To illustrate one thing by its resemblance to another, has been always the most popular and efficacious art of instruction. There is indeed no other method of teaching that of which any one is ignorant, but by means of something already known; and a mind so enlarged by contemplation and inquiry that it has always many objects within its view, will seldom be long without some near and familiar image through which an easy transition may be made to truths more distant and obscure.
Of the parallels which have been drawn by wit and curiosity, some are literal and real, as between poetry and painting, two arts which pursue the same end, by the operation of the same mental faculties, and which differ only as the one represents things by marks permanent and natural, the other by signs accidental and arbitrary. The one, therefore, is more easily and generally understood, since similitude of form is immediately perceived; the other is capable of conveying more ideas, for men have thought and spoken of many things which they do not see.
Other parallels are fortuitous and fanciful, yet these have sometimes been extended to many particulars of resemblance by a lucky concurrence of diligence and chance. The animal body is composed of many members, united under the direction of one mind; any number of individuals, connected for some common purpose, is therefore called a body. From this participation of the same appellation arose the comparison of the body natural and body politick, of which, how far soever it has been deduced, no end has hitherto been found.
In these imaginary similitudes, the same word is used at once in its primitive and metaphorical sense. Thus health, ascribed to the body natural, is opposed to sickness; but attributed to the body politick, stands as contrary to adversity. These parallels therefore have more of genius, but less of truth; they often please, but they never convince.
Johnson: Idler #34 (December 9, 1758)