DESCARTES’ VIRUS: CARTESIAN METHODOLOGICAL SKEPTICISM & THE GROUNDS FOR DOUBT
I realized that it was necessary, once in the course of my life, to demolish everything completely and start again right
from the foundations if I wanted to establish anything at all in the sciences that was stable and likely to last.
— Descartes, Meditations on First Philosophy
The skeptic, being a lover of his kind, desires to cure by speech, as best he can, the self-conceit and rashness of the
dogmatists.
— Sextus Empiricus, Outlines of Pyrrhonism
Ignorance is preferable to error; and he is less remote from the truth who believes nothing, than he who believes
what is wrong.
— Thomas Jefferson
A new scientific truth does not triumph by convincing its opponents and making them see the light, but rather
because its opponents eventually die out, and a new generation grows up that is familiar with it.
— Max Planck, Wissenschaftliche Selbstbiographie
THE BIG IDE A S TO M AS TER
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Epistemic vs. psychological certainty
Broad knowledge skepticism
External world skepticism
Descartes’ three stages of external world
belief-formation
Representationalism
Epistemic bridge principle
Cartesian methodological skepticism
Skeptical counterpossibility
Skeletal skeptical argument
Indefeasible beliefs
Incorrigible beliefs
Self-evident truths
The problem of evidence underdetermination
Descartes’ goal: the building of a new, secure epistemic
foundation
Now that we understand the theories of epistemic warrant
with which Descartes is working—namely, evidentialism and
foundationalism—let us get clear on what types of
propositions he thinks must be in this foundation so it is
capable of doing its intended job, namely, being the best
possible foundation for the acquisition of scientia (i.e.,
genuine propositional knowledge). We can ask, then, what
kind of foundation is capable of doing that? Says Descartes,
only a foundation that includes only certain and indubitable
beliefs.
Certainty and indubitability: Descartes’ criteria for being
a properly basic belief
That Descartes believes that the foundation of knowledge must include only certain and indubitable beliefs
reveals that he is committed to the following epistemological principle regarding properly basic beliefs:
FIP:
A belief B is properly basic iff B is certain and indubitable.
(Call this principle The Foundational Inclusion Principle.) Why is Descartes committed to FIP? Since the goal is
to establish the best possible foundation for the acquisition of scientia, he believes that such a foundation must
be capable of (i) solving the problem of the external world as well as (ii) withstanding any possible challenge
from a philosophical skeptic. According to Descartes, the only type of foundation that can do that is one that
satisfies FIP: that is, one in which the properly basic beliefs—i.e., those that are properly included in the
foundational set—are certain and indubitable. Anything weaker, he argues, will always be subject to the
problem of the external world and attacks from other philosophical skeptics.
Philosophical skepticism & the problem of the external world
On philosophical skepticism
As we said earlier in the course, philosophical skepticism is the view that
Philosophical skepticism. It’s not possible to satisfy the conditions for knowledge.1
As we can see, philosophical skepticism is any type of view that calls into question the standard view of
knowledge. If they are correct, then the first thesis of that view, namely,
SVK1:
We have (propositional) knowledge in one or more particular domain of inquiry (e.g., physics,
theology, mathematics, ethics)
is false. To make this case against SVK1, a philosophical skeptic calls into question our ability to satisfy (one or
more of) the necessary conditions for having propositional knowledge in some particular domain of inquiry.
They do this by arguing that the second thesis of the standard view of knowledge, namely,
SVK2:
We have sources that provide us with (propositional) knowledge
is false. For if the second thesis is false, the first thesis must also be false. For SVK2 is a necessary (though not
sufficient) condition for SVK1.
There are a number of varieties of philosophical skepticism. The particular brand of skepticism with which
Descartes is concerned is broad knowledge skepticism, namely, the view that
BK–SKEPTICISM.
For any proposition p, it is not possible for a person S to know that p is true or that p is
false.
In his Mediations on First Philosophy, Descartes seeks to demonstrate that BK–SKEPTICISM is false. He is not a
broad knowledge skeptic. Why does he think that BK-SKEPTICISM is false? Before we look at his case,
however, we need to understand two things: the so-called problem of the external world and the Cartesian
method for attacking BK–SKEPTICISM.
On the problem of the external world
One particular skeptical worry with which Descartes is concerned is the problem of the external world. What is
that problem? It is simply the problem of trying to determine whether or not philosophical skepticism about
the external world is true. What is philosophical skepticism about the external world? It is the view that
EW–SKEPTICISM.
