PHIL 2100: Introduction to Philosophy
Dr. Robert Lane
Lecture Notes: Thursday June 17, 2010


[4.2.3.] The Method of Doubt vs. A Priori Beliefs


The second group of beliefs to which he applies the method of doubt test is a priori beliefs:


a priori (df.): an a priori belief or statement is one that could be known or justified prior to or apart from sense experience, e.g., “2 + 2 = 4”; “Triangles have three sides”; “All bachelors are unmarried.”


Even if we are dreaming, the images of our dreams are composed of shapes, and can be counted using numbers, that (it seems) cannot be imaginary...


… Physics, Astronomy, Medicine and all other sciences which have as their end the consideration of composite things, are very dubious and uncertain; but that Arithmetic, Geometry and other sciences of that kind which only treat of things that are very simple and very general, without taking great trouble to ascertain whether they are actually existent or not, contain some measure of certainty and an element of the indubitable. For whether I am awake or asleep, two and three together always form five, and the square can never have more than four sides, and it does not seem possible that truths so clear and apparent can be suspected of any falsity [or uncertainty]. (Med I, Sober 208-209)


But even these sorts of belief fall victim to the Method of Doubt test, since there is always a possible reason to doubt any mathematical belief, or any belief about shapes and colors, namely, that other people have believed themselves to have perfect knowledge but have been wrong:


…as I sometimes imagine that others deceive themselves in the things which they think they know best, how do I know that I am not deceived every time that I add two and three, or count the sides of a square, or judge of things yet simpler, if anything simpler can be imagined? (Med I, Sober 209)


Descartes’ point is that even the most obviously true belief, e.g., that 2 + 3 = 5 or that the color blue is real, is not beyond doubt. The feeling of certainty that we have about such beliefs (i.e., our psychological certainty) is no guarantee that they are true (i.e., no guarantee of epistemic certainty).


So a priori beliefs, including the beliefs of mathematics, also fail the Method of Doubt test.



[4.2.4] The Evil “Genius.”


But Descartes thinks that these considerations are not enough to keep him from slipping back into his old habits of thinking. He wants to “give in” even more to distrust, to deliberately suppose that all of his prior beliefs might be false. He thinks that he has found a reason that will help him do just that, a possible reason for doubting every belief:


There might be an evil demon systematically deceiving us about them.


I shall then suppose … that … some evil genius [this is sometimes translated: “evil demon”] not less powerful than deceitful, has employed his whole energies in deceiving me; I shall consider that the heavens, the earth, colours, figures, sound, and all other external things are nought but the illusions and dreams of which this genius has availed himself in order to lay traps for my credulity; I shall consider myself as having no hands, no eyes, no flesh, no blood, nor any senses, yet falsely believing myself to possess all these things; I shall remain obstinately attached to this idea, and if by this means it is not in my power to arrive at the knowledge of any truth, I may at least do what is in my power [i.e., suspend my judgment], and with firm purpose avoid giving credence to any false thing, or being imposed upon by this arch deceiver, however powerful and deceptive he may be. But this task is a laborious one, and insensibly a certain lassitude leads me into the course of my ordinary life. And just as a captive who in sleep enjoys an imaginary liberty, when he begins to suspect that his liberty is but a dream, fears to awaken, and conspires with these agreeable illusions that the deception may be prolonged, so insensibly of my own accord I fall back into my former opinions, and I dread awakening from this slumber, lest the laborious wakefulness which would follow the tranquillity of this repose should have to be spent not in daylight, but in the excessive darkness of the difficulties which have just been discussed. (Med I, Sober 210)


The 21st century analogue of Descartes’ evil demon story is: it is possible that you are not really a college student but instead a brain in a vat—a brain kept alive through artificial means and whose experiences are generated by a super-computer to which it is connected. (This is very similar to the premise of the movie The Matrix.)


So the first Meditation ends with Descartes certain of nothing.



[] The Representational Theory of Perception.


At the end of Meditation I, Descartes is faced with the possibility that all of his beliefs about the world are false.  It is possible that none of his ideas about the world represent anything that actually exists. There might be no connection between his ideas, the way he thinks the world is, and the actual world. Descartes thinks that such a situation is possible because he approves of a certain theory about perception, i.e., of how human beings perceive the world…


The Representational Theory of Perception

1.      We have no direct access to things in the world; we only have direct access to our own ideas.

2.      “Ideas” means all the contents of the mind (perceptions, images, memories, concepts, beliefs, decisions, etc.)

