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René Descartes: Discours de la méthode (Discourse on the method)

By Levi Clancy for Student Reader on

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René Descartes released the Discourse on the method in 1637, as an introduction to his views on the sciences.

The Discourse was written against the backdrop of the 17th century Scientific Revolution, when Galileo, Kepler, Newton, Boyle and dozens of others were transforming science. Though called the Discourse on the method, in fact that was just one of four discourses on method, optics, meteorology and geometry. This exegesis analyzes the first.

It took him seventeen years to publish the Discourse, starting around 1620. He said then when he started, it took nine years before people said he was complete. Then he withdrew eight more years, avoiding personal contact.

Table of contents

Descartes' four discourses on the method, optics, meteorology and geometry are each largely self-contained books.

The Discourse on the Method has a preface and six parts, outlined below.


The preface briefly describes the six parts as being on:

Part I - Science
Part II - Rules of the method.
Part III - Morals derived from the method.
Part IV - God and god's relationship to the soul.
Parts V and VI - Physics, medicine and science in general.

This is accurate, though the first part is mainly autobiographical, and the second part has significant autobiography intertwined as it also details how Desecartes' method arose from his experiences.

Part I (Autobiography)Mostly autobiographical.
Part II (Method)Rules of the method
Part III (Morals)Morals derived from method.
Part IV (Metaphysics)God and the soul.
Part V and VIOn general, natural science.

Part I

Common sense is the best distributed thing in the whole world. Everyone thinks they are well endowed with it, so that even those who are most difficult to please in every other respect do not usually wish to have more than they already possess. It is unlikely that everyone is wrong about this.Discourse on the Method, §1 ¶1

Descartes believes that humans are equally endowed with an ability to make good judgment about truth and falsity.

He refers to this ability as common sense, and describes it as neither more or less abundant in different people. It is this ability to reason that distinguishes human beings from animals (alluding to the classical definition of humans as rational animals).

As such, reason is complete in all people because we are members of our rational species; nobody is more or less human.

Different beliefs arise not from unequal ability, but from thinking differently. Thus, ability is not enough: a correct method is needed.

That is why his book is Discourse on the method.

Philosophy provides a way of speaking plausibly on all matters and making oneself admired by those who are less scholarlyDiscourse on the Method, §1 ¶7

Perhaps Descartes is not disparaging philosophy in and of itself, just philosophy as it was practiced.

Rather than assuming that Descartes is flat-out disparaging philosophy, he may consider himself as bringing in a new philosophy that's worth more than plausibility and admiration.

Descartes insists that his method was meant as a manual for himself, not for others. But is he serious?

First of all, he took the trouble to write and publish it in French, which was more widely read than Latin, the lingua franca of technical philosophical works. It seems unlikely that he did not intend it as a guide for others.

Perhaps he said this solely to soften his tone, in light of church sensitivities.

Descartes' beliefs on reason and method are drawn from his experiences.

Thus, he provides his experiences autobiographically so that people may critique them. (Similarly, Augustine's Confessions was an autobiographical theological philosophical record.)

Searching books

Descartes first describes his bookish education.

At first he had believed that all necessary knowledge could be drawn from books. However, he was overcome by the errors he saw in himself and others. He could not be certain if he had at all learned anything of value. (This is not entirely true, as he absorbed much on ancient languages, literature, rhetoric, mathematics, theology, medicine, history, poetry, ethics, law and philosophy.)

Moreover, he decides that he is suspicious of a bookish education because it innately omits much of life. Reading is good, but one can have too much of a good thing.

When one is too curious about things which went on in past ages, one usually lives in considerable ignorance about what goes on in this one. In addition, fables make us imagine several totally impossible events as possible, and even the most faithful histories, if they neither change nor increase the importance of things to make them more worth reading, at the very least almost always omit the most menial and less admirable circumstances, with the result that what is left in does not depict the truth.Discourse on the method, §1 ¶7

Searching the world

Thus, Descartes turns from books and decides to seek knowledge from the world and himself.

