Another problem which might well be examined by the student of dreams is one which we all at times tried to solve for ourselves: where does the actual border-line lie? For we never know the moment when we cross over it, or even the moments when we approach it most nearly. In childhood, and long after childhood ended, most of us have tried very hard, but tried in vain, to keep a watch so vigilant, as we approached the border, that we should know the moment of our crossing it.

It is indeed strangely tantalising that this mystery of sleep should happen nightly, without our getting any nearer to a consciousness of its actual oncoming. Every night our normal mind abdicates its power, and the dream mind comes to its own, waking into activity, and taking the reins into its own hands. But the moment of that mysterious transition is always effectively veiled from us. The nearest I have ever been able to get to a realisation of it has been when I have been reading rather late into the night Sleep is approaching; the dream mind has already started on its activities and has set in motion a train of thought or dream story. The page of the book that I am reading still lies open before my eyes, and though it is becoming rather indistinct, it has not yet wholly gone from my sight; my mind is fast "losing hold," but I am not yet asleep. Suddenly, as though it were in the middle of the printed page, I read a sentence which does not belong there at all, an alien sentence wholly disconnected from the subject or sense of the book. To what does this belong? I am conscious that it has come from elsewhere, that it was part of a definite sequence of ideas or story which was being carried on on another "floor" of my mind, by another part of my brain.

This story or thread of ideas was interrupted by the normal mind momentarily resuming its functions and supremacy. The two quite different strains of thought which were being carried on simultaneously have crossed each other, and I am for the moment aware of both. One night when this occurred, the book I was reading lay at a little angle to my eyes and I noticed that, whilst this made the lines of the printed page slope upwards from left to right, the interposed alien sentence seemed to be written at an obtuse angle to these, cutting diagonally across the printed lines. "It is like a weaver's warp and woof," I thought; and this is indeed the truest image that can be made of these interwoven and crossing strands of thought Instances of such interweaving could be multiplied indefinitely by anyone sufficiently interested to make note of them at once, but the memory of the interposed idea is so evanescent that it fades away with extreme quickness and cannot be recalled. Drifting slowly towards sleep one night I was thinking over some public work that had taken up all the time and energy of the day.

Suddenly across these thoughts there "came through" a clear-cut sentence belonging to a wholly different set of ideas, intercepted - as a scrap of a conversation is overheard on the telephone, or a portion of an alien "wireless" message is "tapped" by a Marconi operator. The intercepted fragment ran thus: " Haunted by the pirate ship." For one instant before the memory of it faded out I remembered the context that this belonged to; I remembered a ship with sails. I knew that it was somehow connected with piracy on the high seas, and that the story had to do with ships engaged in the oil-carrying trade, but the memory of it all faded away almost instantaneously, as indeed always happens; only the words of the intruding sentence remaining for a little while printed on my mind.

Another similar note was made at about the same hour of the night. My mind had been dwelling on the anxieties of the war and on certain war work that was occupying me. The words that "came through" were as remote as they well could be from these thoughts; they were - "newly fledged birds on a tree - all grey," and with these words, just for one brief second, the picture they referred to also came back. I remembered a row of very tiny birds perched on a grey bough - "not a tree," I said to myself, "it was a bough only." They were small fluffed-out tilings, their breasts ruffled by a little gust of wind which disturbed the downy feathers, making little waves like the waves that the wind makes when lightly blowing over grass. "All grey and white," I thought, "and the bough grey too." And then I realised that all this was an intercepted bit, taken out of quite a different train of thought, and that the context to which the words and the grey picture belonged had wholly disappeared.

It is probably true that, not only in the twilight time between sleeping and waking, but also by day, the mind and the subconscious mind are often at work simultaneously on different trains of thought; but if this be the case the normal mind is generally so dominant that no message can penetrate through to interrupt it; and only at times, when a drowsy condition causes it to lose its grip and mastery, can the working of the subconscious mind be perceived. The rare moments when we thus become aware of this duplicate working may have a certain value in the study of dreams, and we have to glean what we can from them, for the difficulty of tracing dream origins is great, and the sources of our knowledge about the dream mind are so limited that we cannot afford to disregard any clue that may give us further insight into them.

There are, for instance, many dreams whose central idea is gathered from a book, generally a book that has lately been read, or that we are reading at the time. The book, like everything else that the dream mind makes use of, will be completely metamorphosed; but some leading idea or some character taken from it will be carried on into the dream. It is sometimes possible to study the actual process by which this is effected in the state where waking and sleeping shade into one another.

I was reading late at night one of Arnold Bennett's chronicles of English midland life, and as I read sleep must have approached. Between sleeping and waking my mind wandered from the book, and a new and different story superimposed itself and ousted the other. The hero of the book remained the principal actor; his name and characteristics were unchanged, but he was placed in an entirely new setting. A wild and unsettled prairie country with steeply undulating outlines now made the background of his adventures. Great caves with ramifying passages sheltered a group of men - pur-sued or pursuing - against whom he was pitted in a fierce but unequal struggle. I took no part personally in the drama, but was following it step by step, when the thread of it broke suddenly. The light in the room seemed to grow brighter; my eyes still rested on the open page where the hero's name lay before me. For the moment the prairie setting was so much more present to me than the scenery of the "Five Towns" that I could not convince myself quickly that it was not to be found in the book at all and was simply the creation of the dream mind, although sleep had not actually closed my eyes.

The intensification of light had acted, as it so often does, as a warning signal just when the moment of true sleep was approaching, and the story that belonged to the borderland of dreams was brought to an abrupt close.

Now, these methods of the dream mind, the qualities of its imagination, its habit of seizing upon part of an argument or part of a story to weave into something new and strange, all these are quite unlike the ways of the normal mind that is familiar to me by day. The difference in their methods of working is so sharply defined that I should seldom have to question which was the author of a particular train of thought. They work differently and they arrive at different results; and herein lies undoubtedly a great part of the unexpectedness and charm of the dream mind. We find in it something of the attraction that is to be found in friendship with one whose outlook is not quite like our own and who brings to it qualities of mind that delight us and that are not ours.