462 THE HUMAN BODY. 



observation apart from a nervous system, and so are pre- 

 sumably, in some way, endowments of it; we are, therefore, 

 justified in calling them properties of the nervous system; 

 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 phe- 

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

 W hether masses of protoplasm, before the differentiation of 

 definite nerve-tissues, possess some ill-defined sort of con- 

 sciousness, 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 doc- 

 trine 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 never- 

 theless true that many laws of thought have been esta- 

 blished concerning which our present knowledge of the- 

 laws of the nervous system gives us 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 successions and connections of mental states, as estab- 

 lished by observation, they are merely descriptions, and not 

 explanations in a scientific sense: we know that so many 

 mental phenomena have necessary material antecedents and 

 concomitants in nervous changes, that we are justified in 

 believing that all have such, and in continuing to seek fcr 

 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; bat 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 knoAV how a nervous change causes a 

 mental state, but we have not explained the mental state 

 until we have 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 consciousress that we know much of the necessary 

 physiological antecedents, and among these our sensations 

 are the best investigated. As regards such mental pheno- 



