SENSATION AND SENSE-ORGANS. 489 



and their examination, especially with respect to what nerve- 

 parts are concerned with different mental states, and what 

 changes in the former are associated with given phenomena 

 in the latter, forms properly a part of Physiology. Whether 

 masses of protoplasm, before the differentiation of definite 

 nerve-tissues, possess some ill-defined sort of consciousness, 

 as they possess an indefinite contractility before they have 

 been modified into muscular fibres, may for the present be 

 left undecided: though those who accept the doctrine of 

 evolution will be inclined to assent to the proposition. 



' While, however, the physiologist has a right to be heard 

 on questions relating to our mental faculties, it is neverthe- 

 less true that many laws of thought have been established 

 concerning which our present knowledge of the laws of the 

 nervous system gives as no clue; the science of Psychology 

 has thus a well-founded claim to an independent existence. 

 But, in so far as its results are confined merely to the succes- 

 sions and connections of mental states, as established by 

 observation, they are merely descriptions, and not explana- 

 tions in a scientific sense: we know that so many mental phe- 

 nomena have necessary material antecedents and concomi- 

 tants in nervous changes, that we are justified in believing 

 that all have such, and in continuing to seek for them. We 

 do not know at all how an electric current sent round a bar 

 of soft iron makes it magnetic; we only know that the one 

 change is accompanied by the other; but we say we have 

 explained the magnetism of a piece of iron if we have found 

 an electric current circulating around it. Similarly, we do 

 not know how a nervous change causes a mental state, but 

 we have not explained the mental state until we nave found 

 the nervous state associated with it and how that nervous 

 state was produced. 



As yet it is only with respect to some of the simplest 

 states of consciousness that we know much of the necessary 

 physiological antecedents, and among these our sensations 

 are the best investigated. As regards such mental phenom- 

 ena as the Association of Ideas and Memory, physiology 

 can give us some light; but so far as others, such as the Will 

 and the Emotions, are concerned, it has at present little to 

 offer. The phenomena of Sensation, therefore, occupy at 

 present a much larger portion of physiological works than 

 all other mental facts put together. 



