4 THE POPULAR SCIENCE MONTHLY. 



That which determines us, from given premises, to draw one in- 

 ference rather than another, is some habit of mind, whether it be con- 

 stitutional or acquired. The habit is good or otherwise, according as 

 it produces true conclusions from true premises or not; and an infer- 

 ence is regarded as valid or not, without reference to the truth or fal- 

 sity of its conclusion specially, but according as the habit which 

 determines it is such as to produce true conclusions in general or not. 

 The particular habit of mind which governs this or that inference 

 may be formulated in a proposition whose truth depends on the va- 

 lidity of the inferences which the habit determines ; and such a for- 

 mula is called a guiding principle of inference. Suppose, for example, 

 that we observe that a rotating disk of copper quickly comes to rest 

 when placed between the poles of a magnet, and we infer that this 

 will happen with every disk of copper. The guiding principle is, that 

 what is true of one piece of copper is true of another. Such a guid- 

 ing principle with regard to copper would be much safer than with 

 regard to many other substances brass, for example. 



A book might be written to signalize all the most important of 

 these guiding principles of reasoning. It would probably be, we 

 must confess, of no service to a person whose thought is directed 

 wholly to practical subjects, and whose activity moves along thor- 

 oughly-beaten paths. The problems which present themselves to 

 such a mind are matters of routine which he has learned once for 

 all to handle in learning his business. But let a man venture into an 

 unfamiliar field, or where his results are not continually checked by 

 experience, and all history shows that the most masculine intellect 

 will ofttimes lose his orientation and waste his efforts in directions 

 which bring him no nearer to his goal, or even carry him entirely 

 astray. He is like a ship in the open sea, with no one on board who 

 understands the rules of navigation. And in such a case some gen- 

 eral study of the guiding principles of reasoning would be sure to be 

 found useful. 



The subject could hardly be treated, however, without being first 

 limited ; since almost any fact may serve as a guiding principle. 

 But it so happens that there exists a division among facts, such that 

 in one class are all those which are absolutely essential as guiding 

 principles, while in the others are all which have any other interest as 

 objects of research. This division is between those which are neces- 

 sarily taken for granted in asking whether a certain conclusion fol- 

 lows from certain premises, and those which are not implied in that 

 question. A moment's thought will show that a variety of facts are 

 already assumed when the logical question is first asked. It is im- 

 plied, for instance, that there are such states of mind as doubt and 

 belief that a passage from one to the other is possible, the object of 

 thought remaining the same, and that this transition is subject to 

 some rules which all minds are alike bound by. As these are facts 



