658 - SCIENCE PROGRESS 



hypotheses based on the known process of rotation about a fixed axis. 

 After it had been shown that the forecasts thus obtained were not "suffi- 

 ciently accurate, Newton applied the hypothesis of gravitation, suggested 

 by the known process of the acceleration of iron toward a magnet, and of 

 heavy bodies toward the earth. The forecasts made on this latter hypo- 

 thesis have corresponded remarkably well with subsequent experience. 4j 



Lastly, what is the relation of knowledge to causation and to external reality ? 



Present ideas of causation rest upon the theory of energy, to which 

 Newton gave the first definite shape in the scholium to his third law of 

 motion. His enunciation is paraphrased by Lord Kelvin in the words : 

 " The whole work done, in any limited material system, by applied force 

 is equal to the whole effect, in the forms of potential and kinetic energy, 

 produced in the system, together with the work lost in friction." (Natural 

 Philosophy, Thomson & Tait, § 287). 



About the middle of the nineteenth century, however, it was shown that 

 each unit of work has its exact equivalent- in a unit of heat, the two being 

 interchangeable ; so that work employed in overcoming friction is not lost, 

 but only changed into the form of heat ; or putting the theory into more 

 general terms, that energy always remains the same in amount, however 

 much it may change its form. 



For modern thought this is almost as important as the older hypothesis 

 of the constancy of the quantity of matter ; it supplies an intelligible con- 

 nection between the changes in a physical process by giving definition to 

 the idea of causation, which had previously been extremely vague. 



The connection is of two slightly different kinds. In one the cause and 

 the effect are simultaneous, and the idea of the transformation of energy 

 forms a mental link between two different aspects of a single process, which 

 is called the cause when it is regarded as a change from the initial condition, 

 while as a change into the final condition it is called the effect. For example, 

 the combination of carbon and oxygen is the cause, whose effect is the pro- 

 duction of carbon dioxide ; the link between these aspects being the idea 

 of the liberation of energy which is transformed and given dut as heat. 



In connections of the other kind the idea is that energy is set free by 

 one process, called the cause, for transformation in a subsequent process 

 called the effect. In this sense the pulling of a trigger may be the cause of 

 the discharge of a gun. 



The question of our knowledge of substance, or external reality, has 

 been much discussed, and has generally been treated as though it differed 

 from all other questions, or was something mysterious. Locke says that 

 we neither have nor can have the idea of substance by sensation or reflec- 

 tion (Essay on Human Understanding, Bk. I, iv. § 18), yet he also says 

 that we have the ideas of its " primary qualities " as they really are (Bk. II, 

 xxxv. § 2). Berkeley asserted that the external reality corresponding to 

 our idea was a thought in the mind of his deity, and had no other existence. 

 Reid and the Scotch metaphysicians denied this, but their positive views 

 about our knowledge do not seem to have been much clearer than those of 

 Locke. Kant again deals with reality in phrases concerning categories and 

 schemata and the-thing-in-itself, which could never have afforded any con- 

 firmation of the world-wide hypothesis of external reality. On this subject 

 neither the writers mentioned, nor any others that I know of, have put their 

 views in such form that they can be tested by comparison with experience. 



In the solution of the problem two considerations are necessary. The 

 first concerns the meaning of reality, which in its ordinary, non-metaphysical 

 sense has reference to the fulfilment of forecast. Just as we say that gold 

 is real if the forecasts be fulfilled which are made from a particular hypo- 

 thesis about its nature, so we must admit that substance is real if the fore- 

 casts be fulfilled which are made from the hypothesis of its existence. 



