From what has been said, it is easy to observe what has been before remarked, viz. that the names of simple ideas are, of all others, the least liable to mistakes, and that for these reasons.  First, Because the ideas they stand for, being each but one single perception, are much easier got, and more clearly retained, than the more complex ones, and therefore are not liable to the uncertainty which usually attends those compounded ones of substances and mixed modes, in which the precise number of simple ideas that make them up are not easily agreed, so readily kept in mind.  And, Secondly, Because they are never referred to any other essence, but barely that perception they immediately signify:  which reference is that which renders the signification of the names of substances naturally so perplexed, and gives occasion to so many disputes.  Men that do not perversely use their words, or on purpose set themselves to cavil, seldom mistake, in any language which they are acquainted with, the use and signification of the name of simple ideas.  White and sweet, yellow and Bitter, carry a very obvious meaning with them, which every one precisely comprehends, or easily perceives he is ignorant of, and seeks to be informed.  But what precise collection of simple ideas modesty or frugality stand for, in another’s use, is not so certainly known.  And however we are apt to think we well enough know what is meant by gold or iron; yet the precise complex idea others make them the signs of is not so certain:  and I believe it is very seldom that, in speaker and hearer, they stand for exactly the same collection.  Which must needs produce mistakes and disputes, when they are made use of in discourses, wherein men have to do with universal propositions, and would settle in their minds universal truths, and consider the consequences that follow from them.

19.  And next to them, simple Modes.

By the same rule, the names of simple modes are, next to those of simple ideas, least liable to doubt and uncertainty; especially those of figure and number, of which men have so clear and distinct ideas.  Who ever that had a mind to understand them mistook the ordinary meaning of seven, or a triangle?  And in general the least compounded ideas in every kind have the least dubious names.

20.  The most doubtful are the Names of very compounded mixed Modes and Substances.

Mixed modes, therefore, that are made up but of a few and obvious simple ideas, have usually names of no very uncertain signification.  But the names of mixed modes, which comprehend a great number of simple ideas, are commonly of a very doubtful and undetermined meaning, as has been shown.  The names of substances, being annexed to ideas that are neither the real essences, nor exact representations of the patterns they are referred to, are liable to yet greater imperfection and uncertainty, especially when we come to a philosophical use of them.