3.      Ideas serve as representations of things other than themselves. When you perceive a tree, what you are directly perceiving is your idea of a tree—something like a mental image. That idea represents the actual tree that is outside your mind.

4.      Many of the things that ideas represent, they represent as being external to the mind. For example, your idea of the tree represents that tree as being something that is apart from you and your experiences of it.

5.      It is possible that ideas represent these things correctly, but it is also possible that they represent them incorrectly.[1]



[4.2.5.] The Cogito.


Toward the beginning of Meditation II, Descartes hits upon a belief about which he thinks he can be certain:


I suppose, then, that all the things that I see are false; I persuade myself that nothing has ever existed of all that my fallacious memory represents to me. I consider that I possess no senses; I imagine that body, figure, extension, movement and place are but the fictions of my mind. What, then, can be esteemed as true? Perhaps nothing at all, unless that there is nothing in the world that is certain.

But how can I know there is not something different from those things that I have just considered, of which one cannot have the slightest doubt? Is there not some God, or some other being by whatever name we call it, who puts these reflections into my mind? That is not necessary, for is it not possible that I am capable of producing them myself? I myself, am I not at least something? But I have already denied that I had senses and body. Yet I hesitate, for what follows from that? Am I so dependent on body and senses that I cannot exist without these? But I was persuaded that there was nothing in all the world, that there was no heaven, no earth, that there were no minds, nor any bodies: was I not then likewise persuaded that I did not exist? Not at all; of a surety I myself did exist since I persuaded myself of something [or merely because I thought of something]. But there is some deceiver or other, very powerful and very cunning, who ever employs his ingenuity in deceiving me. Then without doubt I exist also if he deceives me, and let him deceive me as much as he will, he can never cause me to be nothing so long as I think that I am something. So that after having reflected well and carefully examined all things, we must come to the definite conclusion that this proposition: I am, I exist, is necessarily true each time that I pronounce it, or that I mentally conceive it. (Med II, Sober 210-11)



Even if Descartes is being fooled about everything by an evil demon, it still must be the case that he, Descartes, exists. After all, how could he be deceived, how could he doubt, how could he have the thoughts that he is having right now, unless he exists? Thus, there is a belief of which there is no possible reason to doubt: “I exist.”


In the Discourse he expresses the same idea as, “I think, therefore I am,” or (in the Latin translation) Cogito ergo sum.


We can think of the cogito as combining two separate claims: “I am thinking” and “I exist”.


So here at last Descartes has found two basic beliefs, two beliefs that are indubitable, in the sense that, so long as Descartes has them, they cannot possibly be false. These beliefs pass the Method of Doubt test, and they will serve as part of the foundation for the entire structure of knowledge that Descartes wants to build.



[4.2.6.] What is the “I” of the cogito?


Having concluded that he must exist, Descartes “I exist” must be true whenever he is thinking it, he now asks: what is this “I”?


He considers the thoughts he had on the subject before he began trying to doubt: he thought that he was a body (something which takes up space and can be perceived by the senses) and a soul, although his idea of what the soul is was somewhat vague:


In the first place, then, I considered myself as having a face, hands, arms, and all that system of members composed on bones and flesh as seen in a corpse which I designated by the name of body. In addition to this I considered that I was nourished, that I walked, that I felt, and that I thought, and I referred all these actions to the soul: but I did not stop to consider what the soul was, or if I did stop, I imagined that it was something extremely rare and subtle like a wind, a flame, or an ether, which was spread throughout my grosser parts. (Med II, Sober 211)


After the method of doubt, he can no longer be certain that he has a body, and he can no longer be certain about certain characteristics of his soul: that it nourishes the body, that it moves the body, or that it perceives via the sense organs (since that requires a body).


Finally he discovers the one thing that he cannot doubt that he is:


What of thinking? I find here that thought is an attribute that belongs to me; it alone cannot be separated from me. I am, I exist, that is certain. But how often? Just when I think; for it might possibly be the case if I ceased entirely to think, that I should likewise cease altogether to exist. I do not now admit anything which is not necessarily true: to speak accurately I am not more than a thing which thinks, that is to say a mind or a soul, or an understanding, or a reason, which are terms whose significance was formerly unknown to me. I am, however, a real thing and really exist; but what thing? I have answered: a thing which thinks. (Med II, Sober 212)


So Descartes is certain that HE IS A THING THAT THINKS. Here “think” includes a number of different mental activities:


But what then am I? A thing which thinks. What is a thing which thinks? It is a thing which doubts, understands, [conceives], affirms, denies, wills, refuses, which also imagines and feels. (Med II, Sober 213)


One of Descartes’ central points, here and throughout the Meditations, is that he is not identical with his body. He is a mind, a soul, and his mind/soul and his body are two completely different things. To be sure, they interact with one another, but each is capable of existing apart from the other. We will return to this aspect of Descartes’ philosophy when we study the philosophy of mind, next week.



