Arthur Conan Doyle 
The Valley of Fear 

“I am inclined to think—” said I.
“I should do so, ” Sherlock Holmes remarked impatiently.
I believe that I am one of the most long-suffering of mortals; but I'll admit that I was annoyed at the sardonic interruption. “Really, Holmes, ” said I severely, “you are a little trying at times.”
He was too much absorbed with his own thoughts to give any immediate answer to my remonstrance. He leaned upon his hand, with his untasted breakfast before him, and he stared at the slip of paper which he had just drawn from its envelope. Then he took the envelope itself, held it up to the light, and very carefully studied both the exterior and the flap.
“It is Porlock's writing, ” said he thoughtfully. “I can hardly doubt that it is Porlock's writing, though I have seen it only twice before. The Greek e with the peculiar top flourish is distinctive. But if it is Porlock, then it must be something of the very first importance.”
He was speaking to himself rather than to me; but my vexation disappeared in the interest which the words awakened.
“Who then is Porlock?” I asked.
“Porlock, Watson, is a nom-de-plume, a mere identification mark; but behind it lies a shifty and evasive personality. In a former letter he frankly informed me that the name was not his own, and defied me ever to trace him among the teeming millions of this great city. Porlock is important, not for himself, but for the great man with whom he is in touch. Picture to yourself the pilot fish with the shark, the jackal with the lion—anything that is insignificant in companionship with what is formidable: not only formidable, Watson, but sinister—in the highest degree sinister. That is where he comes within my purview. You have heard me speak of Professor Moriarty?”
“The famous scientific criminal, as famous among crooks as—”
“My blushes, Watson!” Holmes murmured in a deprecating voice.