92 THE SCIENCE OF LOGIC. 



individuals, of grouping or classifying the classes in question 

 under the wider class of &quot; living things &quot;. 



&quot;There are two contrasting ways,&quot; writes Mr. Joseph, 1 &quot; in which we 

 may attempt to construct a definition. We may take instances of that which 

 is to be defined, and try to detect what they have in common, which makes 

 them instances of one kind, and on the strength of which we call them by the 

 same name. This is the { inductive method. 2 We might thus define 

 * snob, comparing those of our acquaintance to whom we could apply the 

 name, or those whom Thackeray has drawn for us ; and if we thought that 

 among all their differences they agreed in prizing rank or wealth above char 

 acter, we might accept that as our definition. The other method is that of 

 dichotomy [60], and in this we try to reach our definition rather by working 

 downwards from a genus, than upwards from examples. Some genus is taken 

 to which the subject we wish to define belongs. This genus we divide into 

 what possesses and what does not possess a certain differentia. The differ 

 entia taken must be something predicable of the subject to be defined ; and 

 if genus and differentia together are already commensurate with that subject, 

 the definition is reached ; if they form only a subaltern genus predicable of it, 

 this subaltern genus must be again divided in the same way : until we reach 

 a commensurate notion. At every stage of our division the differentia taken 

 must if possible be a modification of the differentia next before it, it must at 

 least be capable of combining with those that have preceded it in the con 

 struction of one concept in such a way that we are throughout specifying the 

 general notion with which we started [62] ; and there should be as many steps 

 of division as there are stages which our thought recognizes as important in 

 the specification of this concept. At every stage also we precede by dicho 

 tomy because we are only interested in the line that leads to the subject we 

 are defining ; all else contained within the genus we thrust aside altogether, 

 as what does not exhibit the differentia characterizing that subject.&quot; Thus, 

 to revert to Aristotle s example, we work downwards to the definition of 

 &quot; three &quot; from the genus &quot; number &quot; by dividing the latter into &quot; odd &quot; and 

 &quot;not-odd,&quot; and then dividing &quot;odd&quot; into &quot;prime&quot; and &quot;not-prime&quot;. 

 Porphyry s tree, leading from the summum genus, &quot; substance,&quot; to the defini 

 tion of &quot; man,&quot; is another example of this method. 



But in ultimate analysis the two methods are not fundamentally different : 

 both alike involve comparison, division, classification. When, for instance, 

 by the inductive method, we take examples of &quot; snobs &quot; and look for that 

 which &quot; differentiates &quot; them, we are really trying to divide the genus 

 &quot; man &quot; into snobs and people who are not snobs. &quot; Let us take the prizing 

 of rank or wealth ; if that by itself does not constitute a snob, we need some 

 further differentia, to distinguish snobs from other men who prize rank or 

 wealth ; say they are distinguished by prizing these beyond character ; we 

 then have a definition of a snob, but in getting it, we have taken note of a 

 wider class of men within which they are included.&quot; 3 



50. NATURE OF DEFINITION : PER GENUS ET DIFFEREN- 



l op. cit., pp. in, 112. 



2 It was extensively employed and illustrated by Socrates and Plato. 



3 ibid., p. 114. 



