_Mental Faculties not immediately dependent on the Will._--If any of the mental faculties can be shown to be entirely dependent on the will for their activity and operation, so as to have no power to act except by its order or permission, then it would follow that when the will is no longer in possession of the throne, when its sway is for the time suspended as in sleep, the faculties thus dependent on it must lie inactive. But with regard to most if not all mental operations, we know the reverse to be true. They are capable of _spontaneous_, as well as voluntary action. Nay, some of them, it would seem, are not subject, in any case, directly to its control. It is not at our option whether to remember or forget, whether to perceive surrounding objects, whether such or such a thought shall, by the laws of association, follow next in the train of ideas and impressions. Some mental operations are more closely connected with and admit of a more direct interference on the part of the will than others, but it cannot be shown, I think, that any faculty is so far dependent on the will as not to be capable of action, irrespective of its demands. Indeed, facts seem to show that where once a train of mental action has been set in operation by the will, that action goes on, for a time, even when the will is withdrawn, or held in abeyance, as in sleep, or profound reverie. _Whence this Suspension of Power of the Will._--The question may occur, whence arises this suspension of the power of the will over the mental operations in sleep? What produces it? Does it, like the loss of voluntary power over the physical frame, result from the inactivity of the nervous apparatus? The fact that it always accompanies this, and is found in connection with it, that whatever produces the latter seems to be the occasion, also, of the former, as in the case of disease, delirium, mesmeric influence, stupefying drugs, inebriation, etc., and that the degree of the one, whether partial or complete, is in proportion to the degree of the other--these facts seem to me to favor the idea now suggested. _Summary of Results._--These, then, seem to be the principal phenomena of sleep: loss of sensational consciousness, loss of voluntary power over the body, loss of voluntary power over the operations of the mind.