The phenomena of sleep have engaged the speculative and philosophic consideration of psychologists from the earliest periods. And although deep-searching investigation has made no approximation toward the fundamental fact of what sleep is, still a vast store of knowledge has been accumulated, from which material may be drawn to define its operations, and determine the laws by which this condition of our being is governed.

The ultimate principles - or what sleep is, and what may be its cause - being, through Infinite wisdom, involved in obscurity so profound, that it can only be dissipated by disclosing the mysterious fountains of life, we pass to the consideration of its existence as a component of our mental and physical being. The phenomena of memory, as presented in connexion with sensation during sleep, will also engage our especial attention.

We shall, in this Essay, endeavor to show, That during sleep, the mental faculties are as active as during wakefulness; That memory is no criterion by which to judge the mind in sleep ; and That the mind is dependent upon the integrity of the organs of external sensation for a remembrance of what transpires during this state.

To establish these propositions, it becomes necessary to examine such of the pathological and normal phenomena of our compound being as bear relation to the subject. The mysterious as well as the more evident, the complex and the comparatively simple states in which our existence is involved, are made in some degree the subjects of our inquiry.

The doctrine, that the mind is as active during our sleep as when we are awake, has long been maintained by some physiologists. A writer upon dreams in the London Penny Cyclopaedia says, "It is a question, whether sleep operates on the mind as well as on the body; whether while it suspends the action of the body, it also, either through the body or otherwise, suspends the action of the mind. This is a question on which we cannot speak positively, and on which our opinion can be determined only by the greater probability of the one side or the other." That the greater probability is on the side of mental activity, we hope, in the course of our remarks, most fully to establish.

Whether memory is dependent on the integrity of the organs of external relation, in the waking as well as in the sleeping state, may be questionable; but many circumstances give plausibility to the suggestion that it may be. Do we remember anything not transmitted to the sensorium through the senses ? If not, any mental operation, independent of sensational organs for its primary stimulus, cannot be retained. When the mind is so riveted upon an idea, that these organs do not convey to the centre of perception external stimuli, there will be no recollection of things felt or seen, although they may have been even powerfully impressed. The fact that memory is a faculty of the mind not fully comprehended, renders it a fit subject of speculation, and thus it may be as. Dr. Reed says, "when philosophers have piled one supposition on another, as the giants piled the mountains in order to scale the heavens, it is all to no purpose - memory remains unaccountable ; and we know as little how we remember things past, as how we are conscious of the present."

In examining the phenomena of mind, with a view to establish the proposition, 'that the mind is dependent upon the integrity of the organs of external sensation, for a remembrance of what transpires during sleep,' it becomes necessary to present the psychological foundation upon which this conclusion is based. We should remark, however, that with the doctrine of essences, or ontology, we have little to do ; but that in the science of mind, in its more limited acceptation as regards its psycho-physiological phenomena, is to be found the sphere of our observations.

The mind, then, in the field of our labors, we regard as a unit, and as acting and being acted upon in one direction, through sensation, by external influences; and in the other, by the innate powers of the organ of its own manifestation. The brain, notwithstanding the unity and immateriality of the mind, is so organized that particular parts are designed as fit instruments to reveal the properties of mind separately considered. The faculty of perception, the flow of feeling, and the power of thought, are elementary modes of organic activity by and through whose agency we know our position, exert our energy, and comprehend our relations. By a combination of these elements we are enabled to realize the outward world, and to grasp the results which arise from a contemplation of its phenomena. It is to this intermediate, this connecting link between the world of mind and the world of matter, that our philosophy is confined. How matter is so sublimated that the spirit principle is enabled to realize it, or how mind is so conjoined to matter as to be cognisant of the operations connected with our material being, are problems to be solved, if ever solved, in the higher walks of metaphysical research.

External and internal sensations, and the powers of reflection, are the stimuli to these elementary principles or their special organs. Of these several sources of knowledge, we hold external sensation alone, as contributing to the permanency of our mental acquirements. It is at least doubtful whether the understanding or reason contributes any knowledge whatever. The facts acquired through sensation, are only combined and arranged by reason, and the results are brought into practical existence by the power of the will.

In the process of recollecting, the reflective faculty casts its stimulating influence through the mass of ideas already accumulated by observation, until it falls upon some one bearing relation to the subject in question, when, by association, that particular catenation is aroused, and in this manner the idea sought is quickly brought to view. This rationale having relation to ideas of outward existence alone, it may be asked, whether the mind has no power of recalling into mental being the memory of previous impressions from the vast field of the emotions? We answer, it has not. It may create a new emotion, but cannot recall one which has passed. When by the power of recollection we bring to view pictures of material objects not then present to the senses, we merely bring up what was previously deposited in the mind; the idea of the impression - not the impression itself; it is a re-presentation - not a creation. This is a memory of the fact: but through the reappearing of the idea, the feelings may be stimulated in their activity, and an emotion produced identical with the one caused by the impression on which the idea was originally formed, and which may again be incorporated in the mental process. This is not the memory of an idea, but the creation of an emotion. An organ of feeling has but one definite action; and whenever excited, it is the creation of a new emotion, not the memory of one gone by. Its action can neither be induced nor varied by any direct intellectual effort, but may be indirectly stimulated through association with the perceptions, as it requires a repetition of the same stimulus to excite the emotion which originally induced it. In the re-excitement of an organ of internal sensation, we have its mode of action, which presents to the mind an actual existence - the thing itself. Memory is only a recall of ideas already formed - not a creation of them. On the contrary, when recollecting, the mind can re-assemble the ideas that originated in perception of material impressions, in proportion to the perfection of the ideas, and within the range of each individual capacity. In this recall of ideas without reference to their origin, we recognise a distinct power of the mind, operating through the organs of perception, which we consider to be memory.

Memory, as before stated, is a mode of activity of the organs of perception stimulated by external sensation; but recollection or voluntary memory, i. e. the power of recalling into thought the memory of past perceptions and their associate ideas, is an attribute distinct from what may be termed passive memory, or the mere remembrance of things unconnected with collateral circumstances. That volition may again and again bring up all these associations by an act of recollection, is undoubtedly true, for they have become the property of the mind, to be used as occasion may demand.

To distinguish consciousness from memory, we would say that the former does not depend upon the latter for existence, because consciousness is ever present, ever living. The consciousness of one period, and the consciousness of another, are connected by a variety of circumstances; and it is an attribute of memory, through these circumstances, to combine these different periods of existence, making our lives one harmonious whole. When by disease the memory becomes impaired, it does not affect the consciousness. In such instances, the sources of memory, not consciousness, are involved. In many cases of insanity, the individual imagines himself to be another person ; and through impairment of memory, it is probable that identity, not consciousness, is lost: identity being the result of memory and consciousness combined. We are acquainted with a lunatic irrecoverably deranged, who believes himself to be the Lord Mayor of London, being originally from that city. But notwithstanding his imaginary social elevation, he knows himself; he answers to his name; and in some particulars is aware of his actual condition. The chain of his identity is deranged and broken, but his consciousness is perfect, because it is a resistless, ever-present fact, like a full gushing fountain, springing unceasingly into existence.