Thus far the procedure of the understanding has been purely passive.  But besides the capacity for passively receiving simple ideas, it possesses the further power of variously combining and extending these original ideas which have come into it from without, of working over the material given in sensation by the combination, relation, and separation of its various elements.  In this it is active, but not creative.  It is not able to form new simple ideas (and just as little to destroy such as already exist), but only freely to combine the elements furnished without its assistance by perception (or, following the figure mentioned above, to combine into syllables and words the separate letters of sensation).  Complex ideas arise from simple ideas through voluntary combination of the latter.

Perception is the first step toward knowledge.  After perception the most indispensable faculty is retention, the prolonged consciousness of present ideas and the revival 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.