“In like manner, in the course of an alarming dream we are sometimes conscious of making an exertion to save ourselves by flight from an apprehended danger; but in spite of all our efforts we continue in bed. In such cases we commonly dream that we are attempting to escape and are prevented by some external obstacle; but the fact seems to be that the body is at that time not subject to the will. During the disturbed rest which we sometimes have when the body is indisposed, the mind appears to retain some power over it; but as even in these cases the motions which are made consist rather of a general agitation of the whole system than of the regular exertion of a particular member of it with a view to produce a certain effect, it is reasonable to conclude that in perfectly sound sleep the mind, although it retains the power of volition, retains no influence whatever over the bodily organs. “In that particular condition of the system which is known by the name of _incubus_, we are conscious of a total want of power over the body; and I believe the common opinion is that it is this want of power which distinguishes the _incubus_ from all the other modifications of sleep. But the more probable supposition seems to be that every species of sleep is accompanied with a suspension of the faculty of voluntary motion; and that the incubus has nothing peculiar in it but this--that the uneasy sensations which are produced by the accidental posture of the body, and which we find it impossible to remove by our own efforts, render us distinctly conscious of our incapacity to move. One thing is certain, that the instant of our awaking and of our recovering the command of our bodily organs is one and the same. “2. The same conclusion is confirmed by a different view of the subject. It is probable, as was already observed, that when we are anxious to procure sleep the state into which we naturally bring the mind approaches to its state after sleep commences. Now it is manifest that the means which nature directs us to employ on such occasions is not to suspend the powers of volition, but to suspend the exertion of those powers whose exercise depends on volition. If it were necessary that volition should be suspended before we fall asleep, it would be impossible for us by our own efforts to hasten the moment of rest. The very supposition of such efforts is absurd, for it implies a continued will to suspend the acts of the will.