DEFINITION. 107 



extremity of a straight line revolving z/; a plane around the other 

 extremity fixed&quot; . &quot;A cusp is a curve traced by some fixed point in 

 a circle travelling along a straight line? &quot;A ring is the solid 

 figure obtained by allowing a circle to revolve around a fixed axis 

 in its own plane but outside it&quot; In definitions of this class, it is 

 fairly obvious that the reference is to the thing rather than to 

 the name. 



Again, it is evident from the nature of a property in the strict 

 sense (47) that an object may be identified by assigning a pro 

 perty, though perhaps not always so easily x as by assigning the 

 differentia. A statement which enables us to identify an object by 

 assigning one or more of its properties (instead of its differentia], is 

 called a Distinctive Explanation. For example, &quot; Man is an 

 animal endowed with the faculty of articulate speech&quot; ; or &quot;Man 

 is a biped who cooks his food&quot; . Such &quot; definitions &quot; are most com 

 monly met with in the physical and natural sciences chemistry, 

 botany, zoology, etc. their purpose being to identify individuals 

 by the possession of certain natural properties. 



Were it the sole aim of definition to secure unfailing accuracy in the 

 identification of the objects defined, any of the devices now under consideration 

 would suit as well as strict definition per genus et differentiam; and our 

 ultimate test of the adequacy of any such device would be &quot; to try whether by 

 conceivable variation of circumstances we can cause it to break down. . . . 

 Thus to recur to a very venerable old logical joke, when it was proposed to 

 define &quot; man &quot; as &quot; a featherless biped,&quot; a plucked cock was exhibited by way 

 of confutation &quot;. a 



We may also identify an object by assigning, instead of its 

 differentia, one or more of its inseparable accidents, or such a col 

 lection of its separable accidents as is found in that object alone. 

 This process is called Description. It is the only way we have of 

 defining individuals. Though of great practical utility, the de 

 vice is a precarious one, for it is liable to be vitiated by change 

 of circumstances. Thus, we may describe &quot; midday&quot; as &quot;the 

 time when the sun is due south,&quot; but the description will be 

 correct only in the northern hemisphere. 



The process sometimes called Physical Definition or Physical 

 Division or Physical Partition is nothing more than the description 

 of a physical object by an enumeration of the parts of which this 

 is composed. For example: &quot;A ship is an object composed of 



1 C/. JOSEPH, op. cit., pp. 78, 83 sqq. 2 VENN, op. cit., p. 304. 



