The book cover for A Discourse on the Method by René Descartes

Book Notes: A Discourse on the Method by René Descartes


René Descartes was an 18th century scientist, mathematician, and thinker who had a direct effect on western philosophical thinking. Descartes was known for questioning everything he knew, by realizing that all ideas could be doubted and therefore potentially false. He wanted a system for cutting through the grand narratives of society so that he could identify incontrovertible truths. The full title of this text is A Discourse on the Method of Correctly Conducting One’s Reason and Seeking Truth in the Sciences. It’s a short work, divided in six parts, that introduces his approach to intellectual, moral, and scientific ideas. He does not present his frameworks as universal truths, but rather as methods that have worked well for himself.

Direct Quotes:

P.5 – It’s Not Enough to Possess a Good Mind
“For it is not enough to possess a good mind; the most important thing is to apply it correctly. The greatest minds are capable of the greatest vices as well as the greatest virtues; those who go forward but very slowly can get further, if they always follow the right road, than those who are in too much of a hurry and stray off it.”

P. 9 – Learn to Think Clearly
“Those who reason most powerfully and are the most successful at ordering their thoughts so as to make them clear and intelligible will always be best able to persuade others of what they say, even if they speak in the thickest of dialects and have never learned any rhetoric.”

P. 11 – Free Your Mind from Error
“I learned not to believe too firmly in anything that only example and custom had persuaded me of. So it was that I freed myself gradually from many of the errors that can obscure the natural light of our minds, and make them less able to see reason.”

P. 55 – The Challenge of Overcoming Errors in Knowledge
“For to try to overcome all the problems and errors that prevent us attaining knowledge of the truth is indeed to engage in battle.”

Analysis of Key Passages:

Part 1: Intellectual Autobiography

P. 5 – Good Sense is Practiced Through Perspective
“Good sense is the most evenly distributed thing in the world; for everyone believes himself to be so well provided with it that even those who are the hardest to please in every other way not usually want more of it than they already have. Nor is it likely that everyone is wrong about this; rather, what this shows is that the power of judging correctly and of distinguishing the true from the false (which is what is properly called good sense or reason) is naturally equal in all men, and that consequently the diversity of our opinions arises not from the fact that some of us are more reasonable than others, but solely that we have different ways of directing our thoughts, and do not take into account the same things.”

Analysis: “Good sense” is a character trait that we all share, but we see things differently because we come from different backgrounds, cultures, and ways of thinking. One person is not more reasonable than another – he or she is looking at the problem from a different angle and to them, that makes good sense. This is why diversity is so important.

P. 6 – Create a System for Learning
“I have fashioned a method by which, it seems to me, I have a way of adding progressively to my knowledge and raising it by degrees to the highest point that the limitations of my mind and the short span of life allotted to me will permit it to reach.”

Analysis: Descartes created system for learning that he could trust. He needed a way to break down complex ideas into simple building blocks that he could analyze. Descartes accomplished a lot because he cared about his thinking system.

P. 8 – Reading Too Much
“But I then decided that I had devoted enough time both to the study of languages and to the reading of the books, histories, and fables of the classical world. For conversing with those of another age is more or less the same thing as travelling. It is good to know something of the customs of different peoples in order to be able to judge our own more securely, and to prevent ourselves from thinking that everything not in accordance with our own customs is ridiculous and irrational, as those who have seen nothing of the world are in the habit of doing. On the other hand, when we spend too much time travelling, we end up becoming strangers in our own country; and when we immerse ourselves too deeply in the practices of bygone ages, we usually remain woefully ignorant of the practices of our own time.”

Analysis: Descartes felt that too much reading prevented critical thinking. While reading is good, if you never give yourself time to consider the texts and form your own opinions, then you’ve lost your way. Reading and thinking (for yourself) must be combined for the greatest output.

P. 10 – Philosophy Can’t Be Trusted
“I shall not say anything about philosophy except that, when I realized that it had been cultivated by the best minds for many centuries, and that nevertheless there is nothing in it that is not disputed and consequently is not subject to doubt, I was not so presumptuous as to hope to succeed better than others; and that seeing how different learned men may defend different opinions on the same subject, without there ever being more than one which is true, I deemed anything that was no more than plausible to be tantamount to false.

As for the other disciplines, in so far as they borrow their principles from philosophy, I concluded that nothing solid could have been built on such shaky foundations.”

Analysis: There are many branches of philosophy, each of which have many of the best thinkers arguing their claims. The problem is that they all disagree with each other which makes Philosophy as a whole, questionable. It lacks a foundation that we all agree on. At best, you can learn critical thinking skills.