[4.2.7.] First-Person Psychological Beliefs


Having identified the cogito as something about which he can be absolutely certain, and having come to understand that the “I” of the cogito refers to a thing that thinks, Descartes now thinks of a flood of beliefs about which he can also be certain… beliefs about his own mind.


Am I not that being who now doubts nearly everything, who nevertheless understands certain things, who affirms that one only is true, who denies all the others, who desires to know more, is averse from being deceived, who imagines many things, sometimes indeed despite his will, and who perceives many likewise, as by the intervention of the bodily organs? Is there nothing in all this which is as true as it is certain that I exist, even though I should always sleep and though he who has given me being employed all his ingenuity in deceiving me? (Med II, Sober 213)


So the list of absolutely certain beliefs, in addition to “I think” and “I exist,” now includes:

·         “I doubt some things.”

·         “I understand some things.”

·         “I affirm some things.”

·         “I desire some things.”

·         “I am averse from some things.”

·         “I imagine some things”

·         “I perceive some things.”


He proceeds to identify a few more of these sorts of belief:


Finally, I am the same who feels, that is to say, who perceives certain things, as by the organs of sense, since in truth I see light, I hear noise, I feel heat. But it will be said that these phenomena are false and that I am dreaming. Let it be so; still it is at least quite certain that it seems to me that I see light, that I hear noise and that I feel heat. That cannot be false; properly speaking it is what is in me called feeling; and used in this precise sense that is no other thing than thinking. (Med II, Sober 213)


Even if the evil demon is fooling you into thinking that the sky is blue, or that two and two make four, you can still be certain that it seems to you that the sky is blue and that you believe that two and two make four.


So another sort of belief gets added to the list:



All of the epistemically certain beliefs that Descartes has identified are…


first-person psychological beliefs (df.): beliefs that a person has about his or her own mind, its activities and contents.


Descartes’ view seems to be that all of one’s own first-person psychological beliefs pass the method of doubt test, since no one could believe them unless they were true. Descartes thought that if you have this sort of belief, then you cannot be mistaken. This view is called…


the thesis of the incorrigibility of the mental (TIM) (df.): The idea that first-person psychological beliefs cannot be mistaken. [“incorrigible” means


Is TIM true? Many psychologists don’t think so [e.g., Sigmund Freud (1856-1939)—see Sober’s description of the Oedipus complex on pp.161-62, but note that many non-Freudians also reject TIM].[2]



[4.2.8.] The Foundations of Knowledge.


So Descartes believed that the foundations of knowledge (basic beliefs) are first-person psychological beliefs, because (he thought) we cannot be wrong about them.


And our beliefs about the external world can be “based on” or “built up from” these basic beliefs.








 | knowledge







All beliefs about the world outside the mind. E.g.

“There is a book in front of me.”

“There is a knife in my thigh.”


1st Person Psychological Beliefs


E.g. “I seem to see a book in front of me.”

“I feel a pain in my thigh.”

“I believe that p.”



But of course, this is just a metaphor. We need a more literal explanation of the relationship between basic and derived beliefs.


Is the relationship that of a deductive argument?  I.e., are derived beliefs deductively implied by basic beliefs? For example,


basic                It seems to me that there is a book in front of me.

derived                        So, there really is a book in front of me.


This isn’t a deductively valid argument (and that’s the lesson Descartes’ method of doubt is supposed to teach: it is always possible for a belief about the external world to be false).


Still, Descartes thinks that derived beliefs are deductively implied by basic beliefs… It’s just that there’s one basic belief that we haven’t yet discovered. Descartes thinks that once we add that basic belief to all the other (first-person, psychological) beliefs, we’ll be able to derive beliefs about the external world… and those beliefs will be certain, since they’ve been deductively derived from certain beliefs.



[4.2.9.] The Wax Passage.


