It is customary to divide dreams into two classes, namely presentative dreams and representative dreams. The first or presentative group are dreams instigated by some stimulus reaching the brain from outside the body or the. physical structures per se. The second or representative group are memory dreams, or dreams which result from mental activity per se.

While stimuli arising outside the mind itself may instigate dreams, comparatively few dreams can be traced to these sources. Doubtless such influences may provoke dreams, but in order to do so they must be powerful enough to make a marked impression on sleeping consciousness. If powerful they may interrupt the dream that is already going on and a new dream may be built about them, just as in waking life something sufficient to attract our attention may dislodge the present thought and instigate a new line of thought. Feeble stimuli are generally disregarded, or fail to make a notable impression on sleeping consciousness. It may happen, however, that a stimulus of moderate intensity may be worked into the dream. For example, I may be dreaming about my office, and imagine that an electric fan is in the room; my dream, however, is not concerned with the fan but with other things. On awaking I may hear a fly buzzing, which will explain the origin of the electric fan. However, the common mistake in such a case would be to attribute all the dream to the fly, where as a matter of fact the fly was responsible for but one incident of the dream.

When one says that such and such a dream was caused by indigestion or other cause, we should bear in mind that the stimulus no more explains that particular dream than dropping a hook into a pond containing miscellaneous fishes would explain why we caught a particular fish. The stimulus merely makes an impression on sleeping consciousness; what the result of this impression will be, depends upon how the mind interprets it. According to the interpretation, certain memories will be called forth, and these memories will vary with the individual. The stimulus is merely a key which opens the door of memories; the same key opens different doors for different persons. The stimulus is responsible only for calling forth a memory; it has not made the memory. While certain specific stimuli may evoke specific dreams in various persons, e.g., dreams of flying due to respiratory and skin disturbances, the number of such are few; in most cases, the same stimuli evoke different dreams. For example, the buzz of a fly may make me dream of an electric fan; another person may interpret the buzz as due to a buzz saw; to another it may cause a dream of a hurricane. We cannot specifically explain why there should be such a difference in interpreting the stimulus, no more than we can explain why the sight of an indifferent object should suggest, when awake, one thing to one person and an entirely different thing to another.

Again, when we speak of certain things as instigating dreams we should not ascribe dreaming as dependent upon these stimuli. Many persons think that unless some impression is made on the mind by an external or physical cause, or by a pronounced mental cause, dreaming is abeyant. We have before given our reasons for believing that dreaming goes on constantly. Accepting this view, stimuli from without or within the body merely govern, more or less, the nature of the dream. They cause dreaming it is true, but no more so than looking at a tree, for example, causes us to think of a forest, a hunt, etc. Thinking was going on before we looked at the tree; likewise dreaming was going on prior to the impression made on the mind by the stimuli.

As regards the power of external influences on the dream, there is scarcely a factor that may not be cited. For instance, one's posture in sleep. Many persons attribute dreams, particularly unpleasant ones, to sleeping on the back, and that this operates to some extent is true. Other persons, who go to sleep on the right side, claim that they have unpleasant dreams if they sleep on the left side; others, used to going to sleep on the left side, say they dream if they sleep on the right side. Doubtless, suggestion plays a part in the latter cases. No matter what side we go to sleep on, we change position many times during the night.

An uncomfortable posture in sleep is mentioned as A cause of distressing dreams, accompanied by a sense of oppression, and nightmare. In changing posture, the contact of one foot with the other has been said to cause dreams of being manacled; raising the arm, fighting; drawing the leg upwards, going up stairs; pushing the leg down, going down stairs. The falling of an arm outside the bed or from an upright position may suggest falling from a height. It is well to note, however, that the position of the limbs may have resulted from the dream, rather than be a cause of the dream. For example, if one is dreaming of going down stairs the legs may carry out the act; in such a case a minor somnambulism would be present. Considering that we shift positions often in sleep, and considering the rarity of dreams like the above, we are justified in discounting somewhat the influence of changes of position of the limbs as instigators of dreams.

The condition of the bed and bedclothes may have an influence. If the bed sags a dream of sinking or falling may be instigated. Dreams of being crushed under a heavy weight have been attributed to the sleeper's head

! being covered by the pillow. Excessive bedclothes, by their weight, may suggest dreams of oppression; by their warmth, fire. Insufficient bedclothes may suggest freezing, or of being in a snow storm; in the latter case, possibly the whiteness of the bedclothes make an impression on the individual prior to his going to sleep. The entanglement of the feet in the bedclothes has been held responsible for dreams of being bound. If the body becomes uncovered, dreams of insufficient clothing may be instigated; a dream of being tortured by savages who were using a giant electric fan was attributed to the exposure of the feet. A tight nightshirt has been ascribed as a cause of dreaming of being hanged; a tight nightcap, of being scalped by Indians; a hot water bottle to the feet, of ascending a hot volcano.