We've learned that inferences to the best explanation, are inductive arguments.
They're not valid. And they are defeasible.
But still, they might be strong and good arguments.
They might give you good reasons to believe their conclusions.
Great, they might be, sure. But, how do you tell whether they really
are strong arguments and good arguments? We need standards for determining when an
inference to the best explanation is a good argument.
And to understand those standards, we're going to view another skit made by my
students a few years ago. It's time for class.
It's obviously the cult of the Magic Dragon here.
Right. Do you know all those songs are about
drugs? Was Peter and Paul manning drugs?
Yeah, I know. Whoa, where is everybody?
Beats me. It's Monday, isn't it?
Yeah, first day of the week. Remember the weekend?
Hm. The weekend?
. Yeah, I remember, vaguely.
Good times. Now, it's back to work.
It's Monday. But, what time is it?
It's 11:15.15. Well, isn't that when class is supposed
to start? .
Here's the start of a good explanation. There are surprising circumstances that
need to be explained. Yeah, maybe my watch is fast.
It has been giving me some trouble lately.
Do you check yours? I don't wear a watch.
I just ask you when I need to know the time.
Great! So, I'm your designated time keeper.
kind of, but there's usually a clock around.
One right there. Okay, so we know its 11:1515.
Right. In any entrance for the best explanation,
it is crucial to get an accurate picture of what you're trying to explain.
In this case, if it were 11:10 instead of 11:15, then the explanation may be
different, or, there might be nothing at all to explain.
So, where is everybody? Well, maybe everybody else is late.
After all, the professor does show up a few minutes late.
He told me that he doesn't like to start teaching right away, because he doesn't
like when he's talking and students walk in.
Oh, that makes me mad. So maybe, they're coming in later,
because they don't want to wait around for him to start talking.
It's one of those vicious circles. Okay, maybe that's it.
But then some students should start showing up.
So, let's just wait and see. 'Kay.
. Enough is enough.
If people are coming they'd be here by now.
I mean, the professor's been late, but he's never been this late.
Anne refutes Bob's explanation by pointing out new facts that falsify Bob's
hypothesis that all the students are late.
A good explanation has to be compatible with all the facts, not just the
particular ones that it's trying to explain.
Bob's hypothesis can't be the best explanation of why the students aren't
there at 11:1515, if they're still not there by noon.
. Well that brings us right back to where
we started. .
Why isn't anybody else here? Your guess is as good as mine.
Well, well my guess is that everybody else is here.
We just can't see or hear them. I bet they're laughing at us right now.
[LAUGH]. I bet the professor's up there jabbering
away right now, like he always does. [uuu].
Oh, I hope this material is not on the test.
[LAUGH], Congratulation's, you've proved me wrong.
Your guess is not as good as mine. Why?
Humans can't become invisible. How do you know?
Have you ever seen it? No.
They're invisible, that's why I've never seen it.
Okay. But the laws of physics don't allow
humans to be come silent and invisible. Anne dismisses Bob's new hypothesis,
because it's not conservative. It conflicts with well established prior
beliefs in physics. If you start believing that humans can
become invisible, you'll have to give up everything you know about how light and
matter work. .
Besides, if they were here, the seats wouldn't be popped up like this.
Nope. Because, besides being invisible and
silent, these students are also weightless.
No smell either. Okay, so there is no way to tell whether
or not they're students here. You're catching on.
But the laws of physics still don't allow people to be weightless, invisible, and
silent. Well, the laws of physics are based on
observation. And there is no way these students can be
observed at all. The laws of physics can't apply to
something that can't be observed, or these students.
So then nothing could possibly tell whether or not there's students in these
chairs? Exactly.
Hey, Ed, we're here live with an unidentifiable, unobservable student.
What's it like to be unobservable? Okay, back to you Ed.
Pretty sweet, huh? No, it is not sweet. The only reason why
your idea isn't falsified, is because it's not falsifiable.
If you can't prove it then it's useless. I mean, you might as well say there's
invisible elephants floating around. .
Ann rejects Bob's new explanation on the grounds that it's not falsifiable.
It's compatible with anything that could possibly happen.
But that means that it can't explain why one thing happens rather than another.
It's completely empty. .
I was just joking. This is a philosophy class after all.
Okay, I don't mind jokes, but, let's get serious,
because it's doesn't help us with our problem.
Why isn't anybody here? Speaking of joking, maybe they're all
playing a trick on us, maybe they decided not to show up.
[LAUGH]. Okay but why would they do that?
Is it April Fool's Day? No, it's January.
Oh. Well then I don't know why they're
tricking us. They just are.
Okay, I can't believe that unless you can tell me why they would be so silly.
Anne is criticizing Bob's new explanation, because it is not deep.
It's shallow, because it depends upon a principle that
is not itself explained, and needs to be explained.
I don't know. I, I guess the professor just decided to cancel class because he
didn't feel like teaching today. Maybe, but then other students wouldn't
know so they would be here. Unless, he emailed the entire class, then
they would know. Did you check your email recently?
