OF THE UNDERSTANDING 371 



Of all parts of our organisation, that which most readily undergoes 

 modifications, due to some acquired habit of thought, idea, or action, 

 is our organ of intelligence. Now the region which is modified is neces- 

 sarily that concerned with the ideas or thoughts which habitually 

 occupy us. I repeat, then, this region of our intellectual organ when 

 vigorously exercised acquires a development which may finally 

 become noticeable by external signs. 



We have dealt with the general principles concerning the organ 

 which gives rise to intelUgence ; we shall now pass on to a enquiry 

 into the formation of ideas. 



Formation of Ideas. 



My purpose here is not to undertake any analysis of ideas, nor to 

 show how they become compounded and increased, nor, in short, how 

 the understanding is perfected. Many celebrated men since Bacon, 

 Locke, and Condillac, have dealt with these matters and greatly 

 illuminated them. I need not therefore stay to consider them. 



My purpose is simply to indicate by what physical causes ideas may 

 be formed, and to show that comparisons, judgments, thoughts, and 

 all operations of the understanding are at the same time physical 

 acts, which result from the relations between certain kinds of matter 

 in action, and which are carried out in a special organ which has 

 gradually acquired the faculty of producing them. 



All that I have to say on this important subject is entirely a matter 

 of probabiUties. All is a product of imagination ; limited however by 

 the necessity for admitting nothing but physical causes compatible 

 with the known properties of matter, nothing, in short, but causes 

 which may be and probably are correct. With regard to the physical 

 acts which I shall endeavour to analyse, none of them can be witnessed 

 and none therefore can be proved. 



I should mention that ideas are of two distinct kinds, viz. : 



Simple or direct ideas ; complex or indirect ideas. 



By simple ideas I mean those which spring immediately and ex- 

 clusively from the noticed sensations that are impressed upon us by 

 objects either within or without us. 



By complex ideas I mean those which are found within us as a result 

 of some operation of our understanding, performed on previously 

 acquired ideas ; they therefore require no immediate sensation. 



Ideas of all kinds are the result of images or special outUnes of objects 

 which have affected us ; these images or outUnes only become ideas 

 for us, when they have been traced on some part of our organ ; and 

 the nervous fluid then passes over them and carries back the result 

 to our inner feehng, by which we become conscious of it. 



