700 THE PRINCIPLES OF SCIENCE. [CHAP 



It is a peculiar property of equilateral triangles that they 

 are equiangular, and vice versd, it is a peculiar property of 

 equiangular triangles that they are equilateral. It is a 

 property of crystals of the regular system that they are 

 devoid of the power of double refraction, but this is not a 

 property peculiar to them, because liquids and gases are 

 devoid of the same property. 



An Accident, the fifth and last of the Predicables, is any 

 quality which may or may not belong to certain objects, 

 and which has no connexion with the classification adopted. 

 The particular size of a crystal does not in the slightest 

 degree affect the form of the crystal, nor does the manner 

 in which it is grouped with other crystals ; these, then, are 

 accidents as regards a crystallographic classification. With 

 respect to the chemical composition of a substance, again, 

 it is an accident whether the substance be crystallised or 

 not, or whether it be organised or not. As regards botan- 

 ical classification the absolute size of a plant is an accident. 

 Thus we see that a logical accident is any quality or cir- 

 cumstance which is not known to be correlated with those 

 qualities or circumstances forming the definition of the 

 species. 



The meanings of the Predicables can be clearly explained 

 by our symbols. Let A be any definite group of qualities 

 and B another quality or group of qualities ; then A will 

 constitute a genus, and AB, Ab will be species of it, B 

 being the difference. Let C, D and E be other qualities 

 or groups of qualities, and on examining the combinations 

 in which A, B, C, D, E occur let them be as follows : 

 ABODE AbCdE 



ABCVe AbVde. 



Here we see that wherever A is we also find C, so that 

 C is a generic property; D occurs always with B, so that it 

 constitutes a specific property, while E is indifferently 

 present and absent, so as not to be related to any other 

 letter ; it represents, therefore, an accident. It will now be 

 seen that the Logical Alphabet represents an interminable 

 series of subordinate genera and species ; it is but a concise 

 symbolic statement of what was involved in the ancient 

 doctrine of the Predicables. 



