Rene Descartes: Context and Theory
Crisis of Knowledge
The medieval worldview was undermined in various ways during the Renaissance, Reformation, and
Counter Reformation movements of the 16th century. The science or wisdom of the Middle Ages was
based on three sources: Aristotle, Reason, and Theology. The reliability of each of these comes under
attack in various ways in the 16th century. Since each of these represented a kind of foundation on a
pyramid structure of human knowledge, these attacks created cracks in the foundations of the Western
intellectual tradition. They were responsible for a crisis of knowledge, what we can call an
epistemological crisis. (Epistemology is the study of knowledge; epistemological, having to do with
A. The authority of Aristotle was put into question with the rise of a new science, specifically the
new physics and scientific methodology of scientists such as Copernicus and Galileo. The
critique of geocentric cosmology is an important example of the sort of challenges put to the old
science rooted in the Greek tradition.
B. The authority of reason was put into question with the rise of various forms of skepticism
during the Renaissance. Various skeptical arguments had existed in Greek philosophy
subsequent to Aristotle. Since the Renaissance involved a revival of classical culture, it also
brought into circulation various writings by Greek skeptics (e.g., Sextus Empiricus). One of the
Descartes' contemporaries, Montaigne, was an important defender of skeptical arguments. One
of Montaigne's central claims was that all we could "know" are appearances, not the real nature
C. The authority of theology was put into question with the rise of the Reformation and its
challenge against the Roman Catholic ecclesiastic authority. Also, the Reformation itself
quickly became a very diverse and pluralistic movement, further raising doubts about whether
theological ideas based on the Bible could serve as an authority since there were so many
different interpretations of the Biblical text.
A + B + C = an epistemological crisis for the Western world. It is this crisis that Descartes aims to
Meditation I: Certainty and Indubitability
Descartes' project was to find a secure foundation for the sciences or knowledge. His project was not
simply to fix the foundations, but to actually establish a new foundation. He wants to examine his
beliefs to see which, if any, are certain or indubitable.
Beliefs and Propositions
In philosophy the word "belief" typically refers to the cognitive act of thinking: "something" is true.
It is belief that such-and-such is (or is not) the case. The "such-and-such" is something we can express
by declarative sentences. For instance, I believe that I teach at Santa Margarita H.S.. In other words,
I take it to be true that I teach at Santa Margarita H.S . The declarative sentence "I teach at Santa
Margarita H.S." expresses a meaning. What I believe when I believe that I teach at Santa Margarita
H.S. is not the sentence "I teach at Santa Margarita H.S.," but the meaning expressed by this
sentence. This meaning can be expressed by different sentences in different languages, but it would
still believe the same thing if I believed the sentences in a different language that also asserted "I teach
at Santa Margarita H.S." So the object of belief is the meaning of a declarative sentence.
Philosophers call the meaning of any declarative sentence a proposition (or statement).
There are other uses of the word belief. Sometimes we use it to refer to religious views or faith, where
there may be no evidence for what is believed, or where we don't o can't know what we believe. In
other cases, we speak about belief in something or someone to indicate trust. These will not be the uses
of the term in Descartes. It will be propositonal belief.
We have propositional beliefs of various sorts. There are memory beliefs (e.g., I had eggs from
breakfast yesterday). There are sensory perceptual beliefs (e.g., I see a true). There are introspective
beliefs (e.g., I seem to see a true). There are logical and mathematical beliefs (e.g., A=A, 2 + 2 = 4).
And there are many other sorts of beliefs. Not only do we have various kinds of beliefs, we have plenty
of each sort. We form new beliefs all the time. Not all our beliefs are conscious at a given time, but
they are the sort of thing that we could become aware of fairly easily upon reflection.
Certainty and Indubitability
We use the word "certain" and "certainty" in various ways. Frequently it is used to refer to the state of
believing something with maximal confidence. You are certain of something if you are sure of it. This
is a kind of psychological or subjective certainty. This is not the sort of certainty Descartes is looking
for. There are many beliefs that are psychologically certain that will not be certain in Descartes' sense.
Descartes is concerned with certainty in a more objective sense. A certain belief will have as its object
a proposition that cannot, logically or objectively speaking, be doubted. It is one for which there are no
grounds for doubt, i.e., no objective reason to suppose that it might be false. Such a belief is also called
We can state Descartes general line of reasoning as follows:
1. If we know some proposition p, then we are certain of p.
2. If we are certain of p, then there are no grounds for doubting p.
3. Therefore, if we know some proposition p, then there are no grounds for doubting p.
What is a ground for doubt?
