There is another kind of constancy which should here be noticed. Some humorous sayings survive for long periods, and occasionally are adopted in foreign countries. In some cases they have immortalized a name, in others we know not who originated them, or to whom they first referred. They seem to be the production, as they are the heritage, not of man but of humanity. It is essential to the permanence of humour that it should refer to large classes, and awaken emotions common to many. If Socrates and Xantippe, the philosopher and the shrew, had not represented classes, and an ordinary connection in life, we should have been little amused at their differences.[16] Having mentioned these few first aspects in which humour is constant, we now come to the wider field of its variation. It may be said to vary with the age, with the century, with classes of society, with the time of life, nay, it has been asserted, with the very hours of the day! The simplest mode in which we can demonstrate this character of humour is to consider some of those things which although amusing to others are not so to us, and those which amuse us, but not others; we sometimes regard as ludicrous what is intended to be humorous, sometimes on the other hand we view as humorous what is seriously meant, and sometimes we take gravely what is intended to be amusing. A man may make what he thinks to be a jest, and be neither humorous nor ludicrous, and a man may cause others to laugh without being one or the other; for what he says may be amusing, although he does not intend it to be so, or he may be merely relating some actual occurrence. Occasionally, there is some doubt as to whether we regard things as ludicrous or humorous. This is seen in some proverbs.