It’s not possible that humans have propositional knowledge about the external
world, e.g., what exists, what things that exist are like, etc.
As you can see, if BK–SKEPTICISM is true, then EW–SKEPTICISM must be true as well. To make sense of this,
consider a quote from Barry Stroud:
Since at least the time of Descartes in the seventeenth-century there has been a philosophical problem
about our knowledge of the world around us. Put most simply, the problem is to show how we can have
any knowledge of the world at all. The conclusion that we cannot, that no one knows anything about the
world around us, is what I call ‘scepticism about the external world’, so we could also say that the problem
is to show how or why scepticism about the external world is not correct (Barry Stroud, The Significance of
Philosophical Scepticism, p.1).
Why is this problem said to arise? Why does Descartes take this to be a genuine problem? While we’ll discuss
the answer to these questions in greater detail, we can say simply this: given that our sensory faculties seem to
be the only source of evidence about the constituents and goings-on in the external world and there are
serious questions as to whether or not our sensory evidence (about the world) is any good—i.e., whether our
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Recall that we distinguished philosophical skepticism from methodological skepticism. We’ll discuss the
latter variety shortly.
sensory faculties reliably present the world to us as it is—we cannot be sure that we know what we think we
know. We think we know that the external world is such-and-such, and thus-and-so. That much is obvious and
undeniable. Yet, as we’ll see, what is far from obvious is whether we actually know such things. So, The
Problem of the External World is the problem of trying to show that we know what we think we know and, as I
noted, Descartes thinks that the only way to solve this problem is to find propositional claims that are certain
and indubitable.
Two kinds of skeptical methods: enumerative vs. modular methodological skepticism
How does Descartes seek to undermine EW–SKEPTICISM as well all forms BK–SKEPTICISM? He adopts what
we call Cartesian methodological skepticism. Cartesian methodological skepticism is not philosophical
skepticism; it is something else. Simply put, methodological skepticism is the following view:
Methodological skepticism.
A person S should suspend judgement about the truth-value of a
proposition p until S has sufficient epistemic warrant for accepting that p
is true or that p is false.
This is the kind of skepticism that we expect good thinkers to adopt. And Descartes does in his search for the
best possible foundation for science. In other words, in order to locate that foundation, he must figure out
whether or not BK–SKEPTICISM is true. If BK–SKEPTICISM is true, then the search of the foundations of science
is dead. However, if it isn’t true, if BK–SKEPTICISM is false, then we can know things and we can seek to
determine the foundation that makes that possible.
To do that work, Descartes takes up the skeptic’s case for BK–SKEPTICISM and makes the best defense of it
that he can. His case involves testing his beliefs using the skeptic’s method in order to find out whether or not
he knows anything he thinks he knows. (This is why the skepticism that Descartes defends in his discussion is
referred to as Cartesian methodological skepticism.) If he finds that he actually knows what he thinks he
knows, then BK–SKEPTICISM is in fact false. If, however, he finds that he does not know any of what he thinks
he knows, then BK–SKEPTICISM is in fact true, and as already noted, science is dead. However, there are two
different ways to employ this type of methodological skepticism. They are enumerative methodological
skepticism and modular methodological skepticism.
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Enumerative methodological skepticism: the enumerative methodological skeptic considers each
individual belief and determines whether there are rational grounds for doubting that very belief.
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Modular methodological skepticism: the modular methodological skeptic considers each belief-forming
module (i.e., the faculty from which a given kind of belief arises) and determines whether there are rational
grounds for doubting the reliability of the evidence given by that module.
Descartes employs the latter type: the modular skeptical method. He calls into question each of his beliefs by
calling into question the reliability of his belief-forming modules. As such, we can say that the important
principle that Descartes is using for modularity testing is this:
MODULAR–DOUBT.
If a person S has grounds for doubting the reliability of the source s from which S
comes to believe that p, then S has rational grounds for doubting all s-based
beliefs.
For instance, if Jones has grounds for doubting the reliability of his visual faculties, then Jones has grounds for
doubting all vision-based beliefs. With MODULAR–DOUBT explicitly formulated, Descartes is now ready to
pursue his goal.
Descartes’ grounds for doubt: skeptical arguments & skeptical counterpossibilities
In pursuit of this secure epistemic foundation, Descartes tests every one of his belief-forming modules to
determine whether or not those types of beliefs are to be excluded from the foundational set. He does this by
subjecting those modules to the skeptic’s main weapon: the skeptical counterpossibility.
On the concept of a skeptical counterpossibility. We will understand the notion of a skeptical
counterpossibility as follows:
SKEPTICAL-CP.
