THE FACTS OF SCIENCE 73 



impressions, but slightly differing from the last, for the 

 sun has left some leaves and fallen on others, and the 

 wind is still ; but there is a sameness in the great majority 

 of the sense-impressions of the two groups, and accordingly 

 I term them one and the same individual tree — the ash- 

 tree in my garden. If any one tells me that the sameness 

 is due to some " thing-in-itself " which introduces the per- 

 manency into the group of sense-impressions, I can as 

 little accept or deny his assertion as he forsooth can 

 demonstrate anything about this shadowy thing-in-itself. 

 He may call it Matter, or God, or Will, or Mind- stuff, but 

 to do so serves no useful purpose, for it lies beyond the 

 field of conception based on sense-impressions, beyond 

 the sphere of logical inference or human knowledge. It 

 is idle to postulate shadowy unknowables behind that real 

 world of sense-impression in which we live. So far as 

 they affect us and our conduct they are sense-impressions ; 

 what they may be beyond is fantasy, not fact ; if indeed 

 it be wise to assume a beyond, to postulate that the surface 

 of sense-impressions which shuts us in, must of necessity 

 shut something beyond out. Such unknowables do not 

 assist us in grasping why groups of sense-impressions 

 remain more or less permanently linked together. Our 

 experience is that they are so linked, and their association 

 is at the present, and may ever remain, as mysterious as 

 is now the process by which the impresses of past sense- 

 impressions are involuntarily linked together in the brain. 

 Why is the thought " garden " in my mind invariably 

 followed by the thought " cats " ? The psychical basis of 

 the association is not what I mean. I recognise it in the 

 repeated experience of the havoc which the feline race 

 has wrought in my own garden. But what is the physical 

 nexus between the two conceptions as impresses in my 

 brain ? No one can say ; and yet this problem should 

 be easier to answer than that of the nexus between the 

 immediate sense -impressions we term objects. When 

 physiological psychology has answered the former problem, 

 then it will perhaps cease to be foolish for us to discuss 

 the latter. Meanwhile let us confess our ignorance 



