FIXATION OF IDEAS. 59 



degree of inherited tendency favourable to it. To fix an 

 idea firmly, it should receive our undivided attention, be 

 slowly repeated many times, and be associated in every 

 possible way with cognate ideas. It is a good practice to 

 make notes of all important ideas, and read them occasion- 

 ally. The greater degree of interest we feel in an idea, 

 the more attention, study, and time we give to it ; and the 

 more intimately we associate it with other important ideas 

 with which we are familiar, the more firmly do we acquire 

 it. What we best understand fixes itself best in the 

 memory. But of the way in which a physical sensation is 

 converted into a mental perception or idea and recorded, 

 or vice versa, we know scarcely anything ; and this is nob 

 surprising, when we consider that we know as yet but little 

 of the way in which any of the physical forces or actions 

 of nature are converted into each other ; but it is highly 

 probable that when the latter are discovered (as they 

 probably will be), a similar explanation will be made by 

 analogy (or for other reasons) of the former, and that it 

 will be found to include a conversion of modes of mole- 

 cular motion. There is strong reason for believing that 

 many permanent impressions are made upon the brain and 

 mind, without our perceiving them at the time ; and, 

 under suitable conditions of subsequent excitement (by 

 disease or otherwise), they become evident. Some remark- 

 able instances, affording evidence which supports this con- 

 clusion, have been recorded. 1 



Different ideas require very different degrees of time 

 and attention in order to realise them with equal distinct- 

 ness, and some cannot under any conditions be as clearly 

 conceived as others. The' perception of a sensation, or of 

 an idea of the simplest and least abstruse kind, requires 



1 See Carpenter's Mental Physiology, p. 437. 