P. 10 – Some Things Can’t Be Learned from Books
“I spent the rest of my youth travelling, visiting courts and armies, mixing with people of different character and rank, accumulating different experiences, putting myself to the test in situations in which I found myself by chance, and at all times giving due reflection to things as they presented themselves to me so as to derive some benefit from them. For it seemed to me that I could discover much more truth from the reasoning that we all make about things that affect us and that will soon cause us harm if we misjudge them, than from the speculations in which a scholar engages in the privacy of his study, that have no consequence for him except insofar as the further they are from common sense, the more he will be proud of them, because he has had to use so much more ingenuity and subtlety in the struggle to make them plausible.”

Analysis: Descartes found that somethings can’t be learned from books. The school of hard knocks is a valuable way to get an education as long as you’re paying attention.

Part 2: Descartes’s Framework for Logic

P. 12 – The Poor Quality from Working in Groups
“I spent the whole day shut up in a small room heated by a stove, in which I could converse with my own thoughts at leisure. Among the first of these was the realization that things made up of different elements and produced by the hands of several master craftsmen are often less perfect than those on which only one person has worked. This is the case with buildings which a single architect has planned and completed, that are usually more beautiful and better designed than those that several architects have tried to patch together, using old walls that had been constructed for other purposes.”

Analysis: It’s better to work alone on a large project. The vision and craftsmanship of a creator is lost in a group where conflicting ideas, negotiation, and unclear roles cause the work to suffer. The group dynamics are often detrimental to the outcome when leadership is either missing or abused. The Denial of Death talks about group mentality as analyzed by Sigmund Freud.

P. 15 – Many People Aren’t Capable of Critical Thinking
“The world is made up almost entirely of two sorts of minds to which such a course of action is wholly unsuitable. First, there are those who, believing themselves cleverer than they are, cannot stop themselves jumping to conclusions, and do not have enough patience to govern their thoughts in an orderly way, with the result that once they have allowed themselves to doubt accepted principles and stray from the common path, they would never be able to keep to the road that one must take to proceed in the right direction, and would remain lost all their lives. Second, there are those who, having enough sense or modesty to realize that they are less capable of distinguishing the true from the false than certain others by whom they could be guided, must content themselves with following the opinions of these others rather than seeking better ones from themselves.”

Analysis: There are mostly two kinds of people in this world, those who jump to conclusions and those who are not strong enough to make conclusions. Neither are good.

P. 15 – Differing Opinions from Diversity
“But having already discovered at school that there is no opinion so bizarre and incredible that has not been uttered by some philosopher or other, and having come later in the course of my travels to the realization that all those who have opinions that are diametrically opposed to ours are not on that account barbarians or savages, but that among their number there are many who make use of their reason as much or more than we do; and having considered how a given man with a given mind, brought up since childhood among the French or the Germans, develops differently from the way he would if he had always lived among the Chinese or among cannibals, and how even down to our fashion of dress, the very thing that pleased us ten years ago and may perhaps please us in ten years’ time at present seems outlandish and ridiculous to our eyes (this is because we are much more swayed by custom and example than any certain knowledge; and yet the majority view is of no value as proof of truths which are difficult to discover, because they are much more likely to be discovered by one man by himself than by a whole people); for all these reasons, I could not choose any one person whose opinions struck me as preferable to those of others, and I found myself forced, as it were, to provide for myself my own guidance.”

Analysis: Our background plays an important part in how we view the world. Just because someone has a different opinion, that doesn’t make them wrong. Where you come from and how you were raised are important variables in how you decide.

P. 17 – Descartes Four Rules of Logic
“I came to believe that in the place of the great number of precepts that go to make up logic, the following four would be sufficient for my purposes, provided that I took a firm and unshakeable decision never once to depart from them.

The first was never to accept anything as true that I did not incontrovertibly know to be so; that is to say, carefully to avoid both prejudice and premature conclusions; and to include nothing in my judgements other than that which presented itself to my mind so clearly and distinctly, that I would have no occasion to doubt it.

The second was to divide all the difficulties under examination into as many parts as possible, and as many as were required to solve them in the best way.

The third was to conduct my thoughts in a given order, beginning with the simplest and most easily understood objects, and gradually ascending, as it were step by step, to the knowledge of the most complex; and positing an order even on those which do not have a natural order of precedence.

The last was to undertake such complete enumerations and such general surveys that I would be sure to have left nothing out.”

There are four rules that Descartes followed for logical reasoning:

  1. Never blindly accept something as true.
  2. Break ideas into their individual parts for better analysis.
  3. Start with the simple and progress to the complex.
  4. Look at the argument from all sides to avoid confirmation bias.

