THE FACTS OF SCIENCE 65 



immediate sense-impression. Thus the distinction of what 

 is " outside " and what is " inside " myself at any instant 

 depends entirely on the amount of immediate sense - 

 impression. This has been very cleverly represented by 

 the well-known German scientist, Professor Ernst Mach. 

 In the accompanying sketch our professor may be seen 

 lying on his back, and having closed his right eye, the 

 picture represents what is presented to his left eye : — 



"In a frame formed by the ridge of my eyebrow, by 

 my nose, and my moustache, appears a part of my body, 

 so far as it is visible, and also the things and space about 

 it. . . . If I observe an element, A, within my field of 

 vision, and investigate its connection with another element, 

 B, within the same field, I go out of the domain of physics 

 into that of physiology or psychology, if B, to use the 

 apposite expression that a friend of mine employed upon 

 seeing this drawing, passes through my skin." ^ 



From our standpoint, neglecting for simplicity the 

 immediate contributions of any other senses than that of 

 sight, the picture represents that part of the professor's 

 sense-impressions which for the instant forms his " outside 

 world " ; the rest was " inside " — existed for him only as 

 a product of stored sense-impresses. 



There is no better exercise for the mind than the 

 endeavour to reduce the perception we have of " external 

 things " to the simple sense-impressions by which we know 

 them. The arbitrary distinction between outside and 

 inside ourselves is then clearly seen to be one merely of 

 everyday practical convenience. Take a needle ; we say 

 it is thin, bright, pointed, and so forth. What are these 

 properties but a group of sense-impressions relating to 

 form and colour associated with conceptions drawn from 

 past sense-impressions ? Their immediate source is the 

 activity of certain optic nerves. These sense-impressions 

 form for us the reality of the needle. Nevertheless, they 

 and the resulting construct are projected outside ourselves, 

 and supposed to reside in an external thing, " the needle." 



^ "The Analysis of the Sensations — Anti-metapliysical," The Afoiiist, 

 vol. i. p. 59. 



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