Detectives and Abductive Reasoning

Sherlock Holmes is often thought to engage in deductive reasoning. He even says so himself. However, Ivar Fahsing points out in an article on how to think like a detective that what Sherlock did was actually abductive reasoning (in contrast to deductive or inductive reasoning). This surprised me since, even though I am somewhat versed in logical analysis, I was not consciously aware that there is a word for this form of reasoning.

From Wikipedia: “Abductive reasoning (also called abduction,[1] abductive inference,[1] or retroduction[2]) is a form of logical inference formulated and advanced by American philosopher Charles Sanders Peirce beginning in the last third of the 19th century. It starts with an observation or set of observations and then seeks to find the simplest and most likely conclusion from the observations. This process, unlike deductive reasoning, yields a plausible conclusion but does not positively verify it. Abductive conclusions are thus qualified as having a remnant of uncertainty or doubt, which is expressed in retreat terms such as “best available” or “most likely”. One can understand abductive reasoning as inference to the best explanation,[3] although not all usages of the terms abduction and inference to the best explanation are exactly equivalent.[4][5]” (End from Wikipedia.)

I good example given by Fahsing is medical diagnosis. A good doctor will collect data on symptoms and consider many different diagnoses and look for the diagnosis that best fits the symptoms. You don’t start with an assumption of what the health problem is and look for symptoms to support it. Bringing in any form of bias or ignoring a symptom is potentially deadly.

What struck me about this reasoning method and most of Fahsing’s article was how it is a variant of what skeptics and scientists do. This is illustrated by the ABC principle presented by Fahsing:

  • Assume nothing
  • Believe nothing
  • Challenge and check everything

This is an excellent description of starting from doubt. I often describe doubters and skeptics as making decisions based on reason and evidence. Skeptics are wont to say they challenge (or doubt) everything. This effectively requires starting with a blank slate that contains no assumptions. The collection of evidence and the use of reason is how you check everything. This process leads to working conclusions.

When I think of deductive logic I generally think in terms of formal proof. Starting with a well-defined set of assumptions, rules of logic are applied so as to systematically show how a conclusion must follow from the assumptions. Inductive logic is kind of in-between deductive and abductive reasoning. Induction uses rules of logic, but can start with premises not assumed or known to be true. So it seems to me that inductive reasoning is sometimes used in abductive reasoning although I think intuition based on experience plays are large role as well. But in the sense that a conclusion obtained abductively is not considered to be “proven”, deductive reasoning is not truly part of abduction.

The concept of “inference to the best explanation” is quite precisely the foundation of science and is consistent with my philosophical foundation of modelism. I don’t think there is any choice but to assume that I exist and that something other than I exists. But beyond that, the model which I develop as a consequence of input though my senses is a less than perfect representation of objective reality. My subjective model is the best fit of my sensory input that I’ve been able to infer. Similarly, this is how science works. If new evidence shows up that doesn’t fit the current scientific model, the model is adjusted to fit this new evidence.

This leads to the question of when do we claim to “know” something to be true? A detective works within the context of a legal system. In the U.S. there is the concept that a verdict has to be found “beyond a reasonable doubt.” It is certainly possible to go down the rabbit hole of defining “reasonable doubt”, but consider that this criterion is regularly and successfully applied in court rooms. It isn’t impossible to come up with an acceptable definition – at least in specific cases. (Unfortunately, even though this criterion is the best we seem to have been able to come up with, it still doesn’t always lead to the correct conclusion.)

I think “reasonable doubt” is a useful criterion for science and daily living as well. For example, I think most people consider the existence of gravity beyond a reasonable doubt. But perhaps surprising to some, science has ways of quantifying doubt and even “reasonable” doubt.  I’ve discussed the fact that every measurement has an error. The quantification of this error effectively quantifies doubt. In statistics, the acceptable level of error is identified by applying a limit to the p-factor. (Of course, and more kudos to science, the applied limit and even the value of using the p-factor is under debate.) Other quantified limits relate to the number of sigmas (or standard deviations) or the number of nines (the number of decimal places in a measurement’s error that are 9’s).

In general, thinking of starting from doubt and science as detective work is spot on.

(The cover image does not require a credit, but here’s where I got it from: Pixabay.)

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