At this point Descartes has identified a large class of beliefs—first-person psychological beliefs—to serve as the foundation of knowledge. But he still has not overcome skepticism about the existence of the world outside his mind, including the entire physical world and all of its contents.


He now turns his attention to beliefs about “corporeal things”—physical objects—outside the mind.


Still, he says, he cannot help but feel that he has a more distinct understanding of those things than he does of his own mind—very strange, since he is certain of his own existence but not at all certain of the existence of those corporeal things.


So at this point he focuses on those (allegedly existing) objects. He uses as an example a hunk of bees wax.


Let us take, for example, this piece of wax: it has been taken quite freshly from the hive, and it has not yet lost the sweetness of the honey which it contains; it still retains somewhat of the odour of the flowers from which it has been culled; its colour, its figure, its size are apparent; it is hard, cold, easily handled, and if you strike it with the finger, it will emit a sound. Finally all the things which are requisite to cause us distinctly to recognise a body, are met with in it. But notice that while I speak and approach the fire what remained of the taste is exhaled, the smell evaporates, the colour alters, the figure is destroyed, the size increases, it becomes liquid, it heats, scarcely can one handle it, and when one strikes it, no sound is emitted. Does the same wax remain after this change? We must confess that it remains; none would judge otherwise. What then did I know so distinctly in this piece of wax? It could certainly be nothing of all that the senses brought to my notice, since all these things which fall under taste, smell, sight, touch, and hearing, are found to be changed, and yet the same wax remains. (Med II, Sober 214)


Descartes thinks that he knows many things about the wax: that it has a certain color, smell, taste and texture, that it makes a specific noise when thumped with the finger… All of this (alleged) knowledge is a result of sense experience. (It is all  a posteriori.)


But all of these properties can change completely when the wax is exposed to the heat of a fire. The wax itself remains (it hasn’t disappeared), but it has a different taste, smell, color, and texture, and it no longer makes a noise when thumped. So none of the properties that he took the wax to have at first can really be properties of the wax… and his beliefs that the wax has those properties can’t really count as knowledge.


Once all of the merely temporary properties are “stripped away” from the idea of the wax, all that remains is the idea of an “extended thing which is flexible and movable.” (Med II, Sober 214)

·         This idea does not result from the senses.

·         It does not even result from his imagination, since there is an infinite number of ways in which the wax can be changed and moved.

So from where does this idea come?:


[W]hat must particularly be observed is that its perception is neither an act of vision, nor of touch, nor of imagination, and has never been such although it may have appeared formerly to be so, but only an intuition of the mind, which may be imperfect and confused as it was formerly, or clear and distinct as it is at present, according as my attention is more or less directed to the elements which are found in it, and of which it is composed.

solely by the faculty of judgment which rests in my mind, I comprehend that which I believed I saw with my eyes. (Med II, Sober 214-15)


His clear and distinct idea of the wax must result from his understanding. We grasp the true nature of the wax with the understanding. When it stems wholly from sensation, our perception of the wax is “imperfect and confused.” But our knowledge of the wax that is the result of “a purely mental inspection” can be clear and distinct.


The general point is that even physical objects, which we believe exist outside the mind, are most clearly understood via the use of the understanding rather than by the use of the senses. This is a reflection of Descartes’ rationalism and his rejection of empiricism.


Descartes thinks that if even with regard to physical objects, clear and distinct knowledge of those objects is available only through mental inspection rather than sensation, then knowledge of what we are must also be approached in this way. So another aspect of the wax passage is that it weakens our tendency to think of ourselves as what we can sense, viz. our physical body.  It helps to support Descartes’ claim that we are not the same things as our bodies.


...I know for certain that nothing of all that I can understand by means of my imagination [i.e., qualitative characteristics] belongs to this knowledge which I have of myself [viz. that I am a thing that thinks], and that it is necessary to recall the mind from this mode of thought with the utmost diligence in order that it may be able to know its own nature with perfect distinctness. (Med II, Sober 213)


...since it is now manifest to me that even bodies are not properly speaking known by the senses or by the faculty of imagination, but by the understanding only, and since they are not known from the fact that they are seen or touched, but only because they are understood, I see clearly that there is nothing which is easier for me to know than my mind. (Med II, Sober 216)



[4.2.10.] Descartes’ Strategy for Fighting Skepticism About the External World.


