But this abstract idea, being something in the mind, between the thing that exists, and the name that is given to it; it is in our ideas that both the rightness of our knowledge, and the propriety and intelligibleness of our speaking, consists.  And hence it is that men are so forward to suppose, that the abstract ideas they have in their minds are such as agree to the things existing without them, to which they are referred; and are the same also to which the names they give them do by the use and propriety of that language belong.  For without this double conformity of their ideas, they find they should both think amiss of things in themselves, and talk of them unintelligibly to others.

9.  Simple Ideas may be false, in reference to others of the same Name, but are least liable to be so.

First, then, I say, that when the truth of our ideas is judged of by the conformity they have to the ideas which other men have, and commonly signify by the same name, they may be any of them false.  But yet simple ideas are least of all liable to be so mistaken.  Because a man, by his senses and every day’s observation, may easily satisfy himself what the simple ideas are which their several names that are in common use stand for; they being but few in number, and such as, if he doubts or mistakes in, he may easily rectify by the objects they are to be found in.  Therefore it is seldom that any one mistakes in his names of simple ideas, or applies the name red to the idea green, or the name sweet to the idea bitter:  much less are men apt to confound the names of ideas belonging to different senses, and call a colour by the name of a taste, &c.  Whereby it is evident that the simple ideas they call by any name are commonly the same that others have and mean when they use the same names.

10.  Ideas of mixed Modes most liable to be false in this Sense.

Complex ideas are much more liable to be false in this respect; and the complex ideas of mixed modes, much more than those of substances; because in substances (especially those which the common and unborrowed names of any language are applied to) some remarkable sensible qualities, serving ordinarily to distinguish one sort from another, easily preserve those who take any care in the use of their words, from applying them to sorts of substances to which they do not at all belong.  But in mixed modes we are much more uncertain; it being not so easy to determine of several actions, whether they are to be called justice or cruelty, liberality or prodigality.  And so in referring our ideas to those of other men, called by the same names, ours may be false; and the idea in our minds, which we express by the word justice, may perhaps be that which ought to have another name.

11.  Or at least to be thought false.

But whether or no our ideas of mixed modes are more liable than any sort to be different from those of other men, which are marked by the same names, this at least is certain.  That this sort of falsehood is much more familiarly attributed to our ideas of mixed modes than to any other.  When a man is thought to have a false idea of justice, or gratitude, or glory, it is for no other reason, but that his agrees not with the ideas which each of those names are the signs of in other men.