I had a very clear, very persistent and very precise dream, during which I imagined that I was in Brussels (which I had never visited). I was walking quietly along a very lively street, flanked by numerous shops whose colourful posters stretched out their large arms over the passers-by.

"Here is something very singular," I said to myself, "it is really not to be supposed that my imagination invents so many details. To believe, as the Orientals do, that the spirit travels on its own while the body sleeps, does not seem to me to be a hypothesis that can be accepted either. And although I had never visited Brussels, nevertheless there was the famous church of Sainte-Gudule in perspective, which I knew from having seen engravings of it. I do not have the feeling of having walked this street in any city. If my memory can retain such detailed impressions, even without my knowledge, the fact deserves to be noted; it will certainly be the subject of a curious verification. The essential thing is to operate with positive data and, consequently, to observe well."

I immediately set about examining one of the shops with great attention, so that, if I should one day recognise it, I should not have the slightest doubt. It was a hosier's shop, in front of which I imagined I stood, that became the focus of my mind's eye in this imaginary world. I first noticed, as a sign, two crossed arms, one red and the other white, projecting over the street, and surmounted by a huge striped cotton cap like a crown. I read the merchant's name several times, to remember it well; I noticed the number of the house, and the pointed shape of a small door, decorated at the top with an interlaced figure.

Then I shook off the dream with that violent effort of will which one can always make when one becomes aware of being asleep, and, without giving time for these vivid impressions to fade, I hastened to record and draw all the details with great care. I was going to visit Brussels a few months later, and I would spare no pains to elucidate a fact which, at first sight, without being able to help it, inspired me with the most fantastic speculations. I awaited the moment when my family should go to Belgium with indescribable impatience.

When I arrived, I hastened to the church of Sainte-Gudule, which seemed to me an old acquaintance; but when I looked for the street with the multiform signs and the dreamed-of shop, I saw nothing, absolutely nothing, that resembled it. In vain did I methodically wander through all the commercial districts of this attractive city; I had to recognise the futility of my quest and resign myself to abandon it.

To tell the truth, I would have been more frightened than delighted by an unexpected success, which would necessarily have thrown me into the regions of fantasy and wonder. Now that I knew it was only a psychological phenomenon that probably had some explanation, although I did not know whether I could ever grasp it, I resumed more calmly the conscientious analysis of the phenomena accessible to human research.

Several years passed. I had almost forgotten this episode of my adolescent preoccupations, when I happened to travel through various parts of Germany, where I had already been during my younger years.

When I was in Frankfurt, quietly smoking a cigarette after my lunch, and walking without following any itinerary, I entered the Judengasse, and a whole series of indefinable reminiscences began to vaguely take hold of my mind. I tried to discover the cause of this singular impression; I suddenly remembered the purpose of my useless walks in Brussels. Sainte-Gudule was no longer visible in perspective, but it was the same street which I had drawn in my dream-diary; it was the same whimsical signs, the same public, the same movement which had once struck me so vividly in my dream.

One house, as I have already said, had been the object of my closest scrutiny. Its appearance and its number were strongly impressed on my memory. I therefore ran in search of it, not without real excitement. Was I to meet with a new disappointment, or, on the contrary, was I to grasp the last word of one of the most interesting problems I had studied?

One can imagine my astonishment, and at the same time my joy, when I found myself in front of a house so similar to the one in my old dream that it almost seemed to me that I had gone back six years and had not yet woken up.

In Paris, I would have been very lucky if I had found that characteristic door, with its ancient crown or the traditional sign with the merchant's immutable name.

But in Frankfurt, where the demolition fever was far from having wreaked the same havoc, I had the satisfaction of seeing confirmed the opinion I had formed for so long, both of the formation of memory clichés, without the knowledge of the person who picks them up [in modern terms, unconscious memories], and of the sharpness of the images that these clichés can reproduce, in a dream, before our mind's eye.

I had evidently already walked along this street the first time I was in Frankfurt, that is to say, three or four years before the moment of my dream, and, without suspecting it, without being able to explain to myself on what particular arrangement it depended, all the objects exposed to my view were instantly photographed in my memory with admirable precision.

My attention, however, in the usual sense of the word, must have remained oblivious to the mysterious work that was spontaneously unfolding, for I had not retained even the slightest sensible recollection of it.

This is a subject of serious reflection for anyone who wishes to probe into the secret forces of human understanding.