First off, a typical story from both characters. It may be that no story has all of these element and it may be that some stories have none of these elements, but if you look at enough stories and squint this is basically what you get.
Sherlock Holmes and the Adventure of the Silver Bangle
Holmes and I were in Baker Street. Holmes announced we were about to receive a client, there was a knock at the door and an agitated man entered. Holmes did his clothes trick: telling the man his profession, and briefly why he was there, explaining the observations of the man's clothes that let him guess this. The man introduced himself as Lord Symkins and confirmed many of the things Holmes had said.
Lord Symkins explained that there had been a murder in Fontelroy Hall. Holmes and I leapt on a Hansom cab, went to the Hall, found the dead man and got angry at the official police for disturbing the footsteps. He observed the murder scene very closely, found the silver bangle that the police thought was irrelevant. We followed an obvious path and arrived at some other place. Holmes sent and received many telegrams I wasn't told about. Later that evening we spotted a strange looking man and gave chase, but he got away from us.
Holmes and I then went back to the Hall. He told me to get my trusty service revolver ready, he got his hunting crop. We hid in a dark room. A man walked in and there was a fight. The man was captured and said "it's a fair cop".
The man said that he was the murderer and that a full and frank confession of the truth was his best possible defence. He then told us the whole story. His name was John Hawkins, he's never been mentioned before in this story. He told us the full story of the murder. The silver bangle was important in it. When I challenged Holmes on how he knew that the murderer was to be found here he replied that he knew the silver bangle was the sign of a secret society totally unknown to me, he found out a lot of information from his telegrams, which he didn't tell me about, and with that information the case was so easy that Mma Ramotswe could have solved it. He then sent a message to the murderer telling him to go to the darkened room.
Holmes then said that as the murderer was acting on the best intentions and was a noble upper class individual he would let him go as justice didn't require the murderer to be arrested. We went home.
The Blackwood Convention
Poirot was at the country estate of the eccentric billionaire Lord Blackwood. He was at a party for "all the people who stand to gain by my death, also a famous detective." The doors were sealed. After a long series of conversations with everyone present Poirot got a good assessment of everyone. Those present were:
- Lord Blackwood: A man with more enemies than a the billionaire owner of a large mining concern could reasonably be expected to have.
- Lady Blackwood: Having an affair with Roger.
- Roger: Having an affair with Lady Blackwood, wanted the Lord dead to get his wife. Blackmailing Susan over her affair.
- Susan: Having an affair with Martin. Would do anything to protect him. Woman of expensive tastes.
- Martin: In huge gambling debts, secretly heir to Lord Blackwood, needed his money.
- John: Husband of Susan. Well known heir to Lord Blackwood, needed his money to keep Susan happy. Army man, experienced in native South American poisons.
- Jenkins: The butler. Secretly Lord Blackwood's son. Wants his title. Trained as a doctor.
- Mary: The maid. Mistreated by Lord Blackwood, wants him out of the way to become personal maid to Lady Blackwood.
- Rev. Hawkins: Secretly gay. Being blackmailed by John.
They all played bridge in 2 games of 4. The Lord sat alone in a dark alcove. The servants came and went. Hours later: "By Jove, he's dead!" Rev. Hawkins exclaimed, looking over the dead corpse of Lord Blackwood. Much surprise ensued.
Poirot puts his little grey cells to work. He reconstructs exactly the sequence of events that night, detects the rare poison dart in the Lord's neck. Also the blowpipe in the locked draw that only Lady Blackwood and Mary had access to. Also the spilled wine next to the chair that only Rev. Hawkins, Martin or John could have made, the marks on his arm to indicate that the Lord was a prolific drug user.
Poirot suggested staging a reconstruction, everyone repeating the events of the previous night with Mary playing the part of Lord Blackwood. Noticeably several things were different, there were two spoons in "Lord Blackwood's" cup of tea, every time Martin so much as flinched Susan created a huge distraction in an attempt to distract everyone, John played noticeably worse cards when people were scrutinising him closely. One of the things that was not different was the murder. Mary's death was the final clue Poirot needed.
A few hours later the great detective summoned everyone into the drawing room. He explained in great detail all the evidence and explained the motive of each person there before finally concluding that the butler did it as he had medical knowledge and needed to kill Mary as she had noticed him preparing the poisen.
The two give us very clear examples of the differences between Arthur Conan Doyle and Agatha Christie. First notice how unfair I have been. It took me as long to construct the story of the Silver Bangle as it took me to type it. It's a totally boring and meaningless story with no interesting characters. It probably took me less time to write the Silver Bangle than to come up with just the title for The Blackwood Convention. I put a lot more effort into the second. But notice that this is not just me, this is a fact of the styles of the two stories.
There are only 5 characters, including Holmes and Watson, in the Silver Bangle, none of whom need any details of their backgrounds fleshed out in order to produce a convincing Holmes story. To make a Christie however you need at least 10 characters, plus detective and victim. The Silver Bangle needed no elaborate explanation of the motives, in the Blackwood Convention everyone needs a plausible motive. In the Silver Bangle only one person could have done it, in the Blackwood Convention everyone could have done it. Notice also that in the Silver Bangle the evidence was all hidden until the end, the reader cant guess whodunnit. In the Blackwood Convention all the evidence is in front of you. The reader can guess whodunnit.
