comes possible, or by further showing the implications of the assump- 

 tion to be true. The theory of ions also involves the inference that 

 changes in mechanical structure give rise to changes in chemical qual- 

 ity. This inference could be verified by changing the mechanical 

 structure of atoms in our laboratories and noting the consequent 

 changes of chemical qualities ; or by finding numerous instances where 

 there are with differences of mechanical structure, corresponding dif- 

 ferences of chemical quality. Thus we conclude that where a theory 

 is based upon the experimental method, the difficulties in verifying it 

 are simply the difficulties in the way of further observation and experi- 

 ment. 



We, however, meet with peculiar conditions of verification in the 

 theories based upon the method of assumption and deduction. In 

 Hyslop's theory the existence of spirits and their qualities are assumed. 

 The theory can be verified by revealing the existence of spirits by 

 presenting them, if possible, in experience; or by gathering evidences of 

 their existence from experiments. The peculiarity which one finds in 

 the verification of this theory is that in order to verify it, the spirits 

 themselves must be subjected to direct investigation. Being like no 

 other things of which we know, we cannot investigate other things and 

 apply our conclusions to spirits by analogy. Contrast the conditions of 

 verification in the theory of ions. The inference there is that an ion 

 is an electrically charged particle. Now we can experiment with 

 electrically charged particles and can find what their properties are, 

 even though we cannot experiment directly with an ion. In the case 

 of spirits, however, we have nothing like them in experience which we 

 can investigate; and therefore the theory is to be verified only by di- 

 rectly investigating spirits themselves. Until spirits have been di- 

 rectly investigated, and we know what they are like, we cannot infer 

 their existence from any occurrences in experience, because we can- 

 not know until then, that the occurrences are such as spirits are able 

 to produce. 



The same situation presents itself when we attempt to verify 

 Locke's theory. / Locke assumes that the mind is a plastic medium and 

 that ideas are impressed upon it by operations of matter. This, we 

 could verify by producing, if that were possible, an idea experimentally 

 in that way. We can verify this assumption in no other way. The 

 mind is like nothing in our experience, and we therefore cannot deter- 

 mine by analogy what its qualities are. When therefore an idea oc- 

 curs, we cannot say it was conditioned by an impression upon a mind ; 

 for we do not know whether there is a substance which when im- 

 pressed will give rise to an idea. We must impress something ,and 

 thus produce an idea before we can know that impressions of sub- 



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