Resuming our coverage of Descartes’ Meditations with Meditation III…


In the previous Meditation, Descartes discovered a set of beliefs which he thinks are certain (cannot possibly be false): first-person psychological beliefs.


So he thinks he has a certain foundation upon which he will try to build the rest of his knowledge. In particular, he hopes his beliefs about the external world (e.g., the belief that there is a book in front of me) can be based on that certain foundation. In other words, he also wants to defeat skepticism about knowledge of the external world.


His strategy is as follows:


Prove that God exists (and is no deceiver)

in order to

Solve the Problem of the Criterion

in order to

Be certain that beliefs about the external world are true

which amounts to

disproving skepticism regarding knowledge of the external world


Keep in mind this broad outline of what Descartes is up to as we discuss the specifics of each step.



[4.2.11.] Solving the Problem of the Criterion.


the problem of the criterion (df.): the problem of finding some criterion or mark that sets knowledge apart from mere belief—some mark or characteristic that all instances of knowledge have in common and by which we can recognize them as true beliefs.[3]

·         Since knowledge requires truth, finding a criterion or mark of knowledge would amount to finding a criterion or mark of truth.


Descartes is searching for a criterion, not just of knowledge, but of (epistemic) certainty. Since a belief that is certain is (like knowledge) one that has to be true, such a criterion of certainty would also be a criterion of truth.


Remember that in Med II, Descartes discovered a set of beliefs which he thinks are certain (cannot possibly be false): first-person psychological beliefs (beliefs about his own mind). Among these beliefs is that he is a thing that thinks. He asks: what is it about this belief that makes it certain? What distinguishes it from mere belief?


I am certain that I am a thing which thinks; but do I not then likewise know what is requisite to render me certain of a truth? Certainly in this first knowledge there is nothing that assures me of its truth, excepting the clear and distinct perception of that which I state, which would not indeed suffice to assure me that what I say is true, if it could ever happen that a thing which I conceived so clearly and distinctly could be false; and accordingly it seems to me that already I can establish as a general rule that all things which I perceive very clearly and very distinctly are true. (Med III, Sober 216)


His grasp of that belief about himself has a specific quality: it is so clear and distinct that it cannot possibly be false.


So Descartes identifies clearness and distinctness as the criterion of certainty. Any belief that is clear and distinct is certain.



[4.2.12.] Clear and Distinct Ideas.


Descartes means something very specific by “clear and distinct”—he is giving this phrase a new and specifically defined meaning. But to see exactly what that phrase means for Descartes, we need to go outside the Meditations to examine one of his other works: a textbook entitled Principia Philosophiae, or in English, Principles of Philosophy (1644).


a clear idea is one that is “present and apparent to an attentive mind, in the same way as we assert that we see objects clearly when, being present to the regarding eye, they operate upon it with sufficient strength.”

·         For example, seeing a writing pen held in your hand in good light; unless a belief is as clear as that, we are not supposed to accept it as certain.


a distinct ideas is one that is “so precise and different from all other objects that it contains within itself nothing but what is clear.” That is, a distinct idea is one that has only clear parts, and as a result is impossible to confuse with any other idea.

·         For example, my idea of a triangle is distinct from my idea of a square.


So ultimately, Descartes is identifying something about the way he experiences the Cogito— perhaps a feeling he has when he thinks of it—and concludes: whenever I experience a belief in this same way (“clearly and distinctly”), then the belief is certain, infallible, and cannot possibly be false.




Stopping point for Thursday June 17. This is the last day to withdraw from the class with a grade of “W”.  We are now one full day behind the original course schedule. For next time, read all of Descartes’ Meditation IV. Your second test is now tentatively scheduled for next Tuesday, June 22.


[1] I take this list of the constituent claims of the Representational Theory of Perception from Melchert, The Great Conversation.

[2] For more information on Freud, see Stephen P. Thornton, “Sigmund Freud,” Internet Encyclopedia of Philosophy, URL = <>, retrieved June 17, 2010.

[3] These days, “the problem of the criterion” normally refers to a problem concerning knowledge: in our investigation of questions concerning knowledge, we must either

·         begin by assuming that certain specific beliefs we have actually constitute knowledge, and then proceed to ask what criteria distinguish those beliefs from cases of non-knowledge (this is Descartes’ approach); or

·         begin by assuming a specific set of criteria for knowledge, and the proceed to ask what specific beliefs meet those criteria.

The problem is that neither way of proceeding is satisfactory, but it seems like we have to do one or the other.


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