A very slight withdrawal of the attention from surrounding objects is all that is necessary to enable artists and some other persons to call up these images with vivid distinctness, since even in the waking state the image may for the moment appear to be actually before them.  Any one might attain to the same power of verification if the transition from the real to the merely ideal image were not in the waking state so instantaneous and easy; whereas in a dream the state of illusion is uninterrupted, and it is physiologically impossible for the mind to pass immediately from the image, which is believed to be real, to the simply representative idea of the thing.

Even in the waking state, the image and representative idea of the thing naturally tend to become, or to appear to be, actual realities, even in a strictly normal condition of mind and body.  Nor do they only implicitly tend to become such by the innate impulse of the mind, but they actually become so in fugitive moments of which man is scarcely conscious, and they appear to him exactly as they do in dreams.  Hence it follows that there is no hard and fast line between the sleeping and waking states, so far as the nature of images, their source, action, and combinations are concerned, when men are distracted in mind, and the course of their thoughts is not voluntarily directed to some definite object; so that by a psychological process the phenomena of the waking state may be partly transformed into those of dreams.  The vivid character of the image, presented to the senses as if actually there, is common to both phenomena.  The way in which we begin to dream shows how, owing to our physiological conditions, we pass through regular stages from the waking state into that of sleep.

  “Nuovo pensiero dentro a me si mise,
     Dal qual piu altri nacquero e diversi;
     E tanto di uno in altro vaneggiai
     Che gli occhi per vaghezza ricopersi,
   E il pensamento in sogno trasmutai."[33]

So Dante writes in the “Purgatorio” with deep and subtle truth.  Each man can verify for himself the exactness of the great poet’s description.

I myself can readily study the phenomena of dreams, since I never sleep without dreaming so vividly that I remember all the circumstances in the morning.  I have used all sorts of artifices in order to trace the beginning of sleep and dreams, and always with the same result, so that I am certain of the accuracy of experiments which have been repeated a hundred times.  I have examined other persons who have made the same observations, all of whom agree with me.