S has reason to doubt that a proposition p is true when there is some conceivable or
logically possible scenario sk such that:
(i)
S cannot rule out with certainty that sk is actual, and
(ii)
if sk is actual, then p is false.
SKEPTICAL-CP expresses the notion that not only are there alternate possibilities to what we take ourselves
to know or justifiably believe, but that these alternate possibilities are relevant to such a degree that we must
be able to rule them out if we are to be certain, and thus, actually know, what we claim to know. To
demonstrate that a given skeptical counterpossibility undermines any prima facie—that is, on the face of it—
instance of knowledge, the skeptic forms a skeptical argument.
The Skeletal Skeptical Argument. All skeptical arguments can be formulated in the following way. Letting ‘S ’ =
any cognitive subject, ‘p ’ = any proposition and ‘sk ’ = a skeptical counterpossibility, we get the following
pattern of reasoning:
SSA1.
S knows that p (is true) only if S can rule out sk is true.
SSA2.
S cannot rule out that sk is true.
Therefore,
SSA3.
S does not know that p (is true).
With The Skeletal Skeptical Argument in place, Descartes will test what he takes to be the best representative
of every belief-forming module.
Descartes’ (two most important) skeptical arguments
The Dream Argument
Using The Skeletal Skeptical Argument and replacing ‘sk ’ with I am dreaming (that there is a chair in front of
me) and ‘p ’ with there is a chair in front of me, we get the argument,
DA1.
I know that there is a chair in front of me only if I can rule out that I am dreaming (that there is a chair
in front of me).
DA2.
I cannot rule out that I am dreaming (that there is a chair in front of me).
Therefore,
DA3.
I do not know that there is a chair in front of me.
The Evil Genius Argument
Again using The Skeletal Skeptical Argument, if we replace ‘sk ’ with I am being deceived by an evil genius (that
there is a chair in front of me) and ‘p ’ with there is a chair in front of me, we get the argument,
EG1.
I know that there is a chair in front of me only if I can rule out that I am being deceived by an evil
genius (that there is a chair in front of me).
EG2.
I cannot rule out that I am being deceived by an evil genius (that there is a chair in front of me).
Therefore,
EG3.
I do not know that there is a chair in front of me.
In defense of (DA2) & (EG2): the problem of evidence underdetermination
According to the skeptic, (DA2) and (EG2) are true given that (i) it is impossible to have good evidence for
determining whether or not one is awake or dreaming (or not being deceived by an evil genius) since (ii) there
is no first-person criterion for determining whether one is awake or dreaming (or not being deceived by an evil
genius). Call this the problem of evidence underdetermination. It tells us that the evidence we have cannot
determine the truth.
Why do we supposedly face the problem of evidence underdetermination? The skeptic argues that this is so
given that any evidence that guides our believings is undermined in virtue of the following fact:
The best possible evidence one could in principle have fails to be good enough in light of the fact that
we are limited to how things appear, to the so-called “view from somewhere.”
In other words, given representationalism, the evidence that guides our believings is limited to how things are
represented to us by our minds, e.g., at most we know what it feels like to see a zebra, what it feels like to eat a
chocolate chip cookie, what it feels like to burn one’s hand, what it feels like to climb a frozen waterfall, etc.
We only know what zebras, chocolate chip cookies, burned hands, or climbing frozen waterfalls are like given
how our minds represent those things to us. Yet, in order to determine if things are as they appear, we
have to compare how things appear to how they are. But that requires being in a “view from nowhere” and
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that, we’ve already said, is not possible for us.
With that, we can see that the cases for (DA2) and (EG2) are based on the following principle regarding the
nature of evidence:
(UP)
If S believes that p on the basis of some evidence E and E does not make it more likely that p is true
rather than some incompatible proposition q, then S cannot rule out that q is true.
And with (UP) in place, the Cartesian skeptic argues for (DA2) and (EG2) like this:
(A)
How things appear to a person is the only evidence available to them for believing that they are (or
being deceived by an evil genius).
(B)
How things appear in both waking experience and dream experience (or evil genius experiences)
are qualitatively identical.
Therefore,
(C)
The only evidence available to a person for believing that they are awake and not dreaming (or
being deceived by an evil genius) is underdetermining.
Given (C) and (UP), it follows that
(D)
S cannot rule out that S is not dreaming (or being deceived by an evil genius).
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© 2016, Richard G. Graziano. All rights reserved. This material may not be used, or duplicated in part or
whole without express written permission by the author.