A ground for doubt, according to Descartes, will be any possible situation that raises a "question-mark"
over the truth of a belief. And we must place a "question-mark" over any belief is if there is a possible
situation in which our holding the belief would be consistent with the belief being false. Hence, a
ground for doubt does not require proving that a belief is false, but only showing that it could be false
or need not be true. In short: in situation Q, it is possible that <I believe p and p is false>. In such a
case, it is possible that we are mistaken. Hence, we cannot be objectively certain that p is true.
The Method of Doubt Examined
A. Descartes' method of establishing a new epistemological foundation begins with
"Methodological doubt." Descartes begins with the general destruction of his former opinions
with the hope to see what opinions or beliefs can be left standing after subjecting all his
opinions to rigorous philosophical critique.
B. Descartes argues that methodological doubt need not require proving that beliefs are false, but
only that they are not certain or indubitable.
C. Descartes does not aim to examine each of his beliefs individually to arrive at a general
conclusion about his beliefs. He aims to examine the sources of belief or principles by which
we learn. Call this poisoning the cognitive well. If these sources or principles are questionable,
then there will be doubt also about the beliefs we derive from them, in much the same way that
poison in a well contaminates any water taken from the well. To raise doubts about a source of
belief, propose a possible situation that would undermine the reliability of the source for any
belief it could produce.
D. The source of most of our beliefs is sensory experience. At any rate, nearly all our beliefs about
the external world arise directly or indirectly from sense experience. These beliefs are may be
called empirical or a posteriori beliefs. They depend on sense experience, or at least our
knowing them depends on sense experience. Descartes also recognizes reason as a source for
some knowledge (such as mathematical knowledge and logic). What is known on the basis of
reason, in contrast to sense experience, is known a priori in contrast to a posteriori. So there are
two basic sources of belief. The goal of his methodological skepticism is to raise doubt about
the reliability each category of belief.
Grounds for Doubt
Each ground of doubt raises a question mark over a basic source of belief. In essence, each argument
proposes a situation in which are beliefs would not likely be true (i.e., situations that involve
misleading evidence). The inability to eliminate these possibilities implies that we cannot be
objectively certain that our beliefs are true.
A. Grounds for Doubting Empirical or A Posteriori Beliefs
A. 1. Hallucination/Illusion Argument
If one is having a hallucination or there is a perceptual illusion of sort some sort, then the a
posteriori beliefs formed in response to such experiences will be based on misleading or
unreliable evidence. Unless one can eliminate this possibility, the condition remains a
possibility. And thus it also remains a possibility that one believes p and p is false. Hence, one
cannot be certain of p.
But this argument is not comprehensive in scope, for although hallucinations or illusions
provide mislead us in some situations, we can always determine whether we are hallucinating
by increasing the resolution of our experience, by checking something at a closer distance or
using other senses or instruments.
2. The Dream Argument:
Descartes' dream argument, though, is a more comprehensive sort of ground for doubting a
posteriori beliefs. If one is dreaming, then the a posteriori beliefs formed in response to such an
experience will be based on misleading or unreliable evidence. Unless one can eliminate the
possibility that one is dreaming, one cannot be objectively certain that p is true. But Descartes'
thinks that we cannot eliminate this possibility, so we are stuck with its skeptical consequences.
We can represent Descartes' form of argument here as follows:
(P1) If I am dreaming, then the evidence I have for my a posteriori beliefs is misleading or unreliable.
(P2) It is possible that I am dreaming.
(P3) Hence, it is possible that the evidence I have for my a posteriori beliefs is misleading or
From (P3) it follows that in any situation I might find myself in, I might believe some a posteriori
proposition and yet it might be false. For the possibility of dreaming entails that however much
evidence I might have for my empirical beliefs, the evidence will always be consistent with the falsity
of my belief. Hence, the possibility that I am dreaming entails that I will always have a ground to doubt
my a posteriori beliefs.
Potential problem for Descartes' dream argument
Descartes' dream argument appears at points to rest on the assumption that Descartes has had dreams in
the past and that they are so similar to our waking state that we can't really tell them apart. The latter
belief in turn presupposes a belief about there being a distinction between our waking state and the
dream state and that we can compare one to the other to see that they are similar. These, of course, are
themselves a posteriori beliefs. But would Descartes be entitled to use these beliefs as premises in an
argument that aims to raise doubt about a posteriori beliefs. There is a potential self-defeat here.
