Some things and occurrences, especially if they have happened to us only once and hurriedly, will be reproducible by the memory in respect only of a few conspicuous qualities; in other cases those details alone will recur to us which we have met with elsewhere, and for the reception of which the brain is, so to speak, attuned. These last recollections find themselves in fuller accord with our consciousness, and enter upon it more easily and energetically; hence also their aptitude for reproduction is enhanced; so that what is common to many things, and is therefore felt and perceived with exceptional frequency, becomes reproduced so easily that eventually the actual presence of the corresponding external _stimuli_ is no longer necessary, and it will recur on the vibrations set up by faint _stimuli_ from within. {70} Sensations arising in this way from within, as, for example, an idea of whiteness, are not, indeed, perceived with the full freshness of those raised by the actual presence of white light without us, but they are of the same kind; they are feeble repetitions of one and the same material brain process—of one and the same conscious sensation. Thus the idea of whiteness arises in our mind as a faint, almost extinct, sensation. In this way those qualities which are common to many things become separated, as it were, in our memory from the objects with which they were originally associated, and attain an independent existence in our consciousness as _ideas_ and _conceptions_, and thus the whole rich superstructure of our ideas and conceptions is built up from materials supplied by memory.