are all adequate, for they are their own archetypes, are not intended to represent anything other than themselves, are images without originals.  An idea of this kind, however, though perfect when originally formed, may become imperfect through the use of language, when it is unsuccessfully intended to agree with the idea of some other person and denominated by a current term.  In the case of mixed modes and their names, therefore, the compatibility of their elements and the possible existence of their objects are not enough to secure their reality and their complete adequacy; in order to be adequate they must, further, exactly conform to the meaning connected with their names by their author, or in common use.  Simple ideas are best off, according to Locke, in regard both to reality and to adequacy.  For the most part, it is true, they are not accurate copies of the real qualities, of things, but only the regular effects of the powers of things.  But although real qualities are thus only the causes and not the patterns of sensations, still simple ideas, by their constant correspondence with real qualities, sufficiently fulfill their divinely ordained end, to serve us as instruments of knowledge, i.e., in the discrimination of things.—­An unreal and inadequate idea becomes false only when it is referred to an object, whether this be the existence of a thing, or its true essence, or an idea of other things.  Truth and error belong always to affirmations or negations, that is, to (it may be, tacit) propositions.  Ideas uncombined, unrelated, apart from judgments, ideas, that is, as mere phenomena in the mind, are neither true nor false.

Knowledge is defined as the “perception of the connexion and agreement, or disagreement and repugnancy” of two ideas; truth, as “the right joining or separating of signs, i.e., ideas or words.”  The object of knowledge is neither single ideas nor the relations of ideas to things, but the relations of ideas among themselves.  This view was at once paradoxical and pregnant.  If all cognition, as Locke suggests in objection to his own theory, consists in perceiving the agreement or disagreement of our ideas, are not the visions of the enthusiast and the reasonings of sober thinkers alike certain? are not the propositions, A fairy is not a centaur, and a centaur is a living being, just as true as that a circle is not a triangle, and that the sum of the angles of a triangle is equal to two right angles?  The mind directly perceives nothing but its own ideas, but it seeks a knowledge of things!  If this is possible it can only be indirect knowledge—­the mind knows things through its ideas, and possesses criteria which show that its ideas agree with things.