“From a drop of water,” said the writer, “a logician could infer the possibility of an Atlantic or a Niagara without having seen or heard of one or the other. So all life is a great chain, the nature of which is known whenever we are shown a single link of it. Like all other arts, the Science of Deduction and Analysis is one which can only be acquired by long and patient study, nor is life long enough to allow any mortal to attain the highest possible perfection in it. Before turning to those moral and mental aspects of the matter which present the greatest difficulties, let the inquirer begin by mastering more elementary problems. Let him, on meeting a fellow-mortal, learn at a glance to distinguish the history of the man, and the trade or profession to which he belongs. Puerile as such an exercise may seem, it sharpens the faculties of observation, and teaches one where to look and what to look for. By a man’s finger-nails, by his coat- sleeve, by his boots, by his trouser-knees, by the callosities of his forefinger and thumb, by his expression, by his shirt-cuffs—by each of these things a man’s calling is plainly revealed. That all united should fail to enlighten the competent inquirer in any case is almost inconceivable.”

“What ineffable twaddle!” I cried, slapping the magazine down on the table; “I never read such rubbish in my life.”

“What is it?” asked Sherlock Holmes.

“Why, this article,” I said, pointing at it with my eggspoon as I sat down to my breakfast. “I see that you have read it since you have marked it. I don’t deny that it is smartly written. It irritates me, though. It is evidently the theory of some armchair lounger who evolves all these neat little paradoxes in the seclusion of his own study. It is not practical. I should like to see him clapped down in a third-class carriage on the Underground, and asked to give the trades of all his fellow-travellers. I would lay a thousand to one against him.”

“You would lose your money,” Holmes remarked calmly. “As for the article, I wrote it myself.”

“You!”

“Yes; I have a turn both for observation and for deduction. The theories which I have expressed there, and which appear to you to be so chimerical, are really extremely practical—so practical that I depend upon them for my bread and cheese.”

“And how?” I asked involuntarily.

“Well, I have a trade of my own. I suppose I am the only one in the world. I’m a consulting detective, if you can understand what that is. Here in London we have lots of government detectives and lots of private ones. When these fellows are at fault, they come to me, and I manage to put them on the right scent. They lay all the evidence before me, and I am generally able, by the help of my knowledge of the history of crime, to set them straight. There is a strong family resemblance about misdeeds, and if you have all the details of a thousand at your finger ends, it is odd if you can’t unravel the thousand and first. Lestrade is a well-known detective. He got himself into a fog recently over a forgery case, and that was what brought him here.”

“And these other people?”

“They are mostly sent on by private inquiry agencies. They are all people who are in trouble about something and want a little enlightening. I listen to their story, they listen to my comments, and then I pocket my fee.”

“But do you mean to say,” I said, “that without leaving your room you can unravel some knot which other men can make nothing of, although they have seen every detail for themselves?”

“Quite so. I have a kind of intuition that way. Now and again a case turns up which is a little more complex. Then I have to bustle about and see things with my own eyes. You see I have a lot of special knowledge