He pursues an experiential, reflective education where he absorbs experiences. Descartes looked for truth by learning about how people reason what's important to them. He was interested in what people spent most of their time thinking about: practical reasoning, the kind necessary to survive a day (like knowing to not jump off a bridge).

He was less keen on theoretical reasoning, which he felt was remote from common sense and lacking any consequences.

However, Descartes observed that people disagree on customs.

In Descartes' day, tradition was intrinsically trusted and novelty was automatically shunned. Yet as he traveled, he found many different traditions. There was tremendous cultural diversity, with new modes of behavior everywhere he went. Thus, traditions may have tremendous power over people's lives, but something being customary does not elucidate truth or value -- it only means it is customary.

Descartes learned two lessons. First, what was strange to him may be normal to another. Second, what is customary is not certain.

I learned from my college days on that one cannot imagine anything so strange and so incredible that it has not been said by some philosopher and, later, in my travelling, I found out that all those who have views very different from our own are not therefore barbarians or savages, but that several use as much reason as we do, or more. I also considered how much the same man, with the same mind, raised from his infancy on among the French or the Germans, would become different from what he would have been if he had always lived among the Chinese or the cannibals, and how, even in our style of dress the same thing which pleased us ten years ago and which will perhaps please us again ten years from today, now seems to us extravagant and ridiculous. This being the case, we are clearly persuaded more by custom and example than by any certain knowledge.Discourse on the method, §2 ¶ 4

Descartes had turned from books to the world. But when the world did not inform him, he made another turn and delved inward.

Part II

Part II begins with Descartes having resolved to turn inward.

While a soldier in Germany, there were breaks in fighting when he could think along for long periods. This gave him time to reflect on his own thoughts. He did not just think, he thought about thinking. He was critical of his own thought processes and the soundness of his arguments.

These were the reflections of a philosopher and a mathematician.

Descartes asserts that things are best when built as a unified effort overseen by an individual architect.

While traveling throughout Europe, Descartes had experienced many different cities and buildings. He decided that they were best when built by one person rather than many, as a consolidated effort rather than accumulating over time. He then extended physical constructions (urban landscapes) to mental constructions (laws). In his mind, laws too were best made all at once by one legislator, rather than by many people with many goals at various times.

Descartes parallels this to god: one god created the universe all at once.

However, this ideal was not met by the sciences. Descartes is very critical of the intellectual heritage of his day.

Sciences had been built up cumulatively over centuries, with many parts from many people. As such, the sciences were at best likely stories with degrees of likelihood , but utterly lacking the certainty of geometry. Descartes viewed science as he was taught it, as lacking, unclear and incoherent.

He asserts that for the most part, the sciences are less certain than what one can gather by simple observation and reasoning using one's own rational mind.


Descartes says that people often demolish houses in order to rebuild them.

He views his work as having the coherence and design that a good architect would implement. However, in order to rebuild he will first need to demolish. His first effort was to remove all his old beliefs at once.

He would then subject his beliefs to a test of reason, to either replace them with something better or reinstate them if they passed.

However, Descartes insists he is only demolishing his own mental house, not an entire city.

His own house, he says, was already falling apart because it was built on a poor foundation. This was his own private decision. Regarding other people's mental homes, it was best to tolerate their deficiencies. He did not want to wipe out a city of mental homes, nor all of society's.

Doing so would be unmanageable and create a vacuum vulnerable to chaos.

This caution might have been due to church sensitivities.

After all, Galileo was not the only thinker to recently get the church's sour condemnation. So did Jordano Bruno, Lucilio Vanini (burned at the stake), Tommaso Campanella (incarcerated) and others.

Descartes would have preferred following a single, clear voice of authority to follow.

A single person is more likely to have truth than a group. After all, people have different traditions and perspectives. To find truth they would have to form a committee and vote, which would lead to conflict. This would not at all be a reasonable way to pursue truth.

However, Descartes did not find anybody else to guide him

A plurality of voices is not a proof worth anything for truths which are a little difficult to discover, because it is far more probable that one man by himself would have found them than an entire people. Since I could not select anyone whose opinions it seemed to me one should prefer to those of other people, I found myself, so to speak, compelled to guide myself on my own.Discourse on the method, §2 ¶ 4

Thus, Descartes planned to "walk alone in the dark" carefully and slowly.

