Do we have knowledge of the external world?
This book discusses the skeptical arguments presented in Descartes' Meditations 1 and 2, as well
as how Descartes attempts to refute skepticism by building our knowledge of the external world
on the firm footing of clear and distinct ideas. David Hume's "Problem of Induction" shows a
serious problem for Foundationalism.
Dr. Gina Calderone
Table of Contents
•
•
Descartes' Method of Doubt
o I think, therefore I am.
o Plato's Allegory of the Cave
o Does the mind create reality?
Cartesian Foundationalism
• Our Epistemic Situation
• The Edifice of Knowledge
• The Cartesian Circle
• Problem of Induction
o Hume's Argument
o How Might Any Claim Be Justified?
o Are Causal Laws Justified?
o Is PUN Justified?
o Problem of Induction Chart
Descartes' Method of Doubt
Read Descartes' Meditations 1 and 2. For an online copy of the entire text of Meditations on
First Philosophy, click here.
Seeing something like a table is generally taken to be good reason for thinking that the table
exists. Moreover, the information contained in perception is generally taken to be good reason
for asserting truths about the table itself—that it is flat, rectangular, and brown, for example. But
how can we be sure that the table is actually there in the first place? Descartes observes that we
know ourselves capable of dreams, some of which seem to be quite real, peopled with familiar
objects like tables and chairs and acquaintances and strangers and all kinds of action. In short, in
dreams it seems like we are doing things and seeing things but we’re not. Since, at least while
you’re dreaming, you can’t always tell it’s a dream, how can you know you aren’t dreaming
now?
An analogous problem arises when we think about claims we feel very certain about--those that,
as Descartes puts it, are "clear and distinct": that triangles have 3 sides, for example, and 2+2=4.
Is the kind of subjective certainty we feel any indication of the truth of these propositions? One
would think so, but Descartes considers the possibility that he was created by an “evil genius”
bent on deceiving him about absolutely everything. If so, one might be utterly unable to imagine
a triangle that does not have 3 sides but the power of this evil genius is such that we are
nevertheless deceived—that is, in spite of the impossibility of imagining one, there are triangles
with, say, no sides. Since there's no way to tell that we weren't created by this evil genius, we
have no guarantee that, in spite of the impossibility of imagining otherwise, 2+2 is actually 4.
These two arguments seem to cast doubt on all of our a posteriori beliefs (those we know on the
basis of experience are called into question by the “dream argument”), and even our a priori
beliefs (those we know on the basis of reflection alone are called into question by the “evil
deceiver argument”).
Now, if knowledge requires that we cannot be mistaken about what we believe, and these
arguments show that we can always be mistaken about what we believe, then we have no
knowledge whatsoever. The idea that we have no knowledge whatsoever is known as
philosophical skepticism.
I think, therefore I am.
In Meditation 2, Descartes ponders the possibility that he can now doubt even his own existence.
Perhaps he is dreaming or under the influence of an evil deceiver, in which case….
Right. In order to be dreaming or under the influence of an evil deceiver, you must exist. Hence,
Descartes concludes, cogito ergo sum (I think, therefore I exist). That is, as long as one is
thinking, the thinking is itself proof of your own existence; for there is no story you can tell such
that you come to believe (or doubt) the proposition I exist, where according to the story the
proposition is false.
Of course, knowing that you exist is a far cry from the knowledge we think we have about all
kinds of other things, likes tables and triangles and other people. Descartes’ argument so far gets
one no further than solipsism—the possibility that you are the only thing that exists. Worse, it
gets you no further than the existence of your mind alone, since the existence of your hand—
indeed, your entire body—can be called into question by the evil deceiver argument; so this
knowledge is not yet secure.
Is there anything that could secure knowledge of the "external world"—knowledge, that is, of
anything beyond your own thoughts? (For Descartes' answer to this question, proceed to
"Cartesian Foundationalism".)
Plato's Allegory of the Cave
In Plato's Republic, Socrates imagines this problem another way. And puts it to a different use;
but it is still the problem of knowing when one has grasped "reality", where reality is always
filtered through, and perhaps distorted by, the senses.
Have a look at this video (click here) of Plato's Allegory of the Cave.
You're the philosopher: How can one ever know when one has "transcended" the shadows
of the cave (or, as we might put it, the limits and distortions of the senses)?
Does the mind create reality?
Not convinced that the senses are limited and distorting? Click on this link to a great interactive
website full of optical illusions.
Cartesian Foundationalism
Read Meditation 3 of Descartes' Meditations on First Philosophy. For an online copy of the
entire text, click here.
In the quest for objective certainty, one can be sure of this: I exist (at least as long as one is
thinking). In light of the dream argument and the evil deceiver argument, one might still wonder,
what am I? That is, what is it about the “I” I have just proved exists that remains certain in light
of the possibility that I am indeed under the influence of an evil deceiver?
Descartes argues that all that remains is the content of one’s mind: no matter how deceived I may
be about how things actually are, it is beyond doubt that, as I sit here now, it seems that I am at
this table in this room reading this text. If we have genuine knowledge of the external world,
somehow we must be able to certify the link we think there is between the content of the mind
and the outside world.
Our Epistemic Situation
The Edifice of Knowledge
Descartes' strategy is to refute the evil deceiver argument--that is, prove that God exists and is
not a deceiver--in which case, since God would not allow one to be mistaken about absolutely
everything, one can be sure that when one is in possession of a "clear and distinct" idea (or
perception), this idea maps onto the way the world truly is.
Let’s look more carefully at how this is supposed to go. Consider the following argument:
1.
2.
3.
4.
I seem to see a tree.
My perception is clear and distinct.
All clear and distinct ideas are true.
Therefore, there is a tree.
