l of those which have disappeared, or, as it were, have
been put aside. For an idea to be "in the memory" means that the mind
has the capacity to reproduce it at will, whereupon it recognizes it as
previously experienced. If our ideas are not freshened up from time to time
by new impressions of the same sort they gradually fade out, until finally
(as the idea of color in one become blind in early life) they completely
disappear. Ideas impressed upon the mind by frequent repetition are rarely
entirely lost. Memory is the basis for the intellectual functions of
discernment and comparison, of composition, abstraction, and naming. Since,
amid the innumerable multitude of ideas, it is not possible to assign to
each one a definite sign, the indispensable condition of language is found
in the power of abstraction, that is, in the power of generalizing ideas,
of compounding many ideas into one, and of indicating by the names of the
general ideas, or of the classes and species, the particular ideas also
which are contained under these. Here is the great distinction between
man and the brute. The brute lacks language because he lacks (not all
understanding whatever, _e.g._, not a capacity, though an imperfect one, of
comparison and composition, but) the faculty of abstraction and of forming
general ideas. The object of language is simply the quick and easy
communication of our thoughts to others, not to give expression to the real
essence of objects. Words are not names for particular things, but signs
of general ideas; and _abstracta_ nothing more than an artifice for
facilitating intellectual intercourse. This abbreviation, which aids in
the exchange of ideas, involves the danger that the creations of the mind
denoted by words will be taken for images of real general essences, of
which, in fact, there are none in existence, but only particular things. In
order to prevent anyone to whom I am speaking from understanding my words
in a different sense from the one intended, it is necessary for me to
define the complex ideas by analyzing them into their elements, and, on the
other hand, to give examples in experience of the simple ideas, which do
not admit of definition, or to explain them by synonyms. Thus much from
Locke's philosophy of language, to which he devotes the third book of the
_Essay_.

Complex ideas, which are very numerous, may be divided into three classes:
Modes, Substances, and Relations.

_Modes_ (states, conditio