(intro music)
My name is Daniel Greco and
I'm an assistant professor of philosophy at Yale University.
Today's video will concern a topic in epistemology,
which is the branch of philosophy that
deals with the study of knowledge.
In particular, I'll discuss a version of skepticism,
which is the idea that we know a lot less
than we ordinarily take ourselves to.
The sort of skepticism I'll discuss is due to David Hume,
who was an eighteenth-century Scottish philosopher and historian,
and it targets our knowledge of the unobserved.
To get clearer about just what that amounts to,
we'll have to start with some examples.
So we ordinarily take ourselves to know lots
about things that we haven't directly observed.
For instance, I take it that I know that
blue whales are the largest animals on earth.
I bet you know that too.
I take it I know that the Alpha Centauri system
is the nearest star system to our own.
I take it I know that there was a man named Napolean
who conquered much of Europe.
I also take it I know when the next
American presidential election will be.
None of these are things that I've directly observed.
I haven't seen any blue whales.
I certainly haven't seen all other animals
on earth to compare them to.
I never met Napoleon, and I haven't
observed anything in 2016 yet.
And yet I, and I take it you, ordinarily take myself
to know all sorts of things about these matters.
So how do we know these things
about stuff that we haven't yet observed?
In some cases it seems pretty easy.
For instance, I know that all triangles,
even triangles I haven't yet observed, have three sides.
Or I know that next year, if I have two apples
and two oranges, I'll have four pieces of fruit.
What's special about these cases
that makes them so easy to know?
The way that Hume put it, they express relations of ideas.
A relation of ideas is something whose
denial is inconceivable, or self-contradictory.
Try to imagine a two-sided triangle.
I take it you can't do it.
Or try to imagine a situation where I have two apples
and two oranges, nothing else,
but where I don't have four pieces of fruit.
Again, I suspect you're gonna have trouble.
Here's another way of getting at the same idea.
Relations of ideas have to be true,
no matter how the world turns out.
They're necessary truths.
So what does it take to know claims like this?
What does it take to know relations of ideas?
To quote Hume, he said that propositions
of this kind are discoverable by the mere
operation of thought, without dependence
on what is anywhere existent in the universe.
And what does that mean?
Why does he think it's true?
I take it the idea is something like this.
If some claim is a relation of ideas,
then it will be true no matter what the world is like.
So in order to know that it's true,
we don't need to go out and gather evidence
about what the world is like.
The evidence might tell us the world
is this way rather than that way,
but no matter what way the world is like,
all triangles will have three sides.
Two pieces of fruit and another two pieces of fruit
will make four pieces of fruit.
So if anything at all is required to know
that a relation of ideas is true,
it's just understanding.
This is what Hume called the "mere operation of thought."
That's enough to see that it has to be true,
and so to know that it's true.
Okay, contrast relations of ideas with another
class of claims that Hume called "matters of fact."
For example, as I now make this video,
it's raining outside.
Or here's another one: I have a fluffy puppy.
So these claims are true, both of them,
but their denials are not inconceivable or contradictory.
The mere operation of thought isn't enough
to get us knowledge of their truth.
You could easily imagine that it's sunny outside.
In fact, maybe as you listen to this video, it is.
Or you could imagine that I have no pets at all,
including no fluffy puppy, even though, in fact, I do.
So because these claims have
denials that aren't contradictory,
because you can conceive that they're false,
it's not enough to just understand what they mean
to see that they're true.
You have to go out and make some observations,
see that the world is one way, rather than another way.
So these matters of fact contrast with relations of ideas
and what it takes to know them.
This distinction that Hume is getting at,
between relations of ideas and matters of fact,
is closely related to what was later called
"the distinction between the a priori
and the a posteriori," by Immanuel Kant.
Subtitles by the Amara.org community