RESPECTING THE COMPOSITION OF IDEAS.
53. Another grand law, or mode of operation, of the associative power is that by which simple ideas are formed into compound, or complex ideas ; in other words, more generally, by which simple sensorial changes are combined and blended together.—In the conside ration of this law, we shall derive most of our statements from those of Hartley, divestingthem, however, of those peculia rities of expression, which depend for their correctness upon• the truth of the • positions, that the medullary substance of the brain is the sensorium, and that sensorial changes are vibrations of the medullary substance. In order to ex plain this law of association, it is Ole cessary to take a view of the modes in which simple ideas, or ideas of sensation, may be associated.
Case 1. Let the sensation A be often associated with each of the sensations B, C, D, &c. ; that is, at certain times with B, at certain other times with C, and so on : it is evident from what has been be fore stated (§ 21.) that A, when produced alone, will raise a, b, c, d, &c. (the simple ideas of sensation corresponding respec tively with A, B, C, D, &c,) altogether, and consequently will associate them to gether. If a, b, c, d, &c. are distinct in all their parts, then, in the first instance they will be merely connected, so as to make a group (which may be represent ed by a+b-pc+d,) hut if they are not distinct in their parts, they more or less run into each other, so as to form a com plex cluster, (which may be represented by abed.) Now the more frequently the group a+b+c+d, &c. occurs in connec tion, th,e more closely the single ideas are united ; and unless any one has a peculiar degree of vividness, they will by degrees appear to the mind as one idea ; and un less the notice of the mind is particularly directed to the circumstance, thatit is com posed of parts, it appears as much a single idea as originally each of the parts would have done, if the attention had been di rected to them singly. Again, the more the cluster abed, &c. occurs in combination the more completely the parts coalesce, so that by degrees they form a complex idea, the parts of which are scarcely dis tinguishable.
54. Case 2. If the sensations A, B, C, D, &c. be associated together, according to various combinations of twos, or even of threes, fours, &c. then will A raise up b-l
c-Fd, &c. ; also B will raise up a+cd-d, &c. ; and compound or complex ideas will be formed of those combinations, pre cisely as was before stated in the case of sensations singly associated with another sensation. It may happen indeed in both cases, that A may raise a particular idea, as b, preferably to any of the rest, in con sequence of its being more frequently associated with b, or of the greater novel ty of the impression of the corresponding sensation, B, rendering it more vivid, or of some tendency of the sensorium to ex cite b, or of some other cause ; and in like manner that 13 may raise c or d pre ferably to the rest. However, all this will at last be overruled by the recur rence of the associations, so that any one of the sensations will excite the ideas of the rest at the same instant, and there fore associate them together.
55. Case 3. Let A, B, C, D, &c. repre sent succcessive sensations (occurring in contiguous, successive instants,) A will raise b, c, d, &c. B will raise c, d, &c. : and though the ideas do not rise precise. ly in the same instant, yet they collie nearer and nearer together than the sen sations did in their original impression ; so that these ideas are at last associated synchronously, as they were from the first successively.
56. Case 4. All compound impressions, A -1-C D, &c. or A BC D, &c. cording as they are received by different organs, or the same) after sufficient repe tition, leave behind their compound ideas a + b c d, &c. or abed, &c which re cur every now and then by means of sen sations, or ideas, with which the whole compound, or any one or more of the parts, A, B, D, &c. have been associated. Now in these recurrences of compound ideas, the parts are further associated, and more intimately united to one another, agreeably to what was observed above, so as to form a compound or complex idea, which shall appear to the mind as one single idea,—As the same causes produce the re currence of the compounded ideas in what soever way the union was first produced, the same remark may be made under each of the cases as have been under this and the first case, respecting the causes and effects of such recurrence.