INTRODUCTION. 



Examples of truths known to us by immediate consciousness, are 

 our own bodily sensations and mental feelings. I know directly, and 

 of my own knowledge, that I was vexed yesterday, or that I am hun- 

 gry to-day. Examples of truths which we know only by way of 

 inference, are occun-ences which took place while we were absent, the 

 events recorded in history, or the theorems of mathematics. The two 

 fonner we infer from the testimony adduced, or from the traces of 

 those past occuiTences which still exist ; the latter, from the premisses 

 laid down in books of geometry, under the title of definitions and ax- 

 ioms. Whatever we are capable of knowing must belong to the one 

 class or to the other ; must be in the number of the primitive data, or 

 of the conclusions which can be drawn therefrom. 



With the original data, or ultimate premisses of our knowledge ; 

 with their number or nature, the mode in which they are obtained, or 

 the tests by which they may be distinguished; logic, in a direct way 

 at least, has, in the sense in which I coticeive the science, nothing to 

 do. These questions are partly not a subject of science at all, partly 

 that of a veiy different science. 



' Whatever is known to us by consciousness, is known beyond possi- 

 bility of question. What one sees, or feels, whether bodily or men- 

 tally, one cannot but be sure that one sees or feels. No science is 

 required for the purpose of establishing such ti-uths ; no rules of art 

 can render our knowledge of them more certain than it is in itself. 

 There is no logic for this portion of our knowledge. 



But we may fancy that we see or feel what we in reality infer. 

 Newton saw the truth of many propositions of geometry without read- 

 ing the demonstrations, but not, we may be sure, without their flashing 

 through his mind. A truth, or supposed truth, which is really the re- 

 sult of a very rapid inference, may seem to be apprehended intuitively. 

 It has long been agreed by philosophers of the most opposite schools, 

 that this mistake is actually made in so familiar an instance as that of 

 the eyesight. There is nothing which we appear to ourselves more 

 directly conscious of, than the distance of an object from us. Yet it 

 has long been ascertained, that what is perceived by the eye, is at most 

 nothing more than a variously colored surface ; that when we fancy 

 we see distance, all we really see is certain variations of apparent 

 size, and more or less faintness of color ; and that our estimate of the 

 object's distance fi'om us is the result of a comparison (made with so 

 much rapidity tliat we are unconscious of making it) between the size 

 and color of the object as they appear at the time, and the size and 

 color of the same or of similar objects as they appeared when close at 

 hand, or when their degree of remoteness was known by other evi- 

 dence. The perception of distance by the eye, which seems so like 

 intuition, is thus, in reality, an inference grounded on experience ; 

 an inference, too, which we learn to make ; and which we make with 

 more and more correctness as our experience increases; though in 

 familiar cases it takes place so rapidly as to appear exactly on a par 

 with those perceptions of sight which are really intuitive, our percep- 

 tions of color.* 



* This celebrated theorj- has recently been called in question by a writer of deserved 

 reputation, Mr. Samuel BaUey ; but I do not conceive that the grounds on which it has 

 been received by philosophers for a century past, have been at all shaken by that gentle- 

 man's objections. I have elsewhere said what appeared to me necessary in reply to his 

 arguments. — Westminster Review, for October, 1842. 



