, Cold is -disagreeable&quot;; &quot;Knowledge is good, therefore, 

 Ignorance is bad,&quot; etc. It is incorrect to call these inferences. The second 

 proposition of each pair is suggested indeed by the first, but is by no means 

 inferred from it. It is guaranteed only by a quite independent examination 

 of the facts. Even if they were legitimate inferences, they would resemble 

 inversions more closely than obversions. 



1 1 8. CONVERSION is that process of immediate inference by 

 which from a given proposition we infer another having for its sub 

 ject the former predicate and for its predicate the former subject. 



The original proposition is called the convertend^ the inferred 

 proposition the converse. 



1 JOSKPH, op. cit., p. 221. *ibid., p. 222. 



