(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. Subtitles by the Amara.org community