Descartes would need a method before discarding all his beliefs, to allow him to sort the debris and build a solid philosophical house.

He wanted a method as simple as If P then Q. P therefore Q.

Failures of logic, algebra and geometry

Descartes tried logic, algebra and geometry and found them lacking, despite their rationality.

They were too abstract anyways. Descartes wanted to get into issues loaded with more content than anything describable by logic, algebra nor geometry.

So Descartes had to develop a different method.

LogicLogic, essentially Aristotelian in Descartes' time, could not provide a method. It was hard to use because it was controversial to sort out useful from useless rules. Also, logic was specifically intended to have no content. Furthermore, logic could lead to erroneous conclusions: the baker raised his prices by a penny; that penny could have gone toward pet food; my pet loves food loves food; therefore, the baker hates my pet.
Geometry Geometry was all about images -- squares, circles, etc -- that depended on imagination, which was unreliable because it was not reason.
GeometrySimilar to the case with geometry, the symbols and rules of algebra were confusing. (Remember that Descartes revolutionized algebra to something more accessible and similar to what we have today.)

The Four Rules

With logic, algebra and geometry useless, Descartes turned to four rules.
Rule 1Accept as true only what is known so clearly and distinctly that it cannot be doubted.
Rule 2Divide every problem into as many parts possible.
Rule 3Begin with the simplest and easiest to know and proceed to the complex, thus imputing an order even in the absence of a natural order.
Rule 4Proceed so comprehensively that nothing can be omitted.
His method mandates that he use reason for everything and think as clearly and distinctly as possible.

His meaning of clear is, well, not immediately clear. He uses sensory vision as a metaphor, similar to how Aquinas uses evident when he rejects Anselm's proof for god's existence, because it is evident in itself but not evident to us.

What Descartes means by distinct is pretty straightforward: circle, triangles and squares are distinct, and they must be thought of as such.

Part III

At the end of this second part of the Discourse on the Method, before the third begins, remember from the beginning of the first part that common sense is the best distributed thing of the world.

He has what he believes is the method, a method about theorizing, about thinking. But he realizes itmight not be the best method for getting throught he day. For example you believe nothing before you are positively sure that your understanding of it is clear and distinct.

That's a big task before you accept anything. If you live your life like that you might get hit by a truck. So in addition to the Four Rules, he provides rules not about theorizing but about practical morality.

Addition 1Obey the law and abide by traditional religion. [Descartes avoids trouble by not touching church doctrine.]
Addition 2Once a view is adopted, even the most doubtful, stick to it as if it were certain. [When you are considering a possibility, follow it through.]
Addition 3Change oneself, not the world.
Addition 4Use one's whole life to develop reason, applying the method to find the truth.

Skeptical approach

Descartes next dives right into the heart of doubt and questioning. He uses a skeptical method.

First of all, Descartes feels that his philosophical house is crumbling on weak foundations. So he attacks it skeptically. Descartes is a systematic doubter, a systematic skeptic. However he is not skeptical for skeptical ends.

In fact, he is skeptical for anti-skeptical ends.

Descartes does not seek the skeptical ideal, but rather the opposite.

Descartes does not than vie for the Greek Skeptical ideal of total inconclusiveness and tranquility. He does not doubt, suspend judgment and stay undecided just for the sake of it. Skeptics desire permanent indecisiveness because they identify it as tranquil. Descartes seeks solid, foundational principles. Further, Descartes knows that since philosophy is the most general science, his undertaking can provide a foundation for the other sciences.

He does not want indecision. He wants a firm philosophical foundation.

Part IV

For practical purposes (he was a soldier after all), Descartes had to take actions based on uncertain things.

At the theoretical level, however, Descartes rejected anything which could be doubted in even the least possible way. Right off the bat, all sensory information was rejected. If the senses can deceive, they must be assumed entirely unreliable.

Also, reasoning is prone to error. Thus, all rational arguments should be doubted.

Descartes then takes an even deeper skepticism, asserting that he could be dreaming and not even know it.

