(intro music)
Hi! I'm Geoff Pynn, and I
teach philosophy in Northern Illinois University.
In this video, I'm going to talk about abductive arguments.
Abductive arguments rest on an inference to the best explanation .
The simplest way of thinking about this idea
is in terms of "why"-questions.
Suppose you tell your friend that someone has a crush on him,
and his cheeks turn bright red.
Why did that happen?
What's the explanation?
Well, the most natural answer seems to be that it's
because he was embarrassed to find out about the crush.
That's why his cheeks turned red.
This example can be turned into an abductive argument.
The premise is "Charlie's cheeks turned red
"after I told him that Lucy had a crush on him."
And the conclusion is "So, Charlie was embarrassed to learn about Lucy's crush."
Notice that the promise doesn't guarantee that the conclusion is true.
Something else might explain it instead.
Maybe Charlie was eating a jalapeno,
and his cheeks turned red because it was so spicy.
Or maybe he just got scratched by a cat,
and he's having an allergic reaction.
If you let your imagination rip,
you'll be able to think of other possible
answers to the question "Why did Charlie's cheeks turn red?" too.
Nonetheless, given your background knowledge,
C seems like it's the best explanation for P, or at least it's a contender.
If it is, then P gives you good reason to believe C.
That's how abductive arguments work.
We know that some stuff is true (these are the premises),
and reason from that to whatever is
the best answer to the question "Why are these things true?"
Our knowledge doesn't guarantee that the explanation is correct,
but that's OK, because abductive arguments
aren't supposed to be deductively valid.
Abductive arguments are nonetheless extremely common in all walks of life.
It's a very important critical thinking skill to be able to make,
spot, and evaluate abductive arguments.
Think of how a TV detective solves a crime.
Suppose she knows that the murder weapon was found in Smith's trunk,
Smith doesn't have an alibi, Smith had a motive
and Smith failed the lie detector test.
The best explanation for all this evidence
is that Smith's the murder.
And so the detective believes that Smith is the murder, and for good reason.
She passes this argument on to the prosecutor,
who uses it to convince the jury to believe its conclusion too.
Abduction also plays a crucial role in science.
Scientific hypotheses often rest on
inferences to the best explanation for some observed data.
For example, that's how the planet Neptune was discovered.
In the early 1800s, astronomers noticed small discrepancies
between the observed orbit of Uranus
and the predictions that Newton's theory
of motion made about what the orbit should be.
the best explanation for these discrepancies
was that they were caused by another planet that no one had ever observed.
And it turned out that this was correct.
There was another planet, which we know as Neptune today.
So what makes something a good explanation?
Well, there's a lot of debate about this amongst philosophers,
but here are two characteristics of good explanations
that most generally agree about.
First, the more an explanation fits in with everything we already know,
the better it tends to be.
Consider another possible explanation for the
discrepancies between the observed orbit of Uranus
and the predictions of Newton's theory: that Newton's theory was wrong.
To accept that Newton's theory was wrong would require giving up on lots and lots
of other very good explanations,
and so wouldn't fit very well with what astronomers already knew.
The idea that an unobserved planet was causing the discrepancies
fit much better with what they already knew,
and so counted as a better explanation.
Second, other things being equal,
a simpler explanation is better than a complicated one.
Here's another possible explanation for Charlie's blush:
Maybe he misheard you, and thought you said that
Penny had a crush on him, and so he's
embarrassed to learn about Penny's crush, not Lucy's.
This explanation could be right,
but its needlessly complicated.
Since the original explanation is simpler, it's preferable to this more complex one.
Both fit and simplicity come in degrees,
and other factors are also relevant to how good an explanation is.
There's no sure-fire recipe for saying when an explanation is the best one.
One way to challenge an abductive argument
is to try to come up with a better
explanation of the data than what the argument provides.
Another way to challenge an abductive argument
is to look for more evidence to add to the promises.
Suppose the detective also found out that Smith had a very clever nemesis who had
a motive to commit the murder
and had been planning to frame Smith for a long time.
Then Smith's being guilty would no longer
clearly be the best explanation for all of the detective's evidence.
Now there's another, perhaps equally good, contender,
namely, that Smith was framed by his nemesis
It's important when relying on an abductive argument
to make sure that you get all of the evidence that you can
and then consider all of the evidence before drawing your conclusion.
That's because the fact that a conclusion is a good explanation for some
evidence doesn't mean that it's a good explanation for all of your evidence.
So, summing up.
Abductive arguments are a kind of ampliative argument:
their premises don't guarantee their conclusions.
Abductive arguments involve an inference to the best explanation:
their conclusions are supposed to be the best explanations for their premises.
Abductive arguments play a central role in everyday life and scientific inquiry.
Good explanations tend to fit with our background knowledge
and to be simpler than the alternatives.
And finally, you can challenge an abductive argument by coming up
with a better explanation for the premises, or by finding
additional relevant evidence that isn't well-explained by the conclusion.
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