About an hour ago. Then maybe he emailed us the past hour.
I guess, I mean that would explain why nobody's here.
Some professors do cancel class at the last minute, but not him.
He's too strict. I've had five classes with him and he's
never canceled class at the last minute. If you miss even one class in this
course, you will fail. So, maybe this time it's different.
Yeah, and maybe the sun didn't rise. Either way, it's totally out of
character. Anne's point is that Bob's new
explanation is Ad Hoc. It applies only to the very circumstances
that it was invoked to explain, and it doesn't apply to other cases.
That means that it lacks power and breadth.
Okay. But, I've got it!
Classes must have been cancelled for some reason.
What reason? maybe it's a holiday.
Okay, what holiday? Martin Luther King.
That's it, it's Martin Luther King Day. That would explain it.
I mean, nobody goes to class but a fool on a holiday.
Hi, I'm here. I guess that makes us both fools.
Yeah. So I guess that's why he didn't send the
note cause he figured no one would come here.
I bet he wrote it down in the syllabus. It also explains why no one is in the
whole classroom building. Bob's new explanation works because it
has all of the virtues that the other explanations lacked.
It's powerful, broad, deep, simple, conservative and not falsified, and yet
falsifiable. Those virtues make his explanation good
or in his words, sweet. You're right.
You know, I think there's some kind of ceremony today with the President of the
college and some big hot shot from Washington for Martin Luther King Day.
So probably that's where the whole class is.
That's why they're not here. Maybe.
But, even if there weren't a ceremony, the fact that it's Martin Luther King Day
would explain why nobody's here. So you don't really need to add anything
else. Ann rejects Bob's newest explanation
because it's not modest. It commits him to more matters of fact
than are needed to explain the observations at hand.
So I guess the holiday explains it all. Well not really, we still don't know why
that big head keeps popping in on the side.
That's one I can't figure out myself. That's so weird.
It kind of looks like this kid that used to babysit me when I was younger.
This whole exchange can be seen as a single argument that takes the form of a
long inference to the best explanation. The first premise is an observation,
nobody else is in the room. A second premise is an explanation.
The hypothesis that class was cancelled because of the holiday, plus accepted
facts and principles, gives a suitably strong explanation of the observation in
premise one, namely, the observation that nobody else is in the room.
Premise three is a comparison. No other hypothesis provides an
explanation nearly as good as the holiday hypothesis in premise two.
And the conclusion, is that the holiday hypothesis in premise two is true.
The most controversial premise is probably premise three.
It says that no other explanation provides nearly as good an explanation as
the holiday hypothesis. To justify this premise, we need to
compare other possible explanations. And that's exactly what the two students
do throughout the skip. Their discussion can then be summarized
in this background argument that supports premise three.
The hypothesis that the other students are late is falsified by the passing of
time. The hypothesis that the other students
are invisible is not conservative. The hypothesis that the other students
are undetectable is not falsifiable. The hypothesis that the other students
are playing a joke is not deep. The hypothesis that the professor skipped
class is not powerful or broad. The hypothesis that the other students
are at a ceremony is not modest. No other hypothesis seems plausible.
Therefore no other hypothesis provides an explanation nearly as good as the holiday
hypothesis in premise two. Even if the other explanations are
inadequate, the inference of the best explanation fails unless the holiday
hypothesis succeeds at explaining why there's nobody else in the room.
Premise two claims that the holiday hypothesis does explain why there's
nobody else in the room. So we need to analyze why it succeeds.
And to do that we can look at another background argument.
The holiday hypothesis explains why nobody else is there.
Because if classes were cancelled because of the holiday, then nobody else would be
there. That is, nobody else would be in the room
at the usual time. The holiday hypothesis is also broad,
because it explains other actual observations, such as the observation
that the whole building is empty. The holiday hypothesis is also powerful,
because it applies to many separate cases, for example it explains why
students won't be there on future holidays.
The holiday hypothesis is also falsifiable.
Because two students might find out that classes were not cancelled for the
holiday. The holiday hypothesis is not falsified
because the two students do not actually find out that classes were not cancelled
for the holiday. And similarly for other ways to falsify
the hypothesis. The Holiday Hypothesis is also
conservative because it does not conflict with any prior well established beliefs.
The holiday hypothesis is also deep because it does not depend on any
assumptions that need, that lack independent explanation.
So the holiday hypothesis explains why nobody else is there.
And it's broad, powerful, falsifiable but not falsified, conservative, deep.
Therefore the holiday hypothesis, plus accepted facts and principles, gives a
suitably strong explanation of why nobody else is in the room.
These two background arguments use a common list of virtues that are usually
called explanatory virtues. In general, one explanation is better or
stronger than another if it has more of these virtues.
When you understand these explanatory virtues, then you've mastered inference
to the best explanation. Right?
Right. Now we can go on and look at other kinds
of inductive argument.