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The memory also seems to be especially affected by physical conditions. Fatigue and nervous exhaustion for the time being greatly diminish its power; some forms of disease erase its impressions in whole or in part, while the weakness of age first betrays itself in this faculty. Since, then, physical conditions so obviously and directly modify this power, it is but natural to expect that so great a change as that from wakeful activity to sleep might decidedly affect its action. The thoughts which pass through the mind in revery or abstraction often leave very slight traces. Suddenly startled from such a waking dream by a practical claim, we can scarcely, the moment after, recall what it was which so occupied us. These facts are sufficient to overcome the antecedent improbability of continuous mental action, arising from the want of memory, and to leave the way open for proof.

The most obvious facts which go to establish the constant activity of mind are dreams. The memory does testify to a large amount of movement in hours of sleep not to be distinguished by external signs from other periods of repose. Some habitually dream: that is, the play of imagery, the dumb show in the hours of darkness, the spectral troop of the sportive thoughts pass and repass within the scope of mental vision, and the person, on waking, remains mindful of this fleet, flitting assemblage of this masquerade of his thoughts escaping the control of the senses and the voluntary life. Now, though others rarely dream, that is, rarely recall these shadows of the mind, leaving no more visible traces on the external life than do the clouds that fly through the heavens on the earth which they darken for the moment, this fact goes but a little way to weaken the presumption, that they are not very different from their fellows; that the rehearsal of dreams is only a little more interior and close-locked in the one case than in the other.