"Quite so," he answered, lighting a cigarette, and throwing himself down into an armchair. "You see, but you do not observe. The distinction is clear. For example, you have frequently seen the steps which lead up from the hall to this room."

"Frequently."

"How often?"

"Well, some hundreds of times."

"Then how many are there?"

"How many? I don't know."

"Quite so! You have not observed. And yet you have seen. That is just my point. Now, I know that there are seventeen steps, because I have both seen and observed. By the way, since you are interested in these little problems, and since you are good enough to chronicle one or two of my trifling experiences, you may be interested in this." He threw over a sheet of thick, pink-tinted note-paper which had been lying open upon the table. "It came by the last post," said he. "Read it aloud."

The note was undated, and without either signature or address.

"There will call upon you tonight, at a quarter to eight o'clock," it said, "a gentleman who desires to consult you upon a matter of the very deepest moment. Your recent services to one of the royal houses of Europe have shown that you are one who may safely be trusted with matters which are of an importance which can hardly be exaggerated. This account of you we have from all quarters received. Be in your chamber then at that hour, and do not take it amiss if your visitor wear a mask."

"This is indeed a mystery," I remarked. "What do you imagine that it means?"

"I have no data yet. It is a capital mistake to theorize before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. But the note itself. What do you deduce from it?"

I carefully examined the writing, and the paper upon which it was written.

"The man who wrote it was presumably well to do," I remarked, endeavouring to imitate my companion's processes. "Such paper could not be bought under half a crown a packet. It is peculiarly strong and stiff."