The argument from similitude derives its force from a statement about the likeness of two things. To understand how it persuades, we must recall what was said about analogy in our discussion of the devices of logic. We saw that all analogy depends upon a theory that if two things resemble in a certain number of respects, it is probable that they resemble in still further respects. When we argue from similitude, what the major premise does is assert this principle of similarity or comparability. The minor premise then asserts the fact of the similarity and the conclusion asserts the probability of still further similarity. The process may be best seen in the form of a hypothetical syllogism:
As we learned when we examined the theory of analogy, the weight of this probability will depend upon the number and the pertinence of the particulars in which the two instances resemble. Here again we have to observe that the rhetoric of the argument is in direct relation to the soundness of the predication about the world. If the analogy is weak, the argument will have little power to persuade; but if it is strong, the power will be proportionately great.