Fourthly, But, because common use has not so visibly annexed any signification to words, as to make men know always certainly what they precisely stand for: and because men, in the improvement of their knowledge, come to have ideas different from the vulgar and ordinary received ones, for which they must either make new words, (which men seldom venture to do, for fear of being thought guilty of affectation or novelty,) or else must use old ones in a new signification: therefore, after the observation of the foregoing rules, it is sometimes necessary, for the ascertaining the signification of words, to DECLARE THEIR MEANING; where either common use has left it uncertain and loose, (as it has in most names of very complex ideas;) or where the term, being very material in the discourse, and that upon which it chiefly turns, is liable to any doubtfulness or mistake. 13. And that in three Ways. As the ideas men's words stand for are of different sorts, so the way of making known the ideas they stand for, when there is occasion, is also different. For though DEFINING be thought the proper way to make known the proper signification of words; yet there are some words that will not be defined, as there are others whose precise meaning cannot be made known but by definition: and perhaps a third, which partake somewhat of both the other, as we shall see in the names of simple ideas, modes, and substances. 14. In Simple Ideas, either by synonymous terms, or by showing examples. I. First, when a man makes use of the name of any simple idea, which he perceives is not understood, or is in danger to be mistaken, he is obliged, by the laws of ingenuity and the end of speech, to declare his meaning, and make known what idea he makes it stand for. This, as has been shown, cannot be done by definition: and therefore, when a synonymous word fails to do it, there is but one of these ways left. First, Sometimes the NAMING the subject wherein that simple idea is to be found, will make its name to be understood by those who are acquainted with that subject, and know it by that name. So to make a countryman understand what FEUILLEMORTE colour signifies, it may suffice to tell him, it is the colour of withered leaves falling in autumn. Secondly, but the only sure way of making known the signification of the name of any simple idea, is BY PRESENTING TO HIS SENSES THAT SUBJECT WHICH MAY PRODUCE IT IN HIS MIND, and make him actually have the idea that word stands for. 15. In mixed Modes, by Definition.