Descartes’ Discourse on Method

"Rene Descartes," by Mitch Francis
Selections from Descartes' Discourse on the Method of Rightly Conducting One's Reason and of Seeking Truth in the Sciences are from Jonathan Bennett's translation. Some words on his practices:

"[Brackets] enclose editorial explanations. Small ·dots· enclose material that has been added, but can be read as though it were part of the original text. Occasional •bullets, and also indenting of passages that are not quotations, are meant as aids to grasping the structure of a sentence or a thought. Every four-point ellipsis . . . . indicates the omission of a brief passage that seems to present more difficulty than it is worth. Longer omissions are reported between brackets."

"Rene Descartes," by Mitch Francis
Rene Descartes (1596 – 1650) was a philosopher and scientist who probed human knowledge in search of certainty. (Image: “Rene Descartes,” by Mitch Francis.)

* The Discourse was translated into Latin a half decade after Descartes’ death.

In The Discourse, Descartes published his “discovery” of the method by which reason itself could discern truth. In the first sentences, Descartes declared:

•Good sense is the best shared-out thing in the world; for everyone thinks he has such a good supply of it that he doesn’t want more…

If truth beyond our differences of opinion be our aim, we require a higher standard.

[O]ur opinions differ… because we take our thoughts along different paths…. For it isn’t enough to have a good mind; what matters most is using it well.

He published the work in 1637 – not in the then most scholarly of languages, Latin, but in French*, as if symbolizing a break with still-prevalent Medieval tradition. (For more on the radical nature of his project, and a biographical glimpse, see the “Introduction” to his Meditations.)

The first of our two selections from The Discourse is from Part II, where he presents his four methodological principles for discovering truth. In the second, which is Part V in its entirety, he applies the principles to the discovery of truth about nature – including the nature of the difference between people and animals.

from Part II of Discourse on Method

•[O]our convictions come much more from custom and example than from any certain knowledge…. So I couldn’t choose anyone whose opinions seemed to me preferable to  those of all  others, and I found myself pretty much forced to become my own  guide.

But, like  a man walking alone in  the dark, I resolved to  go  so  slowly and to  keep looking around  me  so  warily that even if I didn’t get  far  I would at least be  sure not to fall.   And  I wouldn’t even •start ·my  project of· completely rejecting opinions that had slipped into my  belief system without being introduced there by reason, until I had •first spent long  enough planning the  work  I was undertaking and looking for the true method of getting all the knowledge my mind was  capable of having….

So I thought I should look  for some … method[:]… in  place of the large number of rules that make up logic I would find the following four  to be sufficient, provided that I made and kept to a strong resolution always to obey  them.

(1) The  first was  never to accept anything as true if I didn’t have evident knowledge of its truth: that is,  carefully to  avoid jumping to conclusions and preserving old  opinions, and to include in  my  judgments only what presented itself to  my  mind so vividly  and so  clearly that I had no  basis for  calling it  in question….

(2)  The  second ·was· to divide each of the difficulties I examined into as many parts as possible and as might be required in order to resolve them better.

(3)  The  third ·was· to  direct my thoughts in an orderly manner, by •starting with the simplest and most easily known objects in order to move up gradually to knowledge of the most complex, and •by stipulating some order even among objects that have no natural order of precedence.

(4)  And the last ·was· to make all my enumerations so complete, and my reviews so comprehensive, that I could be sure that I hadn’t overlooked anything.

Part V of Discourse on Method

Part V’s application of the principles to nature yields Descartes’ account of animal behavior – one from which soul is explicitly absent. As you make your way past Part V’s mid-point, consider:

  • What does Descartes hope to explain about the brain’s role?
  • What do the “two very sure signs” indicate?
  • How do you assess his tests?

* This presumably refers to Descartes’s physics, which contradicted the Aristotelian physics still dominant in universities at the time.

I would like to go on with this, and present the whole chain of other •truths that I deduced from these first ones. But this would involve me in discussing here many questions that are being debated among the learned, and I don’t want to get into those quarrels.* So I think it will be better if I don’t follow my inclination and merely say in general what •those other truths are, and leave it to wiser heads to decide whether it would be useful for the public to be told about them in more detail. I have always stuck by my resolve •not to assume any principles except the one I have just used to demonstrate the existence of God and of the soul, and •not to accept anything as true unless it struck me as more open and more certain than the demonstrations of the geometers once seemed to be. And yet I venture to say that ·even under those constraints· it didn’t take me long to satisfy myself regarding all the chief difficulties usually discussed in philosophy. And I discovered certain laws that God has established in nature; he has implanted notions of these laws in our minds, in such a way that after adequate reflection we can’t doubt that the laws are exactly observed in everything that exists or occurs in the world. Moreover, by considering what follows from these laws I have discovered (it seems to me) many truths that are more useful and important than anything I had previously learned or even hoped to learn.

