DEFINITION AND PUOVINCE OF LOGIC. 7 



ing, acquire empirically in the course of their studies. Men judged of 

 evidence, and often very coiTectly, before logic was a science, or they 

 nevei' could have made it one. And they executed great mechanical 

 works before they understood the laws of mechanics. But there are 

 limits both to what mechanicians can do without principles of mechan- 

 ics, and to what thinlcers can do without principles of logic. And the 

 limits, in the two cases, are of the same kind. The extent of what 

 man can do without understanding the theoiy of what he is doing, is 

 in all cases much the same : he can do whatever is very easy; what 

 requires only time, and patient industry. But in the progress of 

 science fi-om its easiest to its more difficult problems, every gi-eat step 

 in advance has had either as its precursor or as its accompaniment and 

 necessary condition, a coiTosponding improvement in the notions and 

 principles of logic received among the most advanced thinkers. And 

 if several of the more difficult sciences are still in so defective a state ; 

 if not only so little is proved, but disputation has not terminated even 

 about the little which seemed to be so ; the reason, perhaps, is, that 

 men's logical notions have not yet acquired the degree of extension, 

 or of accuracy, requisite for the estimation of the evidence proper to 

 those particular departments of knowledge. 



§ 7. Logic, then, is the science of the operations of the understand- 

 ing which are subservient to the estimation of evidence : both the 

 process itself of proceeding from known truths to unknown, and all 

 intellectual operations auxiliary to this. It includes, therefore, the 

 operation of Naming ; for language is an instrument of thought, as 

 well as a means of communicating our thoughts. It includes, also, 

 Definition, and Classification. For, the use of these operations (putting 

 all other minds than one's own out of consideration) is to serve not 

 only for keeping our evidences and the conclusions fi'om them perma- 

 nent and readily accessible in the memory, but for so marshaling the 

 facts which we may at any time be engaged in investigating, as to 

 enable us to perceive more clearly what evidence there is, and to judge 

 with fewer chances of eiTor whether it be sufficient. The analysis of 

 the instruments we employ in the investigation of truth, is part of the 

 analysis of the investigation itself; since no art is complete, unless 

 another art, that of constructing the tools and fitting them for the 

 purposes of the art, is embodied in it. 



Our object, therefore, will be to attempt a correct analysis of the 

 intellectual process called Reasoning or Inference, and of such other 

 mental operations as are intended to facilitate this : as well as, on the 

 foundation of this analysis, and pari passu with it, to bring together or 

 frame a set of rules or canons for testing the sufficiency of any given 

 evidence to prove any given proposition. 



With respect to the first part of this undertaking, I do not attempt 

 to decompose the mental operation^ in qne-stion into their ultimate 

 elements. It is enough if the analysis as far as it goes is coiTect, and 

 if it goes far enough for the practical purposes- of logic considered as 

 an art. The separation of a complicated phenomenon into its compo- 

 nent parts, is not like a connected and interdependent chain of proof. 

 If one link of an argument breaks, the whole drops to the ground ; but 

 one step towards an analysis holds good, and has an independent value, 

 though we should never be able to make a second. The results of 