These rules are a good foundation for critical thinking.

Part 3: Ethics and Moral Code

P. 21 – The First Moral Maxim of René Descartes
“The first was to obey the laws and customs of my country, and to adhere to the religion in which God by His grace had me instructed from my childhood, and to govern myself in everything else according to the most moderate and least extreme opinions, being those commonly received among the wisest of those with whom I should have to live.”

Analysis: To follow the law, his religion, and to avoid extreme opinions. This one surprises me quite a bit. While I understand that the Catholic church was applying incredible pressure on citizens to follow it’s teachings, I’ve got to wonder if Descartes was as “faithful” as he professed. His background in science, which stood in direct conflict to the church, must have weighed heavy on him.

P. 21 – Actions Express a Person’s True Beliefs
“For me to know what their opinions really were, I had to take note of what they did rather than what they said, not only because in the present corrupt state of our morals few people are willing to declare everything they believe, but also because some do not even know what they believe.”

Analysis: Most people haven’t examined their beliefs with a critical thinking lens, and as such, don’t really know what they believe. That’s why you have to pay attention to their behaviors.

P. 22 – The Second Moral Maxim of René Descartes
“My second maxim was to be as firm and resolute in my actions as I could, and to follow no less constantly the most doubtful opinions, once I had opted for them, than I would have if they had been the most certain ones.”

Analysis: Once he had made a decision, he was going to stick with it. To be wishy-washy led to moral implications. This might lead to decision fatigue or low self-confidence.

P 22 – Lost in the Forest of Thoughts
“In this I imitated those travellers who, finding themselves lost in a forest, must not wander in circles first to one side then to the other, and still less stop in one place, but have to walk as straight as possible in one direction, and not alter course for weak reasons, even if it might only have been chance which had led them to settle on the direction they had chosen; for by this means, even if they do not end up precisely where they want to be, they will eventually reach somewhere where they will most likely be better off than the middle of a forest. And, in the same way, as in life we must often act without delay, it is a very certain truth that when it is not in our power to determine which the truest opinions are, we should follow those which are most likely to be true, and even though we might see no more probability in some rather than others, we must nevertheless opt for one set, and thereafter consider them not as being doubtful, insofar as they relate to the practice of life, but as altogether true and certain, because the reasoning that led us to opt for them is true and certain.”

Analysis: When making decisions, it’s best to pick a path and stick with it. Otherwise, you’ll wander in circles, never getting anywhere. Descartes uses the metaphor of being lost in a forest, and suggest that choosing a direction and sticking to it will eventually lead somewhere.

P. 22 – Lost in the Forest of Thoughts
“In this I imitated those travellers who, finding themselves lost in a forest, must not wander in circles first to one side then to the other, and still less stop in one place, but have to walk as straight as possible in one direction, and not alter course for weak reasons, even if it might only have been chance which had led them to settle on the direction they had chosen; for by this means, even if they do not end up precisely where they want to be, they will eventually reach somewhere where they will most likely be better off than the middle of a forest. And, in the same way, as in life we must often act without delay, it is a very certain truth that when it is not in our power to determine which the truest opinions are, we should follow those which are most likely to be true, and even though we might see no more probability in some rather than others, we must nevertheless opt for one set, and thereafter consider them not as being doubtful, insofar as they relate to the practice of life, but as altogether true and certain, because the reasoning that led us to opt for them is true and certain.”

Analysis: When making decisions, it’s best to pick a path and stick with it. Otherwise, you’ll wander in circles, never getting anywhere. Descartes uses the metaphor of being lost in a forest, and suggest that choosing a direction and sticking to it will eventually lead somewhere.

P. 23 – The Third Moral Maxim of René Descartes
“My third maxim was to endeavor always to master myself rather than fortune, to try to change my desires rather than to change the order of the world, and in general to settle for the belief that there is nothing entirely in our power except our thoughts, and after we have tried, in respect of things external to us, to do our best, everything in which we do not succeed is absolutely impossible as far as we are concerned.”

Analysis: This maxim has a hint of Stoicism to it. Here, René Descartes suggests that we cannot control fate or things outside of ourself, and to do so would cause moral implications. It’s best to focus on you in an attempt for self improvement.

P. 24 – The Fourth Moral Maxim of René Descartes
“Finally, as a conclusion to this moral code, I decided to review the various occupations that men have in this life, in order to try to select the best one. Without wishing to pass judgement on the occupations of others, I came to the view that I could do no better than to continue in the one in which found myself, that is to say, to devote my life to the cultivation of my reason and make such progress as I could in the knowledge of the truth following the method I had prescribed for myself.”

