In many of the stories he is assisted by his companion, the practical Dr. John H. Watson, with whom Holmes shared rooms for some time, before Watson's marriage. Watson is portrayed as Holmes' friend and chronicler, i.e., Holmes' stories are actually told as reports, by Watson, of Holmes' solutions to actual crimes. He also has a brother [Mycroft Holmes]? who appears in at least three stories: "The Adventure of the Greek Interpreter," "The Adventure of the Final Problem," and "The Adventure of the Bruce Partington Plans."
In the very first Holmes story, "A Study in Scarlet," something of Holmes' background is given. He is presented as an independent student of chemistry with a variety of very curious side-interests--which turn out all to be single-mindedly bent toward making Holmes superior at solving crimes. In another early Holmes story, "The Adventure of the 'Gloria Scott'," we get more background on what caused Holmes to become a detective: a college friend's father complimented him very highly on his deductive skills.
Holmes' arch-enemy, and popularly-supposed nemesis was Professor James Moriarty, who pushed Holmes over the [Reichenbach Falls]?. Conan Doyle intended the story where Holmes is pushed over the cliff to be the last that he wrote about Holmes; however the mass of mailings he received demanding that he bring Holmes back convinced him to continue. The next Holmes story had Conan Doyle explaining that Holmes did not in fact die, but managed to grab hold of some vegetation on the side of the cliff; this explanation originated the term "cliffhanger."
Modern readers of the Holmes stories are apt to be surprised that he is an avid user of cocaine?, though Watson describes this as Holmes' "only vice." Watson might not have considered vices Holmes' habit of smoking (usually a pipe) like a chimney, or his tendency to bend the truth and break the law when it suits his purposes (in Victorian England these were probably not considered vices as long as they were done by a gentleman for noble purposes). Holmes is not at all a stuffy straight-laced gentleman; in fact, he describes himself and his habits as "Bohemian."
"From a drop of water"--Holmes wrote in an essay described in "A Study in Scarlet"--"a logician could infer the possibility of an Atlantic or a Niagara without having seen or heard of one or the other." Holmes stories often begin with a bravura display of Holmes' talent for "deduction." It is of some interest to logicians and those interested in logic to try to analyze just what Holmes is doing when he performs his deduction. Holmesian (that's the British adjective; Americans say "Sherlockian") deduction appears to consist primarily of drawing inferences based on either straightforward practical principles--which are the result of careful inductive study, such as Holmes' study of different kind of cigar ashes--or inference to the best explanation. In many cases, the inference can be modelled either way.
Holmes' straightforward practical principles are generally of the form, "If p, then q," where 'p' is observed evidence and 'q' is what the evidence indicates. But there are also, as one may observe in the following example, often some intermediate principles. In "A Scandal in Bohemia," Holmes deduces that Watson had gotten very wet lately and that he had "a most clumsy and careless servant girl." When Watson, in amazement, asks how Holmes knows this, Holmes answers:
In this case, we might say Holmes employed several connected principles such as these:
In other instances of Holmesian deduction, it is more difficult to model his inference as deduction using general principles, and logicians and scientists will readily recognize the method used, instead, as an inductive one--in particular, [argument to the best explanation]?, or, in [Charles S. Peirce]?'s terminology, abduction. (That Holmes should have called this deduction is entirely plausible, however, because, in several stories, Holmes is said not to have known anything at all of philosophy.)
The instances in which Holmes uses abduction tend to be those where he has amassed a large body of evidence, produced a number of possible explanations of that evidence, and then proceeds to find one explanation that is clearly the best at explaining the evidence. For example, in "The Sign of Four," a certain man is found dead in his room, with a ghastly smile on his face, and with no immediately visible cause of death. From a whole body of background information as well as evidence gathered at and around the scene of the crime, Holmes is able to infer that the murderer is--not the various people that Scotland Yard has in custody (each of them being an alternative explanation)--but rather...well, we don't want to give away the story, but you get the idea. As Holmes says in the story, "How often have I said to you that when you have eliminated the impossible, whatever remains, however improbable, must be the truth?"
In the latter example, in fact, Holmes' solution of the crime depends both on a series of applications of general principles and argument to the best explanation.
Holmes' success at his brand of deduction, therefore, is due to his mastery of both a huge body of particular knowledge of things like footprints, cigar ashes, and poisons, which he uses to make relatively simple deductive inferences, and the fine art of ordering and weighing different competing explanations of a body of evidence. Holmes is also particularly good at gathering evidence by observation, as well locating and tracking the movements of criminals through the streets of London and environs (in order to produce more evidence)--skills that have little to do with deduction per se, but everything to do with providing the premises for particular Holmesian deductions.
Conan Doyle wrote four novels and fifty-six short stories about Sherlock Holmes. (Or, as some would have it: Dr. John H. Watson wrote four long accounts and fifty-two short accounts of Holmes's cases, while Holmes wrote two of his own and a third, unnamed person -- possibly Conan Doyle -- wrote two more.)
Novels:
Short stories (organized by collection):
Text of some of the stories is available at [221B Baker Street]
Perhaps see also Scotland Yard.