6 INTRODUCTION. 



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 know- 

 ledge ; 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 con- 

 ceive the science, nothing to do. These questions are partly 

 not a subject of science at all, partly that of a very different 

 science. 



Whatever is known to us by consciousness, is known be- 

 yond possibility of question. What one sees or feels, whether 

 bodily or mentally, one cannot but be sure that one sees or 

 feels. No science is required for the purpose 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 intui- 

 tively. It has long been agreed by thinkers of the most oppo- 

 site 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 coloured surface ; that when we fancy we see 

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

 and degrees of faintness of colour; 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 

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

 and colour 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 perception of distance by the 