There are several possible responses to this criticism of Descartes. Here is one. Although Descartes
may have in fact utilized such premises, it is clear that his argument need not make these controversial
assumptions at all. The argument in essence comes down to a claim about the possibility of humans
having a particular kind of experience (which he associates with what people call dreaming) in which
our experience is a misleading guide to truths about the external world. Put this way, the argument does
not depend on Descartes believing that he has had dreams before, much less than he can tell the
difference between dream-experiences and waking-state-experiences. The argument is purely a priori
or conceptual in nature, resting only on a premise about what is possible or non-contradictory. Surely,
we could be having a kind of experience to which we normally refer to by the word "dreaming."
Ground for Doubting A Priori or Conceptual Beliefs
The Evil Demon Argument:
The evil demon argument has the same general structure as the dream argument. This time, though, the
possibility under consideration is the existence of an all-powerful, malevolent being who controls are
minds and makes us have the thoughts we do. He could control our thoughts so most of our beliefs are
false, or at any rate such that all our experiences were misleading. However, unlike the dream
argument, the evil demon argument would extend doubt even to a posteriori beliefs, such as
mathematics. Therefore, we have grounds for doubting whether any of our sensory perceptual beliefs
are true, as well as our various a priori beliefs.
(P1) If there is an all-powerful, evil demon who deceives my mind, then the evidence I have for either
my a posteriori or a priori beliefs is misleading or unreliable.
(P2) It is possible that there is an all-powerful, evil demon who deceives my mind.
(P3) Hence, it is possible that the evidence I have for either my a posteriori or a priori beliefs is
misleading or unreliable.
Why did Descartes give this second argument? Clearly, the dream argument applies only to a posteriori
beliefs, whereas the evil demon argument applies also to a priori beliefs. Hence, the latter argument
provides a more comprehensive skeptical argument. But also, perhaps Descartes saw certain
weaknesses in his dream argument, and thus had need for another skeptical argument. He clearly
recognizes the applicability of the evil demon argument to a posteriori beliefs.
A Potential Problem for Descartes' Evil Demon Argument
Above I noted the danger of self-defeat in the dream argument. I also suggested that Descartes could
state that argument without reliance on any of the sorts of beliefs about which the argument is aimed to
induce skepticism. He could state the argument as a purely a priori matter. However, this problem re-
emerges in the evil demon argument, without the same way of escape. For the class of beliefs this
argument intends to raise doubts about it is broader than empirical beliefs, but includes a priori beliefs.
So it looks like Descartes is faced with a more significant problem of self-defeat at this juncture. How
can Descartes provide grounds or reasons for doubt if the very reasons he adduces to support this
skeptical conclusion can also be retrospectively called into question? Or does this self-defeat only
strengthen the force of Descartes' argument? Or might Descartes distinguish between mathematical a
priori beliefs and more basic a priori beliefs, specifically beliefs about what is possible or non-
contradictory? Strictly speaking his argument does not depend on mathematical beliefs, but on logical
or conceptual ones.
Meditation II: The Cogito and Certainty
In Meditation II Descartes proposes a truth that cannot be undermined by the skeptical arguments of the
previous meditation. The certain truth is: "I am, I exist." Or more completely (as stated in the Discourse
on Method), I think, therefore I am. Descartes recognizes that with respect to any belief that p, either he
is deceived in believing p or he is not deceived. But in either case, he must exist. So the one truth that is
certain is that I exist. Even if there is an all-powerful demon who controls me and implants false beliefs
in my mind, he can never deceive me with respect to my own existence. My existence is a precondition
for my being deceived.
But Descartes asks, "what I am?" This is crucial. For it might be possible to doubt one's own existence
if one conceived of oneself in terms or under categories that could be doubted. For instance, if one
thought of oneself as a human person with a particular physical structure and biological processes. One
could be mistaken about all of this. The evil demon could make you believe that you are a male person
with two arms and legs, etc, even though this is false. Hence, Descartes rejects the Aristotelian
definition of the self as a rational animal just because it raises further questions the answers to which
can be doubted. Only a conception of self as thought can circumvent being subject to doubt. Only if
one conceives of oneself as thought does it follow that one's own existence is indubitable or certain, for
one cannot doubt that one is a thinking thing since doubt itself presupposes thought. So the one certain
truth is that I exist, I exist as a thinking thing.
