What are our own sensations when we _go to sleep_? Thought wanders. Ideas come straying into the mind unbidden and with no apparent association. External objects grow dim to the eye and sounds fall faint upon the ear. The communications of the senses to the brain are dull and uncertain. We are conscious that the power of the _Will_ is relaxed. We strive to retain it. We recover it by an effort. We resume the work on which we were engaged. Vain the struggle. The thoughts wander still. The unbidden pictures flit again before the mind’s eye. We are conscious of the relaxation of the limbs and the closing of the eyelids. Then we cease to be conscious of external existence. We sleep. But we are not conscious of _the act_ of falling asleep—for itself is a suspension of consciousness. With some sleepers sleep is, as they affirm, a condition of entire unconsciousness. These tell us they have no sense of existence until the moment of waking and that, however protracted their slumber, the moment of waking is to them as the moment after having fallen asleep. It is impossible to contradict those who thus affirm, for their mental condition in sleep cannot be read. But if a judgment may be formed from their _actions_ in sleep, as talking and motions of the limbs, the probable explanation will be that they dream but do not remember their dreams. _All_ dreams vanish from _their_ memories as _some_ dreams vanish from the memories of those who habitually dream. If we observe the aspect of a sleeper, we note the features placid, the breathing regular, the pulse soft and even, the limbs relaxed, the skin moist. Occasionally there are quiverings of the limbs and expressions of the face which betray the presence of mental emotions. This is the _physiological_ condition of Sleep. We turn now to its _mental_ condition. CHAPTER III. THE MENTAL CONDITION OF SLEEP. Of all the phenomena exhibited in Psychology and Mental Physiology there is none more marvellous than that which is presented to every one of us every night. It only does not astonish us because it is so familiar. Perhaps the reason why so few have given a moment of reflection to its marvels is because they are seen so often. When the attention of the reader is more closely invited to these phenomena he will doubtless be surprised to find what a world of wonder is opened to him.