Analysis: What you do for a career matters. It needs to suit you and your interests, otherwise, it could have moral implications. If you do what you love all day, there’s little room left to get yourself into trouble. I would argue that hobbies and other passion projects fit here as well. It reminds me of the hero project that Ernest Becker describes in The Denial of Death.

P. 25 – Why Descartes Doubted Everything
“I was not copying those sceptics who doubt for doubting’s sake, and pretend to be always unable to reach a decision; for, on the contrary, the aim of my whole plan was to reach certainty and reject shifting ground in the search for rock and clay.”

Analysis: Descartes was frustrated by magical thinking. It was popular at that time to study alchemy, occultism, etc. I’m sure these trends pushed him even harder to find a scientific method for understanding knowledge.

Part 4: Metaphysics and Epistemology

P. 28 – Cogito, ergo sum (I Think, Therefore I Am)
“I came to think that I should do the exact opposite and reject as completely false everything in which I could detect the least doubt, in order to see if anything thereafter remained in my belief that was completely indubitable. And so, because our senses sometimes deceive us, I decided to suppose that nothing was such as they lead us to imagine it to be. And because there are men who make mistakes in reasoning, even about the simplest elements of geometry, and commit logical fallacies, I judged that I was as prone to error as anyone else, and I rejected as false all the reasoning I had hitherto accepted as valid proof. Finally, considering that all the same thoughts which we have while awake can come to us while asleep without any one of them then being true, I resolved to pretend that everything that had ever entered my head was no more true than the illusions of my dreams. But immediately afterwards I noted that, while I was trying to think of all things being false in this way, it was necessarily the case that I, who was thinking them, had to be something; and observing this truth: I am thinking therefore I exist, was so secure and certain that it could not be shaken by any of the most extravagant suppositions of the sceptics, I judged that I could accept it without scruple, as the first principle of the philosophy I was seeking.”

Analysis: This is the most famous passage of the book, and one of the most famous in all of Philosophy. Here, René Descartes discovers that everything from his senses to his dreams and even geometry can be doubted. He then realized that his doubting proved one, irrefutable fact – that he was doubting. Only a being in existence could do that, so he coined the term, I am thinking, therefore I exist.

P. 29 – Dualism of Body and Soul
“I thereby concluded that I was a substance whose whole essence or nature resides only in thinking, and which, in order to exist, has no need of place and is not dependent on any material thing. Accordingly this ‘I’, that is to say, the Soul by which I am what I am, is entirely distinct from the body and is even easier to know than the body; and would not stop being everything it is, even if the body were not to exist.”

Analysis: Descartes argued that thinking came from his soul and not his physical body. This is the Dualism argument and it separates René Descartes from Aristotle who took a more integrated approach in his work De Anima, arguing that the soul cannot exist without the body. Aristotle believed that the soul is what animated the body. Descartes argument for the separation and rationality of a soul is a pillar idea of his Philosophy.

P. 32 – Descartes Argument for the Existence of God
“I noted also that there was absolutely nothing in them which made me certain of the existence of their object. Thus, for example, I grasped clearly that, supposing a triangle to be given, it was necessary that its three angles were equal to two right angles; yet for all that, I saw nothing in this which made me certain that a single triangle existed in the world. Whereas, going back to the idea I had had of a perfect being, I found that existence was part of that idea, in the same way, or even more incontrovertibly so, that it is intrinsic to the idea of a triangle that its three angles equal two right angles, or to that of a sphere that all its parts are equidistant from its centre; and that, in consequence, it is at least as certain as any geometric proof that God, who is that perfect being, is or exists.”

Analysis: Descartes makes a case for the existence of God here. He aims to show that there are universal laws of perfection like the fact that a triangle’s interior always equals 180 degrees, and that laws like this are proof that a more perfect being exists. René Descartes was a member of the Catholic faith, and despite the fact that The Roman Inquisition made it impossible for him to publish his scientific works, he still held firm to this faith.

Part 5: Physics and Physiology

P. 45 – The Human Body is a Machine
“I had also shown what changes must occur in the brain to cause states of waking, sleeping, and dreaming; how light, sounds, smells, tastes, heat, and all the other qualities of external objects can imprint various ideas on the brain through the intermediary of the senses; how hunger, thirst, and the other internal passions can also transmit ideas to the brain; what must be taken to be the sensus communis in which these are received, the memory which preserves them, and the faculty of imagination, which can change them in different ways, form them into new ideas and, by the same means, distribute animal spirits to the muscles and make the members of this body move, with respect both to the objects which present themselves to the senses and to the internal passions, in as many different ways as the parts of our bodies can move without being directed by our will. This will not appear at all strange to those who know how wide a range of different automata or moving machines the skill of man can make using only very few parts, in comparison to the great number of bones, muscles, nerves, arteries, veins, and all the other parts which are in the body of every animal. For they will consider this body as a machine which, having been made by the hand of God, is incomparably better ordered and has in itself more amazing movements than any that can be created by men.”