Thoughts can happen in dreams, and dreams can seem quite real until one awakens. Thus, Descartes finds it possible that he could be dreaming and not know it. This is a deeper skepticism. All his experiences, thinking, could be just a dream.

However, even though he held his experiences and even his thoughts in doubt, he was always left with an I -- he could not get past I.

Cogito, ergo sum

I think, therefore I am. Cogito, ergo sum.

Descartes reaches one fundamental, unshakable conclusion: I think, therefore I am. In Latin, this is written cogito, ergo sum What's critical is the I, not the verb. It could be: I am (maybe) dreaming, therefore I am; I am confused, therefore I am; I am in doubt, therefore I am.

What Descartes could not eliminate is the I, no matter the verb.

Descartes was mired in a very extravagant, deep doubt.

But even this most extravagant doubting needed a doubter. Because he doubted, he knew that his existence was certain.

Thinking (including doubting) is prior and essential. Therefore, he exists.

Descartes concludes therefore that he is: an existing thing; a substance; whose whole essence is to think.

What Descartes is specifying is that he only know that his self exists. Because he thinks, he knows he is a substance capable of thought. But is careful to include no more: he does not know he has a body, and even if he did, he knows his self better than his body.

His body is in doubt, but he knows certainly he is a thinking substance.

Descartes' proof for the existence of god

Next, Descartes reasons that god exists.

Doubting is not a good condition in which to be, but Descartes is a doubter. However, he wants to know. Yet being a doubter makes him imperfect. However, he can think of a more perfect nature lacking his defects. From where does he get that thought?

He reasons in the fourth discourse that this thought of a more perfect nature must come from a more perfect nature.

Thoughts of a more perfect nature must come from a more perfect nature.

This is very similar to Anselm's proof of god's existence.

External thingsI cannot be certain that any external thing exists. Is there a world? Is there a body? I can think of an external thing even if it does not exist.
False statementsTrue statements are about things. False statements are not about things: they are about a void, that which does not exist.
PerfectionI can think of perfection.
ImperfectionExternal things are imperfect, because I can doubt them. Also, I am imperfect, because I doubt. Thus, when I think of perfection I am not thinking of myself, nor external things.
SourceNeither nothing, nor I, nor things, are potential sources of my thought of perfection. A thought of perfection cannot come from imperfect, even potentially false, things. It is impossible.
GodThe only possibility left is that something more perfect instilled in me the thought of perfection. This more perfect thing is god.

God is not bodily

Further, god is not a bodily thing.
Mind-body compositeIf something were a composite of bodily and mental natures, then as such it would depend on both for its existence.
DependenceDescartes asserts that dependence is a defect.
Non-bodily godIf god is perfect, then god must not be a mind-body composite. God must not have bodily constraints. God must be non-bodily.

God's existence is most certain

Descartes' proof of god is more certain than a geometric proof.

Geometries study bodies, which exist in space. However, a geometric proof involving a body does not prove that body's existence.

A proof involving a triangle, does not prove that triangle exists.

Thinking of a perfect being means thinking that it exists.

Otherwise, it would be imperfect. Since thinking of god necessitates god's existence, but thinking of a triangle does not necessitate a triangle's existence,

Thus, god's existence is more certain than a geometric proof.

True thoughts are from god

Only the existence of god provides assurance of other very fundamental beliefs like having a body.

It is possible to be morally certain but not metaphysically certain. Moral certainty means certain enough to act on, like moral certainty of having a body.

Metaphysical certainty refers to certainty through reasoning.

God is perfectGod exists, is perfect and does not deceive.
Humans are imperfectHumans are imperfect.
Source of thoughtsEverything derives from god, including thoughts. Thus, the ultimate source of a thought is god.
False thoughtsHowever, false thoughts are unclear (obscure) and un-distinct (confused). These are imperfections and cannot be from god. They are from our imperfections.
True thoughtsTherefore, only clear, distinct thoughts can be from god. Descartes sees a relationship that god is the foundation of a true thought.


Dr. Brian Copenhaver. A Historical Introduction to Philosophy. Lecture, UCLA. August 27, 2012.