272 RECENT PROGRESS OF THE THEORY OF VISION. 



minor proposition, and the ' conclusion ' is that the object I am 

 looking for will be found at the spot in question. 



Suppose that I employ the same way of looking, but look 

 into a stereoscope. I am now aware that there is no real object 

 before me at the spot lam looking at ; but I have the same 

 sensible impression as if one were there ; and yet I am unable 

 to describe this impression to myself or others, or to characterise 

 it otherwise than as 'the same impression which would arise 

 in the normal method of observation, if an object were really 

 there.' It is important to notice this. No doubt the physiologist 

 can describe the impression in other ways, by the direction of 

 the eyes, the position of the retinal images, and so on; but 

 there is no other way of directly defining and characterising the 

 sensation which we experience. Thus we may recognise it as 

 an illusion, but yet we cannot get rid of the sensation of this 

 illusion; for we cannot extinguish our remembrance of its 

 normal signification, even when we know that in the case 

 before us this does not apply just as little as we are able to 

 drive out of the mind the meaning of a word in our mother 

 tongue, when it is employed as a sign for an entirely different 

 purpose. 



These conclusions in the domain of our sensible perceptions 

 appear as inevitable as one of the forces of nature, and hence 

 their results seem to be directly apprehended, without any effort 

 on our part ; but this does not distinguish them from logical 

 and conscious conclusions, or at least from those which really 

 deserve the name. All that we can do by voluntary and con- 

 scious effort, in order to come to a conclusion, is, after all, only 

 to supply complete materials for constructing the necessary 

 premisses. As soon as this is done, the conclusion forces itself 

 upon us. Those conclusions which (it is supposed) may be 

 accepted or avoided as we please, are not worth much. 



The reader will see that these investigations have led us to 

 a field of mental operations which has been seldom entered by 

 scientific explorers. The reason is that it is difficult to express 

 these operations in words. They have been hitherto most dis- 



