128 PRESIDENTIAL ADDRESS — SECTION F. 



is actually known to us only in our own sensations of pressure 

 and effort. Again, " a body is a relatively constant sum of 

 touch and sight sensations associated with the same space and 

 time sensations." 



But, while the origin of scientific concepts is to be found in 

 sense-impressions, the ideas directly expressing sense-impressions 

 lack the definiteness and precision required for scientific pur- 

 poses. Science proceeds by accurate measurement. Measure- 

 ment is in some degree possible by sense-'impression itself. Thus 

 the mere succession of sensations, and especially their character 

 and variety, give us a certain estimate of the lajise of time ; our 

 muscular sense gives us a rough estimate of weight or of the 

 intensity of any pressure or force. But, although such immediate 

 feelings of change, effort, etc., constitute our actual individual 

 experience of things, they are useless as objective standards of 

 measurement. They are incapable of defining relations as obtain- 

 ing between things belonging to a common world. They must 

 accordingly be expressed in terms suitable to exact inquiry. 

 Poincare says on this point : " We do not require a definition of 

 force; the idea of force is primitive; irreducible, indefinable; 

 we all know what is is ; of it we have direct intuition. This direct 

 intuition arises from the idea of effort which is familiar to us 

 from childhood." But " this immediate notion of eft'ort is of no 

 use to us in the measurement of force." " The important thing 

 is not to know what force is, but how to measure it. Everything 

 which does not teach us how to measure it is as useless to^ the 

 mechanician as, for instance, the subjective idea of heat and 

 cold is to the student of heat. This subjective idea cannot be 

 translated into numbers, and is therefore useless ; a scientist 

 whose skin is an absolutely bad conductor of heat, and who, 

 therefore, has never felt the sensation of heat or cold, would 

 read a thermometer in just the same way as anyone else, and 

 would have enough material to construct the whole of the theory 

 of heat."^- Mach puts this in almost identical terms : " The cir- 

 cumstances determinative of motion that are best known to us 

 are our own volitional acts — our innervations. In the motions 

 wdiich we ourselves determine, as well as in those to which we are 

 forced by external circumstances, we are always sensible of a 

 pressure. Thence arises our habit of representing all circum- 

 stances determinative of motion as something akin to volitional 

 acts — as pressures. . . . We are able, in a great many cases, 

 to replace the circumstances determinative of motion, wdiich occur 

 in nature, by our innervations, and thus to reach the idea of a 

 gradation of the intensity of forces. But in the estimation o'f 

 this intensity we are thrown entirely on the resources of our 

 memory, and we are also unable to communicate our sensations. 

 Since it is possible, however, to represent ez.'ery condition that 

 determines motion by a weight, we arrive at the perception that 

 all circumstances determinative of motion (all (forces) are alike 



"Science and Hypothesis, pp. 105-6. 



