That then which makes doubtfulness and uncertainty in the signification of some more than other words, is the difference of ideas they stand for.

5.  Natural Causes of their Imperfection, especially in those that stand for Mixed Modes, and for our ideas of Substances.

Words having naturally no signification, the idea which each stands for must be learned and retained, by those who would exchange thoughts, and hold intelligible discourse with others, in any language.  But this is the hardest to be done where,

First, The ideas they stand for are very complex, and made up of a great number of ideas put together.

Secondly, Where the ideas they stand for have no certain connexion in nature; and so no settled standard anywhere in nature existing, to rectify and adjust them by.

Thirdly, When the signification of the word is referred to a standard, which standard is not easy to be known.

Fourthly, Where the signification of the word and the real essence of the thing are not exactly the same.

These are difficulties that attend the signification of several words that are intelligible.  Those which are not intelligible at all, such as names standing for any simple ideas which another has not organs or faculties to attain; as the names of colours to a blind man, or sounds to a deaf man, need not here be mentioned.

In all these cases we shall find an imperfection in words; which I shall more at large explain, in their particular application to our several sorts of ideas:  for if we examine them, we shall find that the names of MIXED Modes are most liable to doubtfulness and imperfection, for the two first of these reasons; and the names of SUBSTANCES chiefly for the two latter.

6.  The Names of mixed Modes doubtful.

First, The names of mixed modes are, many of them, liable to great uncertainty and obscurity in their signification.

I. Because the Ideas they stand for are so complex.

Because of that great composition these complex ideas are often made up of.  To make words serviceable to the end of communication, it is necessary, as has been said, that they excite in the hearer exactly the same idea they stand for in the mind of the speaker.  Without this, men fill one another’s heads with noise and sounds; but convey not thereby their thoughts, and lay not before one another their ideas, which is the end of discourse and language.  But when a word stands for a very complex idea that is compounded and decompounded, it is not easy for men to form and retain that idea so exactly, as to make the name in common use stand for the same precise idea, without any the least variation.  Hence it comes to pass that men’s names of very compound ideas, such as for the most part are moral words, have seldom in two different men the same precise signification; since one man’s complex idea seldom agrees with another’s, and often differs from his own—­from that which he had yesterday, or will have tomorrow.