The cause of dreams has excited various speculations. This waking sleep, or sleeping activity, appeared to Baxter so inconsistent, that he supposed immaterial spirits were amused, or engaged in suggesting these plays of fancy, and sometimes, perhaps, conveying important information. We cannot deny the existence or employment of these spiritual agents; but can scarcely conceive, in the whole circle of creation, beings so useless. They are, however, wholly unnecessary; for, in the pathology of dreaming, we have seen that the cause is purely corporeal; and, indeed, Baxter's opinion is, we believe, consigned to the same oblivion with that which looks to dreams as foretelling future events.

Wolfius supposed a previous excitement of some part necessary to suggest to the fancies during sleep; and, in fact, delivers the fictions of Shakspeare in the garb of sober sound philosophy. This opinion is, however, inconsistent with the phenomena of dreaming; and we have no modern idea on this subject which need detain us, except the opinion we are about to explain, originally derived from Dr. Cullen.

In sleep, Dr. Cullen observes, that there is a partial collapse of the brain, at least so far as respects the animal functions; and this partial diminution of excitement is shown by the delirium which occurs in the interval between the sleeping and waking state. To different degrees of the collapse, dreams, more or less vivid, appear to be owing. Dreams, indeed, are of the nature of delirium. Similar heterogeneous or disjointed ideas constitute both; and the whole must be resolved into that unequal balance of the nervous power in the brain arising from diminished energy. The proof of this is the want of the usual associations; another, the deficiency of judgment; a function, when perfect, which arises, as we have seen, from a free communication between the different parts of that organ. The defect of judgment is seen from a high opinion we form of what we suppose we have written or said in our dreams, which is often ridiculous, and strangely heterogeneous.

Yet, it may be asked, are no ideas suggested in our dreams which deserve the attention of our awakened judgment ? We have heard of some such suggestions, and suspected that we have experienced them; but they consist only of a happy recollection, or a new combination, which we are sufficiently awake to be pleased with, and rouse ourselves to recollect. We remember dreaming of being asked for a motto for an air balloon, and immediately suggested the following:

Tentanda via est qua me quoque possum Tollere humo victorque virum volitare per ora.

Every instance of peculiar genius or supernatural information in dreams may be referred to one-of the sources just mentioned.

Though this partial collapse explains, in some degree, the heterogeneous combinations of dreams, it does not show us how the images are excited. It will be at once obvious that this question includes one to which no answer can probably be given, viz. in what manner do sensible impressions act on the mind to produce ideas, or how, in turn, does the mind act on the brain, by means of volition, to excite action ? We may hazard a few speculative remarks, which, if ill founded, will detain the reader but a short time. We have seen that the power which resides in the brain and nerves is probably a subtile fluid, capable of vibrations, and that its action consists in these vibrations. If, then, such have been excited by sensible impressions, we know that they may be renewed by powers more inconsiderable; and it is not impossible even that the motion of the blood, in those parts of the brain where the mobility of the fluid is considerable, may excite vibrations, and these be attended with the same ideas which originally followed them. In constitutions where the nervous fluid is particularly mobile, or in cases where the circulation is hurried, dreams will be more frequent, more vivid, and often more troublesome.

Dreams, we have said, are useful in affording prognostics in various diseases. When the dreams are hurried and violent, we have often reason to expect delirium: when the mind in fevers is gloomy and distressed, and the dreams frightful, the fever soon becomes dangerous; but when in sleep, the fancy is soothed by pleasing images, the disease is seldom alarming. In these cases, the consequences are probably those of terror on one side, and complacency on the other; and the peculiar states of fever may not be the causes of the dreams. Yet, when we recollect that, in other instances, diseases of the body excite dreams of distress and horror, we must think that the more violent corporeal affections are really the causes of the terrific visions. In either case they arc often useful prognostics.

If any part of the brain is more easily excited to action by having been previously impressed, the mind, on the contrary, becomes more insensible. Thus, the person used to distress is comparatively calm in witnessing the most painful scenes: the mind, once harrowed with horror, will bear common trouble without emotion. This will account for the fancy not always pursuing, in sleep, the painful scenes of the day, or rather for their not being followed by pungent distress, so that they escape the recollection when we are awake.

Is it a fact that the dreams of children are more terrific than those of adults ? Infants certainly dream, and seldom awake in terror; in fact they know not the tendency of objects, and are not capable of appreciating their effects so as to be terrified. When they are farther advanced, the tales of the nursery often inspire horrible images,which may again return in their dreams; or their systems, more mobile and irritable than those of adults, may be more susceptible of violent impressions. We could wish, however, that the fact were more decidedly ascertained.

Animals dream, particularly dogs; but in these visions they are more often pleased than terrified, though sometimes angry. Their bark is different, and the voice of those who talk in their sleep is usually altered. We have no evidence of the feathered race being subject to these nocturnal visitors; and perhaps they are connected, like associations of ideas, with the proportional bulk of the brain.

See Baxter on the Immateriality of the Soul; Lord Monboddo's Works; Hartley on Man; Wolfius's Ontology; Cullen's Physiology; and Lommius.