Dreams are vagaries of the imagination, and in most instances proceed from external sensations. They take place only when our sleep is unsound, in which case the brain and nervous system are capable of performing certain motions. We seldom dream during the first hours of sleep; perhaps because the nervous fluid is then too much exhausted; but dreams mostly occur towards the morning, when this fluid has been, in some measure, restored.
Every thing capable of interrupting the tranquillity of mind and body, may produce dreams; such are the various kinds of grief and sorrow, exertions of the mind, affections and passions, crude and undigested food, a hard and inconvenient posture of the body. Those ideas which have lately occupied our minds or made a lively impression upon us, generally constitute the principal subject of a dream, and more or less employ our imagination, when we are asleep.
Animals are likewise apt to dream, though seldom; and even men living temperately, and enjoying a perfect state of health, are seldom disturbed with this play of the fancy. And, indeed, there are examples of lively and spirited persons who never dream at all. The great physiologist Haller considers dreaming as a symptom of disease, or as a stimulating cause, by which the perfect tranquillity of the sensorium is interrupted. Hence, that sleep is the most refreshing, which is undisturbed by dreams, or, at least, when we have the distinct recollection of them. Most of our dreams are then nothing more than sports of the fancy, and derive their origin chiefly from external impressions; almost every thing we see and hear, when awake, leads our imagination to collateral notions or representations, which, in a manner, spontaneously, and without the least effort, associate with external sensations. The place where a person whom we love formerly resided, a dress similar to that which we have seen her wear, or the objects that employed her attention, no sooner catch our eye, than she immediately occupies our mind. And, though these images associating with external sensations, do not arrive at complete consciousness within the power of imagination, yet even in their latent state they may become very strong and permanent.