Descartes’s treatise The World was completed in about 1632 — five years before the present work. In Part VI of this work, he states his reasons for not publishing it.

I tried to explain the most important of these truths in a treatise that certain considerations prevent me from publishing, and I know of no better way to make them known than by summarizing its contents. I had planned to include in that work everything I thought I knew about the nature of material things at the time when I was poised to start writing it, ·but that turned out to be too much to tackle all at once·. A painter can’t represent all the different sides of a solid body equally well on his flat canvas, and so he chooses one principal side, sets it facing the light, and shades the others so that they are seen only in the course of looking at the favoured side. Well, in just the same way, fearing that I couldn’t put into my discourse everything that I wanted to, ·I started by (so to speak) letting daylight shine on just •one face of it; specifically·, I undertook merely to expound quite fully what I understood about •light, and then, as the occasions arose, ·to let the shaded sides of my object (so to speak) enter the picture, one at a time. That is·, I planned to add something about •the sun and fixed stars, because almost all light comes from them; about •the heavens, because they transmit light; about •planets, comets and the earth, because they reflect light; about •the bodies on this planet, because they are either coloured or transparent or luminous, and finally about •man, because it is he who sees light. But I wanted to keep these matters still somewhat in the shadows, so as to be free to say what I thought about them without having to follow or to refute the opinions of the learned. My plan for doing this was to leave •our world wholly to the learned folk to argue about, and to speak solely about what would be the case in •a new world that would exist if

God now •created somewhere in imaginary spaces enough matter to compose a world; variously and randomly •agitated the different parts of this matter so as to create as confused a chaos as any poets could dream up; and then did nothing but •allow nature to unfold in accordance with the laws he had established.

·I shall sketch what I did with this supposition, in seven episodes·. (1) I described this matter, giving an account of it that I tried — successfully, it seems to me — to make as openly plain and intelligible as anything except what I have just said about God and the soul. ·As part of this search for clarity·, I explicitly stipulated that this matter had none of those ‘forms’ or ‘qualities’ that the scholastics argue about; allowed it to have only properties that our mind knows so naturally that no-one could even pretend not to. (2) I showed what the laws of nature were [= ‘are’?], and arguing solely from the infinite perfections of God I tried to demonstrate all those laws that might have been called into question, and to show that they are such that even if God created many worlds there couldn’t have been any in which these laws didn’t hold. (3) I then showed how •these laws had the result that most of the matter of this chaos had to resolve itself into a certain orderly arrangement that made it resemble our heavens [here = ‘sky’]; and how at the same time •some of the parts of matter had to form an earth, some to form planets and comets, and yet other parts to form a sun and fixed stars. (4)  And here I spread myself on the subject of light, explaining at some length •the nature of the light that had to be present in the sun and the stars, •how from there it travelled instantaneously across the immense distances of the heavens, and •how it bounced off the planets and comets towards the earth. (5) To this I added many points about the substance, position, movement and all the various qualities of these heavens and stars; and I thought I took this far enough to show that any features observed in the heavens and stars of our world would — or at least could — also be features of the ·imaginary· world I was describing. (6) I went on from there to speak of the earth in particular: •how, although I had explicitly stipulated that God had put no gravity into the matter of which it was formed, all its parts nevertheless tended exactly towards its centre; •how, there being water and air on the earth’s surface, the lay-out of the heavens and heavenly bodies and especially the moon had to cause an ebb and flow exactly like that of our tides, and also an east-to-west current of both water and air like the one we observe between the tropics; •how mountains, seas, springs and rivers could be formed naturally there, and •how metals could appear in mines, plants grow in fields, and generally •how all the bodies we call ‘mixed’ or ‘composite’ could come into being there. (7)  Among other things, I worked hard to get out into the open and fully understood everything about the nature of fire, because as far I know it’s the world’s only source of light other than the stars. Thus I showed •how fire is formed, •how it is fuelled, •how sometimes it gives off heat but no light, and sometimes light without heat; •how it can produce different colours and other qualities in different bodies; •how it melts some bodies and hardens others; •how it can consume almost all bodies, or turn them into ashes and smoke; and finally •how it can through the sheer power of its action turn these ashes into glass. I took special pleasure in describing the formation of glass, because it seems to me as wonderful a transmutation as any in nature.

