In order that an organism should maintain itself, it must be in a position to make use of natural resources, and protect itself from adverse conditions of every sort. It can accomplish this only if it possesses a knowledge of these adverse conditions, and of such natural resources as it can use; and it can do this better and more surely the more complete this knowledge is. In the more highly differentiated organism it devolves upon the brain and nervous system to acquire knowledge of the outer world, and to turn that knowledge to the advantage of the organism. Memory makes it possible for the brain to perform its task, and the mechanism by which memory is made to serve the purport of knowledge is the association of ideas. For it is clear that a brain, in which a single perception awakens through the operation of the association of ideas a whole train of connected representations, will recognise, conceive and judge far more rapidly than one in which no association of ideas obtains, and which therefore would form only such concepts as had for their content direct sense-perceptions and such representations as originated in those cells which, by the accident of their contiguity, happened to lie in the circuit of a stimulus-wave. For the brain which works with association of ideas, the perception of a ray of light, of a tone, is sufficient, in order instantly to produce the presentation of the object from which the sensation proceeds, as well as of its relations in time and space, to group these presentations as concepts, and from these concepts to arrive at a judgment. To the brain without association of ideas that perception would only convey the presentation of having something bright or sonant in front of it. In addition, presentations would be aroused which had nothing in common with this bright or sonant something; it could form no image of the exciter of the sense, but it would first have to receive a train of further impressions from several or all of the senses, in order to learn to recognise the various properties of the object, of which at first only a tone or a colour was perceived, and to unite them in a single presentation. Even then the brain would only know in what the object consisted, _i.e._, what it had in front of it, but not how the object stood in relation to other things, where and when it had already been perceived, and by what phenomena it was accompanied, etc. Knowledge of objects thus acquired would be wholly unadapted to the formation of a right judgment. It can now be seen what a great advantage was given to the organism in the struggle for existence by the association of ideas, and what immense progress in the development of the brain and its activity the acquirement of it signified.