40 THE GRAMMAR OF SCIENCE 



that it at once leads us up to a very complex group of 

 properties. In common talk we attribute all these 

 properties to the blackboard, but when we begin to think 

 over the matter carefully we shall find that the real link 

 between them is by no means so simple as it seems to be. 

 To begin with, I receive certain impressions of size and 

 shape and colour by means of my organs of sight, and 

 these enable me to pronounce with very considerable 

 certainty that the object is a blackboard made of wood 

 and coated with paint, even before I have touched or 

 measured it. I infer that I shall find it hard and heavy, 

 that I could if I pleased saw it up, and that I should find 

 it to possess various other properties which I have learnt 

 to associate with wood and paint. These inferences and 

 associations are something which I add to the sight- 

 impressions, and which I myself contribute from my past 

 experience and put into the object — blackboard. I might 

 have reached my conception of the blackboard by impres- 

 sions of touch and not by those of sight. Blindfolded I 

 might have judged of its size and shape, of its hardness 

 and surface texture, and then have inferred its probable 

 use and appearance, and associated with it all blackboard 

 characteristics. In both cases it must be noted that 2. sine 

 qua non of the existence of an actual blackboard is some 

 immediate sense-impression to start with. The sense- 

 impressions which determine the reality of the external 

 object may be very few indeed, the object may be largely 

 constructed by inferences and associations, but some sense- 

 impressions there must be if I am to term the object real, 

 and not a product merely of my imagination. The 

 existence of a certain number of sense-impressions leads 

 me to infer the possibility of my receiving others, and 

 this possibility I can, if I please, put to the test. 



I have heard of the Capitol at Washington, and 

 although I have never been to America, I am convinced 

 of the reality of America and the Capitol — that is, I 

 believe certain sense-impressions would be experienced by 

 me if I put myself in the proper circumstances. In this 

 case I have had indirect sense-impressions, contact with 



