That a dream will sometimes reveal in symbolic form the presence of an illness of which there is no awareness in the waking state has been known from earliest times. Such dreams are called prodromic.

Galen, in his book on Prophecy in Dream, tells of a man who dreamed that his leg was turned to stone, and who developed a paralysis of this leg a few days later. Aristides (died 468 B.C.), the orator, is said to have dreamed in the temple of Euscalapius, where the ancient Greeks were wont to go for inspired dreams, that a bull attacked and wounded him in the knee; on awaking he found a tumour there. Conrad Gessner dreamed that he was stung by a serpent; a plague-boil, so-called, developed on the breast a few days later of which he died. Hammond1 mentions many interesting cases, among them that a man who, for two nights before an attack of hemiplegia, dreamed that he was cut in two down the middle line, from chin to perineum. He was afflicted with paralysis soon after. Another of Hammond's patients dreamt that a man dressed in black and wearing a black mask came and struck him violently on the leg. He experienced no pain, though the man continued to beat him. In the morning he felt nothing but a slight headache. On the fifth day he had a stroke with paralysis involving the limb dreamt of as being struck. Other observers have reported dreams of being gnawed by mice by persons who later showed signs of cancer; dreams of driving sweating, panting horses up hill, or of dying under terrifying conditions by those who later showed heart disease; dreams of suffocation, or flight by those with unsuspected lung disease.

1 Op. cit., p. 159.

H. Addington Bruce 1 describes one of his own dreams which is typical of prodromic dreams. He says:

"At least twenty times during a period of six months I had the same dream - namely that a cat was clawing at my throat. The stage setting and the minor incidents might vary, but always the central episode was the same, and usually the fury of the dream cat's onset was so great that it would awaken me. Naturally, this recurrent dream puzzled me, so much so that I spoke about it. But, ascribing it to indigestion, and classifying it with ordinary nightmares, I did not let it worry me at all.

"Then, one day, the accident of a heavy cold that settled in my throat led me to a medical examination which, much to my surprise, revealed the presence of a growth requiring immediate treatment by the surgeon's knife. Some time afterward it suddenly occurred to me that since the removal of the dangerous growth I had not once been troubled by the cat-clawing dream.

"I had suffered no pain, not even inconvenience, from the growth in my throat. In fact, I had not consciously been aware of its presence. But unquestionably the organic changes accompanying it had given rise to sensations which, slight though they were, had made an impression on my sleeping consciousness sufficient to excite it to activity."

1 Sleep and Sleeplessness, 1915, p. 37-38, Little, Brown & Co.

These dreams are in a sense prophetic, but they are easily explained by natural laws. It is a principle of physiology that strong- stimuli crowd the weaker ones out of consciousness. For example, when there are loud and slight sounds occurring at the same time we are aware only of the loud ones. We may illustrate the principle again by referring to the apparent aggravation of pain at night. During the day there are noises, views, conversations, etc., which either make one unaware of pain, or which obtund it so that it is scarcely felt. With night, however, the noise, tumult, and interests of the day are gone, and thus the pain is able to make a greater-impression on consciousness. To mention another example: During the day the light of the sun makes it seem as if the stars were not present in the sky; when the sun has set we note their presence.

It is, or should be, a well-known fact that disease is insidious in its onset. Indeed, there are countless individuals who believe themselves to be in perfect health who have definite impairments or habits leading to such. These ills may be minor, and correctable if taken in hand early; by putting his finger in a hole in a dike a boy once prevented a flood. Usually it is not until these small impairments have become so aggravated as to cause definite symptoms that we become aware of them, or we learn of their existence more or less accidentally, as when we take a life insurance examination. They do not, at their beginning, make any definite impression on consciousness; the interests of the day, etc., crowd them out. When they become intensified they, in turn, crowd out other stimuli; as a rule, the impairments are fairly well developed by that time.

Sleep is the most favourable time for disease to make known its presence. Vague feelings of discomfort felt during the day, referable to no particular part of the body, are apt to be localized in some particular organ or part by the dream. This is because in sleep we are removed from distractions or stimuli sufficiently strong to attract waking attention; weaker stimuli then have an opportunity to make an impression on consciousness. Whether or not the disease will produce an impression, as a dream, referable to some particular region of the body, depends upon whatever other stimuli reach consciousness. Stimuli reaching the brain from the stomach, for example, may be the stronger, and may blot out that coming from the diseased area; or the stimuli may mix and thus create a more or less confused dream. The ancients were wont to pay particular attention to dreams occurring in the early morning hours, since the stomach was likely to be empty at this time.

It is quite probable that physical ills make themselves known in dreams long before they come to awareness in the waking state. The failure to recognize them may be due to many reasons. Since dreams mean nothing but wild fancies to most people, due mostly to indigestion, no attention is paid them, even though the same dream may occur frequently. Again, unused to the symbols made use of by dreams, the prophetic nature of the dream may not be recognized, even after the disease has made known its existence in waking consciousness. Also, most people forget their dreams, or remember only a few, widely separated details. It is a point worthy of note that some individuals who rarely remember their dreams do so prior to an illness. When a person who usually does not recall his dreams finds that he does so, and vividly, the possibility of some unsuspected disorder being present should be borne in mind; this is especially advisable should the dreams appear to be identical in most respects.