“Truly, the human brain is a very delicate instrument, and one that is easily thrown out of gear.  Before I proceed, just sum up for yourselves the facts that I have mentioned:  a light seen and presently extinguished in an apartment supposed to be uninhabited; and a cat of a remarkable color, which appeared and disappeared in a way that was slightly mysterious.  Now there isn’t anything very strange about that, is there?  Very well.  Imagine, now, that these unimportant facts are repeated day after day and under the same conditions throughout a whole week, and then, believe me, they become of importance enough to impress the mind of a man who is living all alone, and to produce in him a slight disquietude such as I spoke of in commencing my story, and such as is always caused when one approaches the sphere of the unknown.  The human mind is so formed that it always unconsciously applies the principle of the causa sufficiens.  For every series of facts that are identical, it demands a cause, a law; and a vague dismay seizes upon it when it is unable to guess this cause and to trace out this law.

“I am no coward, but I have often studied the manifestation of fear in others, from its most puerile form in children up to its most tragic phase in madmen.  I know that it is fed and nourished by uncertainties, although when one actually sets himself to investigate the cause, this fear is often transformed into simple curiosity.

“I made up my mind, therefore, to ferret out the truth.  I questioned my caretaker, and found that he knew nothing about my neighbors.  Every morning an old woman came to look after the neighboring apartment; my caretaker had tried to question her, but either she was completely deaf or else she was unwilling to give him any information, for she had refused to answer a single word.  Nevertheless, I was able to explain satisfactorily the first thing that I had noted—­that is to say, the sudden extinction of the light at the moment when I entered the house.  I had observed that the windows next to mine were covered only by long lace curtains; and as the two balconies were connected, my neighbor, whether man or woman, had no doubt a wish to prevent any indiscreet inquisitiveness on my part, and therefore had always put out the light on hearing me come in.  To verify this supposition, I tried a very simple experiment, which succeeded perfectly.  I had a cold supper brought in one day about noon by my servant, and that evening I did not go out.  When darkness came on, I took my station near the window.  Presently I saw the balcony shining with the light that streamed through the windows of the neighboring apartment.  At once I slipped quietly out upon my balcony, and stepped softly over the ironwork that separated the two parts.  Although I knew that I was exposing myself to a positive danger, either of falling and breaking my neck, or of finding myself face to face with a man, I experienced no perturbation.  Reaching the lighted window without having made the slightest noise, I found it partly open; its curtains, which for me were quite transparent since I was on the dark side of the window, made me wholly invisible to any one who should look toward the window from the interior of the room.