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Abduction: A Thinking Skill You Can Do But Computers Can’t

A Norwegian police detective fills us in on how to use abduction for better thinking

Ivar Fahsing, detective chief superintendent at the Norwegian Police University College, has “worked on some of the worst crimes in Norway for 30 years.” He had to hone his thinking skills but, he says, many of us have never learned to “make safe judgments under pressure.” He is also convinced that any of us can improve our skills and he offers some help from his experience investigating crimes.

One skill he focuses on is abduction, which was Sherlock Holmes’s favorite method. Yes, Holmes always tells his sidekick and foil, Dr. Watson, that he uses deduction — but he doesn’t:

In the Sherlock Holmes novels, our titular hero continuously assails Dr Watson, a man of science, about the merits of deductive logic. In fact, strictly speaking, Holmes’s favoured logical approach is not deduction, which is reasoning on the basis of known facts, but rather what is known as abductive logic, which is the cognitive process of identifying the best possible explanation for a given set of observations. Abductive reasoning is widely recognised as a powerful mechanism for hypothetical reasoning in the absence of complete knowledge. It’s generally understood as reasoning from effects to causes. Only rarely does Holmes engage in the deduction of which he speaks so highly.

Ivar Fahsing, “How to think like a detective” at Psyche

Fahsing point outs that criminal investigations are generally abductive rather than deductive. The detective is not given a hard arithmetic problem to work out. A computer could do that. Rather, the detective has a series of facts to make sense of. That involves creativity in organizing one’s research and thinking strategies. Computers don’t do creativity.

Here are a couple of the many tips he offers:

“You should always create a short outline of all the possible alternative explanations you can think of for the situation you are trying to solve”

“Try to eliminate as many explanations or lines of inquiry as you can. Just like in science, theories can be truly tested only through falsification.”

The biggest risk he says, is persuading ourselves that the first apparently good explanation we come up with is the correct one. Thus, his most interesting suggestion is, have a devil’s advocate who tries to shoot down our theories. Someone we must convince before we can proceed. That’s one reason so many detective stories feature two sleuths working together, like Holmes and Watson.

You may also wish to read:

Why human creativity is not computable. There is a paradox involved with computers and human creativity, something like Gödel’s Incompleteness Theorems or the Smallest Uninteresting Number.

A type of reasoning AI can’t replace. Abductive reasoning requires creativity, in addition to computation.


No AI overlords? What is Larson arguing and why does it matter? Information theorist William Dembski explains, computers can’t do some things by their very nature.

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Abduction: A Thinking Skill You Can Do But Computers Can’t