Analysis: René Descartes lays out his argument for how the human body is like a machine fashioned by the hand of God. He argues that there are many processes which happen within the body that we do not govern, but that are necessary for our well being. He doesn’t actually be we’re machines, but that we function like them. This idea is based off of his passion for mechanisms.

P. 46 – Robots Can Never Replace Us
“At this point I had dwelt on this issue to show that if there were such machines having the organs and outward shape of a monkey or any other irrational animal, we would have no means of knowing that they were not of exactly the same nature as these animals, whereas, if any such machines resembled us in body and imitated our actions insofar as this was practically possible, we should still have two very certain means of recognizing that they were not, for all that, real human beings. The first is that they would never be able to use words or other signs by composing them as we do to declare our thoughts to others. For we can well conceive of a machine made in such a way that it emits words, and even utters them about bodily actions which bring about some corresponding change in its organs (if, for example, we touch it on a given spot, it will ask what we want of it; or if we touch it somewhere else, it will cry out that we are hurting it, and so on); but it is not conceivable that it should put these words in different orders to correspond to the meaning of things said in its presence, as even the most dull-witted of men can do. And the second means is that, although such machines might do many things as well or even better than any of us, they would inevitably fail to do some others, by which we would discover that they did not act consciously, but only because their organs were disposed in a certain way. For, whereas reason is a universal instrument which can operate in all sorts of situations, their organs have to have a particular disposition for each particular action, from which it follows that it is practically impossible for there to be enough different organs in a machine to cause it to act in all of life’s occurrences in the same way that our reason causes us to act.”

Analysis: While it’s possible that machines could be fashioned to look and act like us, they will never fully replace us for two reasons. First, the robot would never be able to have contextual conversations with us. They could be programmed to respond with the appropriate words, but they couldn’t address the nuance and sensitivity of a real conversation. Second, robots could do many of the tasks we now do, but not all of them. Some tasks require a thinking, loving, emotional human to get done. We would know the fake.

Part 6: Reasons for Not Publishing The Universe

P. 54 – Writing Ideas Down Makes Them Better
“Since that time I have, however, had other reasons to make me change my mind and decide that I had indeed to go on recording everything that I thought of some importance as I discovered the truth about it, and to bring the same care to this task as if I intended to publish my results; as much in order to have more opportunity to examine them (as, without doubt, we always take greater care over what we think will be seen by many people than over what we do only for ourselves; and things which have seemed true to me when I began to think them out, have often seemed false when I have tried to set them down on paper), as to lose no opportunity to benefit the public, if I can, so that, if my writings are of any value whatsoever, those who will come into possession of them after my death will be able to make the most appropriate use of them.”

Analysis: The process of writing your ideas down improves them because they’ll be seen by others (this makes you care more about their quality) and visually seeing them makes you wonder about their validity.

P. 62 – Distorting a Philosopher’s Ideas
“I have called them ‘suppositions’ only to make it known that, while I think I can deduce them from the primary truths that I have explained above, I have expressly decided not to do this, in order to prevent certain thinkers from seizing the opportunity of building some new outlandish philosophy on what they believe to be my principles, for which I should be blamed; thinkers who believe that as soon as one has said only two or three words to them on a given matter, they can know in one day what it would take someone else twenty years to think out; and the more penetrating and lively these thinkers are, the more they are liable to err and the less capable they are of the truth.”

Analysis: René Descartes was concerned that people would hear a sliver of his philosophy and assumed they understood his entire stance. This is a good argument for why you should read an author’s full body of work, and not just the most popular book.


A Discourse on the Method is a short work (63 pages), that does a good job of introducing René Descartes’s approach to logical thinking. My favorite passage is found on page 17, where he lays out the four rules for assessing the logic and validity of an idea.

I do find it interesting that Descartes is the philosopher who doubted everything in favor of this logical foundation, but at the same time blindly followed the precepts of the Catholic church. The Roman Inquisition prevented Descartes from publishing his work The Universe, since it sided with the Copernican view that the Earth revolved around the sun. How could a man who was so scientific and critical in thought give way so easily to the religious community?

I have to wonder if Descartes was faithful to the church as a measure of protection. He watched Galileo publicly side with Copernicus and get sentenced to house arrest for the rest of his life. Was Descartes as faithful as he professed? I think there’s more to his story.