A few examples may next be given to show the workings of this associating power, and the consequences thence arising. The intellectual operations known under the names classification, generalization, induction, and deduction, all proceed upon the discovery of likeness among things lying wide asunder in space and time, and very often veiled by diversity. 'Thus, in order to inolude in one list all the species of the rose, botanists have had to trace the characters of the genus through its various members, wherever they occur, and under the greatest differences in every other respect. It takes a keen identifying faeulty—that is, a strong natural tendency for the resurrection of like to meet like—to see the resemblance of some of these species to the rest; and it has happened in many departments of knowledge that a class has remained incomplete for a time, purely from the disguised character of some of the individuals. So in the process termed induction, by which a general law is arrived at by comparing instances of it everywhere, there must be an attraction of similars, in order to bring together in the mind the collection of particulars that the induction is based upon. Thus, Newton assembled in his view the various transparent bodies that he had found in the course of his experiments to refract or bend light strongly, his only intellectual instrument for doing so being the bond of likeness operating as a power of recall. Having looked at them in company, he saw that some were remarkable for their weight or specific gravity, and others for containing inflammable ingredients; upon which he raised the general induction, connecting these two properties with high refrangibility. Then, deductively, he applied this generalization to the diamond, which refracts light more than any other known substance; and as it is not a heavy material, he extended the other inference to it—namely, that it was made up of some inflammable material, an inference afterwards confirmed by the discovery that it is crystallized carbon. Many of the greatest discov eries in science have turned on the identification of modes of action never before supposed the same, as when Franklin was struck with the resemblance between the atmospheric thunder and lightning and the phenomena of common electricity.
Another wide field for the operation of the same principle is the region of illustrative cornparisoim, whereby two things widely remote are brought together, in the view either to elucidate one another, or for the sake of ornament and poetic effect. Most men of genius in literature and poetry have contributed original illustrations, similes, metaphors, or comparisons in the course of their compositions. Shakespeare carries the palm in this faculty. The writings of Bacon are remarkably rich in those that serve- the purpose of exposition. Science is with him the "interpretation" of nature: final causes are "vestal virgins;" they have no fruit: fallacies are " idols." Edmund Burke, another master of illustrative comparison, has termed revolutions the " medicine" of the state, and regular government its "food." If we inquire into the circumstances that render one mind more prolific in new identifications and comparisons than another, apart from difference of original capacity, we must refer mainly to the fact that the one has had the greater previous familiarity with the class of things thus brought up by the attraction of similarity. A mathematician is
the most likely person to bring up comparisons from mathematics; a botanist is prepared to identify plants; a traveled man provides illustrations from foreign countries; a historian, from history. The sailor is notoriously rich in nautical similes and illustra tions. When any one not specially versed in a subject is yet prone to draw upon it profusely in the way of comparison, we must then refer to great natural endowment as the sole explanation. But our space does not allow us to dwell further on the subject. (For the full exemplification of both the associating principles and of the complications that they give birth to, see Bain on The Senses and the Intellect).
The earliest known attempt to lay down the laws whereby thought succeeds to thought, is that contained in Aristotle's treatise on memory. He enumerates three dif ferent principles of mental resuscitation—viz., similarity, contrariety, and co-adjacency. He has been followed by most other philosophers as regards all the three principles. It is now, however, clearly seen and generally admitted that contrariety is not an inde pendent associating force. When a thing suggests its opposite or contrary, it will be found that the two have been previously together in tire mind, and have therefore acquired a mutual hold by contiguity. Such, for example, is black and white, wet and dry, health and sickness, prosperity and adversity, etc. Contraries, in fact, have a natural inseparability; they are of the class of relatives like father and son, which imply each other necessarily, and have no meaning except by mutual reference. It requires no new principle of our constitution to account for suggestion in this particular case. More over, when things are strongly contrasted with one another, as high position before a fall, the mind is greatly impressed with the shock of trausifion, and so retains a lively recollection of the sequence, having by that means a greater tendency to pass from the one to the other. Thus, then, the enumeration of Aristotle is reduced to the two prin ciples that we have now expounded.
Hobbes recognized the principle of contiguity as the foundation of reminiscence; but the Aristotelian philosoPher,Vives, who wrote in the 14th e., was the first to specify in minute detail the various circumstances that determine the adhesive bond Of recollection. Hume's enumeration is well known to have comprised the three principles of resem blance, contiguity, and causation, which he illustrates as follows: "A picture naturally leads our thoughts to the original [resemblance]. The mention of one apartment in a building naturally introduces an inquiry or discourse concerning the others [contiguity]. And if we think of a wound, we can scarce forbear reflecting on the pain which follows it [causation]." Causation, however, is merely a case of contiguity; so also we may say of order in place, and 'Dicier in time, which have been given as distinct principles.
An attempt has been made to generalize into contiOity, but without success. For a full and critical view of the history of these laws, see Sir W. Hamilton's edition of Reid.