Thirdly, it is not enough that men have ideas, determined ideas, for which they make these signs stand; but they must also take care to apply their words as near as may be to such ideas as common use has annexed them to.  For words, especially of languages already framed, being no man’s private possession, but the common measure of commerce and communication, it is not for any one at pleasure to change the stamp they are current in, nor alter the ideas they are affixed to; or at least, when there is a necessity to do so, he is bound to give notice of it.  Men’s intentions in speaking are, or at least should be, to be understood; which cannot be without frequent explanations, demands, and other the like incommodious interruptions, where men do not follow common use.  Propriety of speech is that which gives our thoughts entrance into other men’s minds with the greatest ease and advantage:  and therefore deserves some part of our care and study, especially in the names of moral words.  The proper signification and use of terms is best to be learned from those who in their writings and discourses appear to have had the clearest notions, and applied to them their terms with the exactest choice and fitness.  This way of using a man’s words, according to the propriety of the language, though it have not always the good fortune to be understood; yet most commonly leaves the blame of it on him who is so unskilful in the language he speaks, as not to understand it when made use of as it ought to be.

12.  Fourth Remedy:  To declare the meaning in which we use them.

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.