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3. Abductive Arguments

 

Wireless Philosophy, Fundamentals, 6:14. Original title: CRITICAL THINKING - Fundamentals: Abductive Arguments

Summary​

Geoff Pynn turns to abductive arguments, which rest on an inference to the best explanation. The simplest way into the idea is through "why" questions.

How abduction works​

You tell your friend Charlie that Lucy has a crush on him, and his cheeks turn bright red. Why did that happen? The most natural answer is that he was embarrassed to learn about the crush. Turned into an argument:

Premise: Charlie's cheeks turned red after I told him that Lucy had a crush on him.
Conclusion: So Charlie was embarrassed to learn about Lucy's crush.

The premise does not guarantee the conclusion. Something else might explain the blush: maybe Charlie was eating a jalapeño, maybe he had just been scratched by a cat and is having an allergic reaction. Let your imagination run and further answers will come. Still, given your background knowledge, embarrassment looks like the best explanation, or at least a serious contender, and if it is, the premise gives you good reason to believe the conclusion.

That is the pattern: we know some things are true, and we reason from them to whatever is the best answer to the question "why are these things true?" Our knowledge does not guarantee that the explanation is correct, but that is no defect, because abductive arguments are not meant to be deductively valid.

Where abduction shows up​

Abductive arguments are extremely common in all walks of life, which makes making, spotting, and evaluating them an important critical thinking skill.

Think of how a TV detective solves a crime. The murder weapon was found in Smith's trunk, Smith has no alibi, Smith had a motive, and Smith failed the lie detector test. The best explanation of all this evidence is that Smith is the murderer, so the detective believes it, and with good reason. She passes the argument to the prosecutor, who uses it to convince the jury.

Abduction is also crucial in science, where hypotheses often rest on inferences to the best explanation of observed data. That is how Neptune was discovered. In the early 1800s astronomers noticed small discrepancies between the observed orbit of Uranus and the predictions of Newton's theory of motion. The best explanation was that another, never-observed planet was causing them, and this turned out to be correct.

What makes an explanation the best one​

Philosophers debate this at length, but two characteristics are widely agreed on:

  • Fit with what we already know. Another possible explanation of the orbital discrepancies was that Newton's theory is wrong. Accepting that would have meant giving up a great many other very good explanations, so it fit badly with what astronomers already knew. An unobserved planet fit much better, and was therefore the better explanation.
  • Simplicity. Other things being equal, a simpler explanation beats a complicated one. Perhaps Charlie misheard you, thought you said Penny had a crush on him, and is embarrassed about Penny's crush instead. That could be right, but it is needlessly complicated, so the original explanation is preferable.

Both fit and simplicity come in degrees, other factors matter too, and there is no sure-fire recipe for deciding when an explanation is the best one.

How to challenge an abductive argument​

There are two routes. One is to come up with a better explanation of the same data. The other is to look for more evidence to add to the premises. Suppose the detective also learns that Smith has a very clever nemesis who had a motive of his own and had long been planning to frame him. Smith's guilt is then no longer clearly the best explanation; being framed has become a perhaps equally good contender.

The lesson is to gather all the evidence you can and consider all of it before drawing a conclusion, because a conclusion that explains some of your evidence well may explain the rest of it badly.

Key takeaway​

Abductive arguments are a kind of ampliative argument: their premises do not guarantee their conclusions, but offer them as the best explanation of what the premises report. Good explanations fit our background knowledge and are simpler than the alternatives, and any abductive argument stands only as long as no better explanation and no further evidence turns up.