6 INTRODUCTION. 



to judge of evidence, and to act accordingly. They all have to ascer- 

 tain certain facts, in order that they may afterwards apply certain rules, 

 either devised by themselves, or prescribed for their gmdance by 

 others ; and as they do this well or ill, so they discharge well or ill the 

 duties of their several callings. It is the only occupation in which the 

 mind never ceases to be engaged; and is the subject, not of logic, but 

 of knowledge in general. Our definition of logic, therefore, will be in 

 danger of including the whole field of knowledge, unless we qualify it 

 by some further limitation, showing distinctly where the domain of the 

 other arts and sciences, and of common prudence ends, and that of 

 logic begins. 



The distinction is, that the science or knowledge of the particular 

 subject-matter fmiiishes the evidence, while logic furnishes the prin- 

 cij)les and rules of the estimation of evidence. Logic does not pre- 

 tend to teach the surgeon what are the symptoms which indicate a 

 violent death. This he must learn fi-om his own experience and obser- 

 vation, or from that of others, his predecessors in his peculiar science. 

 But logic sits in judgment on the sufficiency of that observation and 

 experience to justify his rules, and on the sufficiency of his rules to 

 justify his conduct. It does not give him proofs, but teaches him what 

 makes them proofs, and how he is to judge of them. Logic alone can 

 never show that the fact A proves the fact B ; but it can point out to 

 what conditions all facts must conform, in order that they may prove 

 other facts. To decide whether any given fact fulfils these conditions, 

 or whether facts can be found which fulfil them in any given case, 

 belongs, exclusively, to the particular art or science, or to ovir knowl- 

 edge of the particular subject. 



It is in this sense that logic is, what Bacon so expressively calls it, 

 ars artmm ; the science of science itself. All science consists of data 

 and conclusions from those data — of proofs, and what they prove : now, 

 logic points out what relations must subsist between data and what- 

 ever can be concluded from them — between proof and everything 

 which it can prove. If there be any such indispensable relations, and 

 if these can be precisely determined, every particular branch of science, 

 as well as every individual in the guidance of his conduct, is bound to 

 conform to those relations, under the penalty of making false infer- 

 ences, of drawing conclusions which are not gi'ounded in the realities 

 of things. Wliatever has at any time been concluded justly, whatever 

 knowledge has been acquired otherwise than by immediate intuition, 

 depended upon the observance of the laws which it is the province of 

 logic to investigate. If the conclusions are just, and the knowledge 

 sound, those laws have actually been observed. 



§ 6. "We need not, therefore, seek any further for a solution of the 

 question, so often agitated, respecting the utility of logic. If a science 

 of logic exists, or is capable of existing, it must be useful. If there be 

 rules to which every mind conforms in every instance in which it 

 judges rightly, there seems little necessity for discussing whether a 

 person is more likely to observe those rules, when he knows the rules, 

 than when he is unacquainted with them. 



A science may imdoubtedly be brought to a certain, not inconsider- 

 able, stage of advancement, without the application of any other logic 

 to it than what all persons, who are said to have a sound understand- 



