When an _hypothesis_ has once come to birth in the mind, or gained a footing there, it leads a life so far comparable with the life of an organism, as that it assimilates matter from the outer world only when it is like in kind with it and beneficial; and when, contrarily, such matter is not like in kind but hurtful, the hypothesis, equally with the organism, throws it off, or, if forced to take it, gets rid of it again entire. To gain _immortality_ an author must possess so many excellences that while it will not be easy to find anyone to understand and appreciate them all, there will be men in every age who are able to recognize and value some of them. In this way the credit of his book will be maintained throughout the long course of centuries, in spite of the fact that human interests are always changing. An author like this, who has a claim to the continuance of his life even with posterity, can only be a man who, over the wide earth, will seek his like in vain, and offer a palpable contrast with everyone else in virtue of his unmistakable distinction. Nay, more: were he, like the wandering Jew, to live through several generations, he would still remain in the same superior position. If this were not so, it would be difficult to see why his thoughts should not perish like those of other men. _Metaphors_ and _similes_ are of great value, in so far as they explain an unknown relation by a known one. Even the more detailed simile which grows into a parable or an allegory, is nothing more than the exhibition of some relation in its simplest, most visible and palpable form. The growth of ideas rests, at bottom, upon similes; because ideas arise by a process of combining the similarities and neglecting the differences between things. Further, intelligence, in the strict sense of the word, ultimately consists in a seizing of relations; and a clear and pure grasp of relations is all the more often attained when the comparison is made between cases that lie wide apart from one another, and between things of quite different nature. As long as a relation is known to me as existing only in a single case, I have but an _individual_ idea of it--in other words, only an intuitive knowledge of it; but as soon as I see the same relation in two different cases, I have a _general_ idea of its whole nature, and this is a deeper and more perfect knowledge. Since, then, similes and metaphors are such a powerful engine of knowledge, it is a sign of great intelligence in a writer if his similes are unusual and, at the same time, to the point. Aristotle also observes that by far the most important thing to a writer is to have this power of metaphor; for it is a gift which cannot be acquired, and it is a mark of genius.