[1] But in order to employ analogy with effect more is needful than to make sure that the two objects or acts compared are similar without being identical: the design for which a comparison is made enters as an essential element, and decisively determines its value. Between two given objects an analogy may exist, good for one purpose but worthless for another. Given two balls, spherical in form and equal in size, the one of wood and the other of iron; and let the question be, Do these two objects bear any analogy to each other, real in itself and capable of being usefully employed? The question cannot yet be answered: we must first ascertain for what purpose the comparison is instituted. The two balls are like each other in form, but unlike in material; whether is it in respect of their form or their material that you propose to compare them? If one of them rolls along a gently inclined plane, you may safely infer that the other, when placed in the same position, will follow the same course; for although different in other features they are similar in form. But you cannot infer that because one floats when thrown into the water the other will float too, for in respect to specific gravity there is no similarity between them. Again, let two pieces of wood, cut from the same tree, be brought together, the one a cube, the other a sphere; you may safely conclude, if one swim in water that the other will swim too, because though of diverse forms they are of the same specific gravity; but you cannot conclude, if the one roll on an inclined plane, that the other will roll also, because though of the same specific gravity they are diverse forms. Two objects may be compared for the purpose of inferential analogy, although in nine of their qualities they are wholly dissimilar, if they resemble each other in one, and that the quality with respect to which the comparison is instituted. Again, although two objects be similar in nine of their properties, and dissimilar only in one, no useful analogy can be instituted between them if the object for which the comparison is made save with respect to the one point in which they are dissimilar. An acquaintance with such simple rudiments would go far to correct blunders both in the construction and the exposition of analogies. In suggesting probabilities and throwing out lines of inquiry, analogy is of unspeakable value in every branch of science; in sacred apologetics its specific use is to destroy the force of objections which may be plausibly urged against facts or doctrines otherwise established; but it is as an instrument for explaining, illustrating, fixing, and impressing moral and spiritual truth that we are mainly concerned with it here.