“Cartesian Foundationalism” refers to the view that when, and only when, claims about the
external world (such as, There is a tree) rest upon—that is, are deduced from—“objectively
certain” premises, this is an instance of knowledge. Objectively certain premises are foundational
beliefs—those that, like “I exist”, are immune to doubt. All claims about the content of one’s
mind are foundational in this sense.
Premises 1 and 2 are supposed to be foundational, but 3 says something about the link between
clear and distinct perception and the way the world is. So premise 3 is not foundational. Is it
justified? If it is, it must be deduced from other foundational (or justified) beliefs.
This is why Descartes needs to refute the evil deceiver argument by way of proving that God
exists (and is not a deceiver)—to establish with certainty that all clear and distinct ideas are true.
It’s difficult to say exactly how one arrives at a clear and distinct idea, but Descartes argues that
this criterion is the mark of accurate perception.
The Cartesian Circle
Note that since no claim is justified unless it can be deduced from foundational (or other
justified) beliefs, the claim that God exists must be proved on this basis. Thus, Descartes gives a
version of the Ontological Argument in which he claims that the cause of his idea of God—a
perfect being—must be God. Since this belief isn’t itself foundational—again, it refers to the link
between what’s in the mind and what isn’t—it too must have a proof. Does it?
Descartes’ defense of the claim that his idea of God must be caused by God appeals to its clarity
and distinctness, which assures him of its veracity. But this is problem. In order to prove that
there is a tree, we need to prove that all clear and distinct ideas are true, but in order to prove that
all clear and distinct ideas are true, we need to prove that God exists, but in order to prove that
God exists, we need to prove that the cause of the idea of God is God, but in order to prove this
we must appeal to the clarity and distinctness criterion, which has not yet been established as
guaranteeing truth. This problem has come to be known as “The Cartesian Circle”.
Hume's Problem of Induction
Read the excerpt from David Hume's Enquiry Concerning Human Understanding.
Whereas knowledge seems to require that the believed proposition is true, which, according to
the skeptic, can never be known due to our fallible and limited senses, justification is a weaker
notion consisting in the strength of one's evidence and/or reasons for believing something. Thus,
you may be justified in believing that, for example, there is a computer on the desk, even if you
don't know (since you might be dreaming) that there is a computer on the desk.
Of course, the justification of immediate sense perception is more important when one considers
that the claims of science depend on it. For example, scientists think that there is black hole at
the center of our galaxy; that all life on Earth is related; that the next emerald discovered will be
green; and that the laws of nature hold throughout the universe. Induction is the means we have
of arriving at most of these claims.
Induction--the sort of interest to us here--is a type of inference that moves from evidence to
either a prediction about what will happen, or a generalization about what is always the case.
Hume's Argument
David Hume was interested in the justification of these predictions and generalizations-specifically, to what degree are we justified in believing that, for example, the sun will rise
tomorrow?
No doubt we do believe that the sun will rise tomorrow, and we certainly feel justified in
believing this since the sun has always risen in the past. But Hume points out that we would have
no justification for this prediction if it weren't for the assumption that what connects the past
with future events is the presence of causal laws. (In this case, since gravity holds the Earth in
orbit around the sun and inertia keeps the Earth spinning on its axis, we can be reasonably
assured that the sun will rise tomorrow.) Thus, our inference from the past to what will happen in
the future is only justified if these causal laws are.
Causal laws are generalizations. On what basis are we justified in believing in a causal law?
How Might Any Claim Be Justified?
Here we should consider how any claim might be justified. Every claim, according to Hume, is
either a relation of ideas or a matter of fact. Relations of ideas are always justified since they
follow from definitions. Matters of fact on the hand are of two sorts: observed and unobserved.
Observed matters of fact are justified, since all knowledge stems from this sort of sensory input.
Unobserved matters of fact, however, must be justified by an argument, of which there are two
types: deductive and inductive. Deduction is a form of inference that is truth-preserving; thus,
any conclusion that follows deductively from true premises is justified.
What about induction? Since induction is a type of inference that moves from evidence to a
prediction or generalization, inductive arguments must presuppose that nature is uniform, in the
sense that causal laws hold everywhere and for all time. Thus, inductive arguments are only
justified if the principle of the uniformity of nature is justified.
So now we might ask: how are causal laws justified?
Are Causal Laws Justified?
Hume argues that causal laws are not relations of ideas, since they are certainly not true by
definition. Nor are they observed matters of fact--since they are supposed to be "laws", they go
well beyond the present and past testimony of the senses and say something about how things are
generally. Can one give a deductively valid argument for a causal law?
No. Our reason for believing that, for example, F=ma (a generalization), is that all
experimentation thus far confirms this hypothesis. It is always possible, however, that we will
discover it to be false. Thus, this law is supported by inductive reasoning. Now, as noted, all
induction presupposes that nature is uniform, so causal laws, since they are all generalizations of
this sort, are only as justified as the principle of the uniformity of nature (PUN).
Is PUN Justified?
Here we have this claim that seems to be at the root of all claims of science: that nature is
uniform. We are justified in believing that the sun will rise tomorrow only if we are justified in
believing causal laws which connect past observations with predictions about what will happen
in the future, and we are justified in believing in causal laws only if we are justified in believing
that nature is uniform.
Hume argues that we are NOT justified in believing that they sun will rise tomorrow because we
are not justified in believing that nature is uniform. To show this, you need only run the claim,
"Nature is uniform" through all the possibilities there are for justifying such a claim.
Ultimately, PUN is justified by induction, which presupposes the very thing we are trying to
justify.
You're the philosopher: Use the chart below to demonstrate that PUN is not justified, and
therefore, neither is the claim that the sun will rise tomorrow.