Certainty about All Introspective Beliefs
Other way of seeing how it is that we can be certain of our own existence is the mind is transparent to
itself in a way that the (external) world is not transparent to the mind. Detached from matters of the
external world, the mind can find certainty with respect to itself. Hence, we can be certain, not just that
we exist as a thinking thing, but of every truth about our own mind. More specifically, we can be
certain of all our immediate states of consciousness. It seems to me that there is a car in the driveway. I
feel tired. I am sad. These are all examples of truths that cannot be doubted for the same reason that one
cannot doubt whether one thinks or exists as a thinking thing. Although one may be mistaken about
whether there is a car in one's driveway, one can't be mistaken about it seeming to one that this is so.
Compare the two different propositions:
[A] It seems to me that there is a car in the driveway => truth about one's own mind
[B] There is a car in the driveway => truth about something external to one's own mind.
Descartes thinks that we can be certain of [A], though [B] can be doubted.
Meditation III: From the Self to the World
The problem generated by Meditation II is this. Even if Descartes can be certain of his own existence
and all truths about his own mind, it does not follow from this that he can have certainty about the
external world. But his project was precisely to provide a foundation for human knowledge, where this
includes what is allegedly known in the physical and natural sciences. In short, his goal is to provide a
foundation for knowledge of the external world. At this stage, though, he has provided only self-
knowledge. If this is a foundation for knowledge of the external world, we are still left with the
question as to how he can build knowledge of the external world on such a foundation. Meditation III
attempts to address this problem and to begin to show how one can move from self-knowledge to
knowledge of the external world.
A. The Basic Strategy Outlined
Descartes first makes an observation about an important characteristic of his self-knowledge, namely
that it is clear and distinct.
From (A) he infers a general rule, "Everything I clearly and distinctly perceive is true." With this rule
he can deduce the truth of other propositions if he clearly and distinctly perceives them.
He notes a possible problem with (B), namely that there are many things which he thought were
evident and clear but which turned out to be false. Moreover, on the basis of the evil demon argument
the possibility of deception remains for many things that we regard as clear and distinct. So the rule or
principle in (B) cannot be applied in an unproblematic way. By itself, it is insufficient to ground
certainty of the external world. This will be so just if the evil demon argument remains, as long as it is
possible that there is a powerful being who engages in systematic and global deception of human
Descartes proposes an argument that will eliminate the evil demon hypothesis. This argument is an
argument for God's existence, an argument for the existence of a being who is all powerful, all-
knowing, and all-good. Descartes will argue that if God exists, then the evil demon hypothesis is
eliminated. If and only if this hypothesis is eliminated, we can place our trust in the veracity of clear
and distinct ideas.
B. The Argument for God's Existence
(P1) I have an idea of God's existence.
(P2) If I have an idea of God's existence, then God exists as the cause of my idea of God.
(C) Therefore, God exists as the cause of my idea of God.
Notice that the first premise (P1) is an truth internal to Descartes' own mind. Since Descartes is entitled
only to begin with what is certain for him, he must construct his argument on the basis of truths that are
internal to his own mind. Descartes attempts to provide an argument for (P2) though. The argument is
roughly as follows:
(P3) Ideas owe their existence ultimately to some reality that mirrors the idea (actually or
potentially).(P3) is supported by the following considerations. Descartes thinks that ideas must be
caused by something. Moreover, ideas must be caused by something with at least as much reality as is
represented by idea itself (what it is about). This is for Descartes the application of a more general
principle about causality as such as applied to things in general (not simply ideas of things). With
respect to any thing or substance, there must be as much reality in the cause as in the effect, and the
reality of the cause must be correlated or resemble the reality of the effect. A stone, for instance, can
only come into existence by something that has existed and has existed in such a way that it can
produce a stone object. The cause of a stone must contain in itself stoneness (actually or potentially),
otherwise it could not make a stone. In short, "you can't give what you haven't got." Descartes thinks
that this principle of causality applies not merely to actual or formal existence of things (i.e., things as
they really exist) but also to objective reality (i.e., things as they exist in our minds). Ideas also must be
caused by something (real or another idea) that has the power to generate the idea. So in asking what
can cause an idea of x, we must ask what x is. Moreover, the ultimate source of any idea must be
something real, for every idea were generated from a previous idea there would be an infinite regress of
ideas. But no finite mind can hold an infinite number of ideas.
