2i6 THE POPULAR SCIEJVCE MONTHLY. 



manipulated form, the whole meaning of a long calculation, so in all 

 other kinds of reasoning the symbols which we call words contain, in 

 an abbreviated form, vast bodies of signification. Indeed, any one who 

 investigates this subject cannot fail to become convinced that it is 

 wholly impossible to over-estimate the value of language as thus the 

 handmaid of thought ; for, as we have seen, in the absence of language 

 it would be impossible for thought to rise above the very simplest of 

 abstract ideas, while in the presence of language it becomes possible 

 for us consciously to predicate qualities, and so at last to feel that we 

 are conscious of our own consciousness. 



So much, then, for our classification of ideas. We have, first, simple 

 ideas, or ideas of particular perceptions ; and, secondly, abstract ideas, 

 or ideas of general qualities ; and the latter class I have subdivided 

 into those which may be developed by simple feelings, and those which 

 can only be developed by the aid of signs. 



Now, with regard to ideas themselves, I need only add that they are 

 the psychological units which compose the whole structure intellect- 

 ual. They constitute, as it were, the raw material of thought, which 

 may be elaborated by the reflective faculty into various products of 

 thought. Once formed they present an essential property of occurring 

 in concatenated series ; so that the occurrence of one idea determines 

 that of another with which it has been previously joined. This prin- 

 ciple of the association of ideas, manifested as it is by the ultimate units 

 of intellectual structure, is by far the most important principle in psy- 

 chology : it is the principle which renders possible all the faculties of 

 mind — memory, instinct, judgment, reason, emotion, conscience, and 

 volition. 



We are now in a position to investigate the facts of comparative 

 psychology ; and, in order to do so thoroughly, I shall begin by con- 

 sidering what I may term the physiological basis of mind. There is no 

 reasonable doubt that all mental processes are accompanied by nervous 

 processes ; or, to adopt the convenient terms of Prof. Huxley, that psy- 

 chosis is invariably assodated with neurosis. The nature of this asso- 

 ciation, according to the best lights of our present knowdedge, is prob- 

 ably as follows : Nerve-tissue consists of two elementarj^ parts, viz., 

 nerve-cells and nerve-fibres. The nerve-cells are usually collected into 

 aggregates, which are called nerve-centres, and to these nerve-centres 

 bundles of nerve-fibres come and go. The incoming nerve-fibres serve 

 to conduct stimuli or impressions to the cells in the nerve-centre ; and, 

 when the cells thus receive a stimulus or impression, they liberate a 

 discharge of nervous energy, which then courses down the outgoing 

 nerve-fibres to be distributed either to other nerve-centres or else to 

 muscles. It is in this way that nerve-centres are able to act in har- 

 mony with one another, and so to coordinate the action of the muscles 

 over which they preside. This fundamental principle of neurosis is 

 what physiologists call the principle of reflex action ; and you will per- 



