8 THE PRINCIPLES OF SCIENCE. [CHAP. 



the conditions of thought impressed upon it. Before a 

 mistake can be committed, the mind must clearly dis- 

 tinguish the mistaken conclusion from all other assertions. 

 Are not the Laws of Identity and Difference the prior 

 conditions of all consciousness and all existence ? Must 

 they not hold true, alike of 'things material and immaterial? 

 and if so, can we say that they are only subjectively true 

 or objectively true? I am inclined, in short, to regard 

 them as true both " in the nature of thought and things," 

 as I expressed it in my first logical essay ; l and I hold 

 that they belong to the common basis of all existence. 

 But this is one of the most difficult questions of psychology 

 and metaphysics which can be raised, and it is hardly one 

 for the logician to decide. As the mathematician does not 

 inquire into the nature of unity and plurality, but develops 

 the formal laws of plurality, so the logician, as I conceive, 

 must assume the truth of the Laws of Identity and 

 Difference, and occupy himself in developing the variety 

 of forms of reasoning in which their truth may be 

 manifested. 



Again, I need hardly dwell upon the question whether 

 logic treats of language, notions, or things. As reasonably 

 might we debate whether a mathematician treats of 

 symbols, quantities, or things. A mathematician certainly 

 does treat of symbols, but only as the instruments 

 whereby to facilitate his reasoning concerning quantities ; 

 and as the axioms and rules of mathematical science must 

 be verified in concrete objects in order that the calcula- 

 tions 'founded upon them may have any validity or utility, 

 it follows that the ultimate objects of mathematical science 

 are the things themselves. In like manner I conceive that 

 the logician treats of language so far as it is essential for the 

 embodiment and exhibition of thought. Even if reasoning 

 can take place in the inner consciousness of man without 

 the use of any signs, which is doubtful, at any rate it 

 cannot become the subject of discussion until by some 

 system of material signs it is manifested to other persons. 

 The logician then uses words and symbols as instruments 

 of reasoning, and leaves the nature and peculiarities of 

 language to the grammarian. But signs again must 



1 Pure Logic, or the Logic of Quility apart from Quantity, 1864, 

 pp. 10, 1 6, 22, 29, 36, &c. 