Why Holmes could be the greatest detective ever
Sherlock's importance and power as a detective must never be forgotten. The character revolutionised, and indeed invented, large sections of forensic science. The first case ever to be solved by the use of fingerprint evidence was the case of the Norwood Builder (it should be noted for the fans of biometric science that the police condemned the wrong man on the basis of the thumb-print). The emphasis on trace evidence at crime scenes, and protecting the integrity of the same, is now totally part of modern detective work. A large part of why I predicted (wrongly) that a modern version of the Holmes stories would never work is exactly because the modern detective already uses most of Holmes' methods, Holmes himself, I thought, would be redundant.
His ability to "deduce" relies in almost no sense on logic. It relies on Holmes having an almost infinite supply of facts, habits, customs, stereotypes, and an ability to imagine likely causes for observed data. The supreme example of Holmes' abilities is probably the case of the Blue Carbuncle. Or rather, it isn't. The case itself is a trivial affair, follow the obvious trail of clues and get to the end, no thought required at any point. The really awesome thing is the start to the story. Holmes is presented with a hat. And from this concludes things about the owner. The conclusions are expressed thus:
Given the observations Holmes has made about a third of these conclusions are reasonable and supported by at least the balance of probabilities, the other third are rather nebulous and hard to quantify and the last group are not really fair. The combination works shockingly well as a stage trick where confirmation bias means we will quite happily ignore any stupidity we see in the man so long as he drinks, or vice versa. However, with a bit of tightening up and by getting rid of a lot of things it's reasonable for Victorian Holmes to believe but not reasonable for 21st century Holmes to belive we can certainly get a large body of truth out of this. Holmes has without question the potential to be the greatest detective ever conceived."That the man was highly intellectual is of course obvious upon the face of it, and also that he was fairly well-to-do within the last three years, although he has now fallen upon evil days. He had foresight, but has less now than formerly, pointing to a moral retrogression, which, when taken with the decline of his fortunes, seems to indicate some evil influence, probably drink, at work upon him. This may account also for the obvious fact that his wife has ceased to love him.""My dear Holmes!""He has, however, retained some degree of self-respect," he continued, disregarding my remonstrance. "He is a man who leads a sedentary life, goes out little, is out of training entirely, is middle-aged, has grizzled hair which he has had cut within the last few days, and which he anoints with lime-cream. These are the more patent facts which are to be deduced from his hat. Also, by the way, that it is extremely improbable that he has gas laid on in his house."
Why Holmes isn't
Why then, if Holmes is in theory the greatest detective ever, was it so hard for me to make the Silver Bangle sound like an interesting detective story in the way the Blackwood Convention did? Because Holmes is not, in the Arthur Conan Doyle Stories, featured in a detective story, and cant show off. The Holmes stories are not primarily detective stories, but rather adventure novels, the clue is there in the title. To quote Stephen Moffat, "Other detectives have cases, Holmes has adventures". The difference is key. In a detective story the point is the mystery, the facts are laid before the reader and the joy is to watch the case unravel and think "damn, I should have thought of that." In an adventure novel there is a case to be solved, but far more important than the thinking is the action. The chase, the hunt, the fight, the gruesome murder, these are the elements that make a Holmes story exciting. This is why the Silver Bangle is so empty, I put in a one line description of the key elements of the plot.
There are two reasons why the original stories are like this. One in-universe and one out-universe.
In-universe it is only natural that Watson should chronicle the adventure stories. Doubtless Holmes faces endless cases with real meaty intellectual content. But in such cases Watson and his trusty service revolver are not needed. Watson is most useful when he can run across a moor or hide in a dark room and fight a villain. He's not really needed when Holmes solves the problem by deep reasoning and the actual running around is secondary. Secondly of course Holmes is unnecessarily secretive and hordes his evidence. Had Holmes written the stories himself and not relied on Watson they would have been real deep intellectual puzzles. As it is half of the facts needed to solve the case are in telegrams Watson never sees. Which is frankly unsporting, but at least believable for Holmes.
Out-universe of course Conan Doyle is not *trying* to make interesting intellectual exercises, he's trying to write a thriller. That's the kind of story he likes. He famously hated writing Holmes stories, and anyone who's read one could tell you that he's not good at hiding it. Holmes tells Watson point blank to stop writing several times expressing his disgust at the way Watson writes, this is pure author insertion. Then all the times Holmes retires or is killed, that's not for dramatic effect, that's Conan Doyle trying to give people the hint, "stop reading Holmes, I hate him, read Professor Challenger instead". Add to this the incidents (most dramatically in the Study in Scarlet and the Valley of Fear) where the fact that Holmes and Watson exist at all is simply forgotten and in the middle of a story Conan Doyle starts writing a totally unrelated thriller set in America.
Holmes could have been the undisputed greatest detective to have ever been invented. And if anyone but Arthur Conan Doyle had written him then certainly he would have. This is why Holmes stories not copied from an original can (with a decent writer) be incredible. But Arthur Conan Doyle didn't want to create the greatest detective ever, and so inevitably he didn't.