Why Sherlock Holmes is wrong

"Once you have discounted the impossible, then whatever remains, however improbable, must be the truth"

Why this is such a dangerous fiction.

In a sense, of course, the statement is true - but only if the phrase "whatever remains" is taken in the broadest meaning - i.e. of all possible explanations/causes.

The problem is we can never know that we know all possible causes.

In the medieval period, the behaviour of certain people was classed as being evil - riding on broomsticks for example! - the actual definition of a witch's behaviour changed over time, but in trying to determine why a particular person behaved as they did, no doubt the witch explanation was considered to be the last possible explanation.

The problem with that definition was that it was false. We now have a range of psychological explanations for the kind of behaviour exhibited at the time, and none of them involve 'evil', nor are treated by execution!

A more recent but similarly sinister example is the use of 'expert witnesses' to judge whether a cot death is an unfortunate inexplicable tragedy or the result of action by a parent. This practice (using expert statistical witness evidence) has now been largely discredited thank goodness.

But while it lasted there were several cases where the expert witness would assert that the chance of a cot death not being caused by the parent's action was very small. And the chance of a second one being similarly accidental therefore infinitesimally small. Their conclusion was therefore that the 'cot death' explanation could be eliminated, which left only parental action. Several tragically unfortunate and entirely innocent parents were imprisoned on the basis of this ludicrously flawed reasoning.

It was flawed on two counts;

First, just because the chance of an event happening is very small, the chance of it happening again is NOT the product of the two chances.

For example the chance of winning the Lottery jackpot one week is about one in 45 million. The chances of winning the jackpot again the next week is still one in 45 million (not one in 45 million squared).

If you followed the 'expert witness' argument you could conclude that someone winning the jackpot twice MUST have cheated. But of course they haven't - and it has happened! (https://www.national-lottery.co.uk/life-changing/winner-euromillions-breaking-news)

Secondly, and this is the main point of this article, the Sherlock Holmes principle only applies if you know that you can know ALL possible explanations for the phenomenon under scrutiny. This applies as much to crime as it does to science or airplane crash investigation.

And the one thing we do know about humans is that we cannot possibly know everything!

If there is any doubt whatsoever about our ability to know everything then we cannot apply the Sherlock Holmes principle and to do so is reckless and dangerous.

It's also worth remembering also that Sherlock Holmes is the product of FICTION!