168 FhilosuphijoJ the Human Mind, [Chap. XIL 



jiate ideas, and that they are all derived from two 

 sources, .sensation and inflection, Des Cartes siip- 

 ])Osed that the essence of mind consists in thoiighty 

 and that of matter in extension; while Locke be- 

 lieved that tiie real essence of both is beyond the 

 reach of human knowledge. The British philoso- 

 pher exj)lainc(l more distinctly than any one had 

 done before him the operations of the mind in 

 classing the various objects of thought, and re- 

 ducing them to genera and species. He was the 

 lirst who distinguished in substances what he calls 

 the nominal essence^ or that generic character^ and 

 specific difference, which may be expressed by a 

 definition from the real essence, or internal con- 

 stitution, wliich he supposed could not be known ; 

 and who, by means of this distinction, pointed out 

 the wav of brino'intr to an issue tho^e subtle dis- 

 putes, particularly the controversy between the 

 Nominalists and Realists, which had puzzled the 

 schoolmen for ages. He showed, more satisfacto- 

 rily than j)receding inquirers, how we may form ab- 

 stract and general notions, and the use and neces- 

 sity of them in reasoning. He first expressed the 

 distinction between primary tiudsecondanj qualities, 

 thougli tlie iticas implied in this distinction seem 

 to have been in some measure understood by des 

 Cartes. And, finally, Mr. Locke had much merit 

 l)cculiai- to himself, in exhibiting the ambiguity of 

 . Nvords, and by this means solving many dilKcult 

 questions wliich had tortured the wits of former 

 metaphysicians *. 



* See Esauj,' on the Huivatt Undcrstonding, passim j and Reid's 

 Khsap on the ZntaU^ctuui toxiicm of Man, vol. i, Essay 2, chap. ix. 



