Watchfulness invariably reduces, even in the brute: the wild elephant is tamed by the perseverance of the hunter in keeping it constantly awake. The mind, then, as it is manifested to us (for deeply important is it that we confound not the perfect and pure, because unembodied essence of the soul, with its _combined_ existence in the brain—that union from which a _thought is born_), the mind cannot exert itself beyond a certain period without a sensation of fatigue in the brain, as palpable as the exhaustion from excessive muscular exertion. And this depends on a natural law, that organs after acting a certain given period, flag and lose their energy. Thus the first harbinger of sleep is the _closing of the lids_ from languor, and relaxation of the muscles. Muscular fibre will, however, regain its expenditure by simple rest, requiring a certain period for this re-accumulation, like the charging of an electrical jar. Sleep, however, is not always a sequence of exhausted irritability of muscle; we may be _too tired_ to sleep; and thought and memory also will keep the mind awake, and prevent nervous energy from renewing corporeal vigour. The excitement of thought beyond certain limits is both painful and destructive, evincing its effects by various grades of mental disorder, from simple headache to confirmed mania. Our first ray of hope, in fever, is often the coming on of a quiet sleep, and in the sad cases of _delirium tremens_ we must either _sleep or die_; the effort of philosophical determination to overcome the depression only adding to its intensity, as in the case of a person worn out by labour, in attempting to labour on. This conflict cannot be more pertinently exemplified than by some passages in the life of Collins, by one who knew him well:— “He languished some years under that depression of mind which unchains the faculties without destroying them, and leaves reason the knowledge of right without the power of pursuing it. These clouds which he perceived gathering on his intellects, he endeavoured to disperse by travel, and passed into France; but found himself constrained to yield to his malady, and returned. His disorder was no alienation of mind, but general laxity and feebleness, a deficiency rather of his vital than intellectual powers. What he spoke wanted neither judgment nor spirit, but a few minutes exhausted him, so that he was forced to rest upon the couch, till a short cessation restored his powers, and he was again able to talk with his former vigour.” I believe that sensibility and fatigue of mind, by inducing sleeplessness, may often be the source even of mania.