I’ve been on quite a Sherlock Holmes binge lately, having watched a lot of Elementary, the entire Sherlock series and quite a few of the movies as well. I read all the books when I was a kid. The central part of Sherlock Holmes is his logical reasoning and his incredible memory. Many assume he is some sort of human computer, but that’s not at all what is happening.
Anyone can deduce, most people do. What Holmes do differently is what he refers to as the Art of Deduction (sometimes called the Science of Deduction).
Conventional reasoning is that you look at the evidence you have together with your experience and you try to make a series reasonable assumptions. Whatever evidence that doesn’t match you simply dismiss and whatever assumption that seems most likely you go on.
Art of Deduction differs in that you do not make assumptions at all. Instead you take all possibilities and remove those that doesn’t fit all the evidence at hand. The memory part only plays a role for Holmes, as he do not need to look up a lot of things, he remembers everything he has previously looked up and constantly train his memory to do so. Because of this, he can make his deductions on the scene right away.
Very few people have this sort of memory, and science actually disagrees a bit about if it really exists. Even Holmes himself has on occasion questioned his own memories, at which point he dismisses his own memory until he can confirm it to be true.
So when he steps into a murder investigation where a myth about a ghost dog seems to apply, he looks at everything that has happened and everything that is happening and make his deduction.
Could the ghost dog be true? There has been no evidence that ghost dogs exist, save for the victim’s injuries and the circumstances of the death. That’s why we hear him talk about the possibility of a ghost dog in Baskerville’s Hound, and that’s also where we hear the iconic line
How often have I said to you that when you have eliminated the impossible, whatever remains, however improbable, must be the truth?
This line is key to his method, as it shows the audience that he looks into every single possibility. He then applies the evidence at hand, without assumptions, and remove those that does not fit. At first, this would eliminate about 99.9999% of all possibilities. For example, no sign of a wound from a weapon, so all weapon-related cases go away. No sign of animal tracks, save for the giant paw prints, so trampling is out of the question. No crater, so it wasn’t a meteor. Things like that.
Spoilers for those who haven’t seen or read Baskerville’s Hound, but he does deduce that we are looking at 2 separate events, in light of all the evidence and that these two events are somewhat connected. First, the death of Baskerville was due to an abnormally large dog that had been starved and then given his scent through the boot that went missing in London. Second event was the escaped mental patient, who had been given supplies from a relative living on the Baskerville grounds.
All evidence given was taken into account, and the only possible answer was that these two events took place. But that’s only possible if you can look at all the evidence and remember all the circumstances around how they appeared and when, and connect the dots.
A more recent example is from Sherlock, when Holmes can deduce the location of two missing children from a few stains in a foot print. He could only do this because he knew what sort of concrete, what sort of bricks and what the rare chemical was used for, which he only knew because he had access to that information through his memory. That concrete, that type of brick and that chemical lead to a very specific chocolate factory.
Now, it’s unreasonable to think most police detectives could do something like this, but with the expansion of technology, it could be a future possibility. Though that requires the detectives to think in these patterns, which most people don’t and to identify all possible evidence, which nobody reasonably can. Most people have conventional logic, for a good reason. But there are people out there who have deductive logic, as evident by Doctor Joseph Bell, the inspiration behind Sherlock Holmes and lecturer to Arthur Conan Doyle. He could make these connections, though he often required time to read through books first to get the knowledge needed to connect the dots.
So, to sum up:
The Art of Deduction is not guesswork, nor is it supernatural, nor does it require any genetic trait.
It is a method in which all explanations are derived from all possibilities, reduced (or deducted) by the evidence.