20 INTRODUCTION. 



exist ; the latter, from the premises laid down in books of geometry, under 

 the title of definitions and axioms. 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 from these. 



With the original data, or ultimate premises 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 conceive the science, nothing to do. These ques- 

 tions are partly not a subject of science at all, partly that of a very differ- 

 ent science. 



Whatever is known to us by consciousness is known beyond possibility 

 of question. What one sees or feels, whether bodily or mentally, one can 

 not but be sure that one sees or feels. No science is required for the pur- 

 pose of establishing such truths ; 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. A truth, 

 or supposed truth, which is really the result of a very rapid inference, may 

 seem to be apprehended intuitively. It has long been agreed by thinkers of 

 the most opposite schools, that this mistake is actually made in so familiar 

 an instance as that of the eyesight. There is nothing of which we appear 

 to ourselves to be more directly conscious 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 degrees of faintness of color; that our estimate of the object's 

 distance from us is the result partly of a rapid inference from the muscular 

 sensations accompanying the adjustment of the focal distance of the eye to 

 objects unequally remote from us, and partly of a comparison (made with 

 so much rapidity that 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 evidence. The per- 

 ception of distance by the eye, which seems so like intuition, is thus, in re- 

 ality, 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 pei'ceptions of color.* 



Of the science, therefore, which expounds the operations of the human 

 understanding in the pursuit of truth, one essential part is the inquiry: 

 What are the facts M'hich are the objects of intuition or consciousness, and 

 what are those which we merely infer ? But this inquiry has never been 

 considered a portion of logic. Its place is in another and a perfectly dis- 

 tinct department of science, to which the name metaphysics more particu- 

 larly belongs : that portion of mental philosophy which attempts to deter- 

 mine what part of the fui-niture of the mind belongs to it originally, and 



* This important theory has of late been called in question by a writer of deserved reputa- 

 tion, Mr. Samuel Bailey ; but I do not conceive that the grounds on which it has been ad- 

 mitted as an established doctrine for a centuiy past, have been at all shaken by that gentle- 

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

 ments. {Westminster Review for October. 1842; reprinted in "Dissertations and Discus- 

 sions," vol. ii.) 