(P4) No finite thing can be the cause of the idea of God.
The idea of God is the idea of an unlimited, infinite, or perfect being. Descartes argues that this idea
could not have originated from ourselves. The finite cannot produce the concept of the infinite. There is
more (objective) reality contained in the idea of the infinite than is contained in an idea of the finite.
Human beings could only be the source of finite ideas. We could, for instance, be the source of our
ideas of monkeys or stones. We could arrive at such concepts by reflecting on the concepts given in our
self-knowledge. If we know ourselves as a finite substance, we can think our other substances that are
finite but that differ from us in various ways. We can extrapolate these concepts as modifications of the
concepts involved in our own self-knowledge. The idea of God, however, cannot be derived from
ourselves, for the concept of an infinite being is not a mere modification of any concept given in our
self-knowledge. For the same reason, the idea of God could not come from any finite thing, since no
finite thing could contain the conceptual requirements to derive the concept of the infinite. So the
concept of the infinite can only come from a reality than is itself infinite, but that would be God.
Objection. . . .The obvious objection to Descartes' argument here is that we can derive the concept of
the infinite from the concept of the finite simply by negating the latter. The infinite just means that
which is not finite. But if we know ourselves as finite, surely we can modify this concept by a simply
mental act of negation. We can derive the notion of the infinite from the concept of that which is not-
finite, and this in turn from the concept of the finite.
Descartes' response to this is simple. He argues that we could not possess the concept of the finite
unless we already possessed the idea of the infinite (at least latent in our consciousness). All
recognition of limit presupposes the notion of a boundary. Anytime we recognize that we are imperfect
in some way, this judgment takes place over against an assumed standard of perfection. So the
recognition of the infinite is actually prior in us to the recognition of the finite. We derive the concept
of finite with the help of an antecedent idea of the infinite. We do not derive the concept of the infinite
from the concept of the finite.
So Descartes thinks he proves that the idea of God in us logically implies that the existence of the
object of our thought. God must exist. Moreover, being all knowing, all powerful, and all good, God
would not engage in systematic global deception of his creatures, nor would he allow any other
creature to do this. Descartes requires deception, or permitting deception, as an indication of
imperfection. Hence, God could not be involved in deceiving us, anymore than our parents would
deceive us if they wished good to us.
In this way, Descartes thinks that he eliminates the evil demon hypothesis. If God exists, the evil
demon hypothesis is undermined. The existence of a perfect being as the author of our existence is not
compatible with our being systematically and globally deceived with respect to those things that appear
to us clearly and distinctly. Hence, we can trust the veracity of our clear and distinct ideas.
General Observations on Descartes' Philosophy
A. Descartes' search for certainty leads his to posit his own consciousness as the starting point for
human knowledge. This initiates an important turn towards subjectivity or inwardness that
characterizes much of philosophy after Descartes. There is also an interesting parallel between
Descartes and Plato at this juncture. Plato emphasized the answer being within us. Similarly for
Descartes, the answer to the epistemological crisis was within us, in our immediate consciousness of
our own existence.
B. Although Descartes' philosophy requires a turn inwards, perspective turning in on itself, it also
requires a turn outwards, a transcendent movement. For Descartes, only the existence of God can
bridge the gap between knowledge of self and knowledge of the external world. Although Descartes
begins with himself, he is compelled in his intellectual inquiry to move beyond himself. There is a
parallel here also with Plato. In the Meno, the road of intellectual inquiry, though it involves a look
inward, also must move beyond the inward. Virtue, it is concluded, is acquired by divine dispensation.
C. Descartes, unlike Plato, begins with doubt. Plato and Aristotle held the view that philosophy begins
with wonder, not doubt. But doubt, in Descartes, is intimately tied to a sharp dichotomy between the
inward and the outward, between a realm of epistemic safety in which knowledge is secure and a realm
of epistemic doubt in which claims to knowledge are initially subject to the skeptical arguments of the
Meditation I. In Descartes, this dichotomy implies a risk for the self, the risk of isolation and alienation
from the external world. This risk will become an important aspect of later philosophers such as
Kierkegaard, for whom the basic questions of philosophy deal with the nature of the self.