Still, I didn’t mean to infer from all this that our world was created in the way I had been describing, for it is much more likely that from the beginning God made our world just as it had to be. But if we think of material things as developing gradually out of chaos, their nature is easier to grasp than if we considered them only in their ·present· completed form. And there is nothing wrong with believing that God could have brought them about in that manner, starting with chaos, establishing the laws of nature, and then allowing nature to develop in a normal way in accordance with those laws. In particular, that belief doesn’t malign the miracle of God’s creation of the world. It is certain — and theologians generally agree — that God’s activity in now preserving the world is just the same as his earlier activity of creating it, ·so the ‘miracle of creation’ is with us in full strength even now, whether or not the material world began in chaos·.

A long tradition, going back to Plato, held that plants are special in having ‘vegetative souls’, lower animals in also having ‘sensitive souls’, and humans in having ‘rational souls’ as well as the other two. In this paragraph, Descartes describes a thought-experiment concerning a possible living human body which he takes not to be equipped with a rational soul or any substitute for one; he tacitly rules out the other kinds of soul as well, but allows that something in this human body — namely, fire in the heart — might play the role that earlier thinkers assigned to the vegetative and sensitive souls.

Moving on from •inanimate bodies and •plants, I describe •animals and in particular •men. But I didn’t yet know enough to speak of human bodies in the same way as I did of the other things — that is, by demonstrating effects from causes, and showing how and from what seeds nature must produce them. So I ·settled for a second supposition, comparable with my supposition of a chaotic material world. Specifically, I· supposed that God formed the body of a man exactly like yours or mine both in outward shape and in the internal lay-out of its organs, making it out of nothing but the matter that I had described and not at first putting into it any rational soul or anything else to function as a vegetative or sensitive soul, except for his kindling in the human body’s heart one of those fires without light that I had already explained, and whose nature I understood to be just the same as that of the fire that heats hay when it has been stored while wet. . . . And when I looked into what functions could occur in such a body I found precisely the ones that can occur in us without our thinking of them and hence without any co-operation from our soul — i.e. from that part of us, distinct from the body, whose nature is as I have pointed out simply to think. These functions are just the ones in which animals without reason may be said to resemble us. But I could find ·in my supposed living human body· none of the functions that we have only because we are men, the ones that depend on thought; though these all turned up later on, once I had supposed God to create a rational soul and join it to this body in a certain way that I described.

The next few passages — making nearly half of Part 5 — are devoted to the circulation of the blood, and the function of the heart. They are of only historical interest, and are omitted here, except for three isolated fragments which help to convey the intellectual tone of the whole passage. [In the second of the three, the phrase ‘mathematical demonstrations’ means merely ‘explanations conducted in terms of materialistic mechanism’. Descartes isn’t claiming to have done this work a priori ; he is well aware that it is thoroughly empirical.]

So that you can see how I went about doing this, I shall give my explanation of the movement of the heart and the arteries. •To readers who don’t know any anatomy: before going on, please arrange to observe someone dissecting the heart of some large animal with lungs (for such a heart is in all respects enough like that of a man), and get him to show you its two chambers or cavities. . . . •Those who don’t know the force of mathematical demonstrations and aren’t used to distinguishing true reasons from mere prob- abilities may be tempted to reject this explanation without examining it. To head them off, I would advise them that the movement I have just explained follows purely from

the layout of the organs of the heart, which can be seen with the naked eye, the heat in the heart, which can be felt with the fingers, and the nature of the blood, which can be discovered empirically;

and it follows just as necessarily from those three elements as does the movement of a clock from the force, position, and shape of its counterweights and wheels. . . . •An English physician — ·William Harvey· — must be praised for having made the break-through on this subject. He is the first to teach that at the extremities of the arteries there are many small passages through which the blood they receive from the heart enters the small branches of the veins, from there going immediately back to the heart, so that its course is nothing but a perpetual circulation.

Two technical terms in this paragraph: ‘animal spirits’ is the name for a supposed superfine fluid which acts as the body’s hydraulic system. The ‘common sense’ was a supposed department of the mind in which inputs from different senses come together and are organized in relation to one another.

Descartes' explains how bodily motion occurs.
Illustration of the pain pathway in René Descartes’ Traite de l’homme (Treatise of Man) 1664. The long fiber running from the foot to the cavity in the head is pulled by the heat and releases a fluid that makes the muscles contract.

I had explained all these matters in enough detail in the treatise I had previously intended to publish. And then I had showed •how the nerves and muscles of the human body must be structured if the animal spirits inside them are to convey enough force to move its limbs. . . .; •what changes must occur in the brain as causes of waking, sleep and dreams; •how light, sounds, smells, tastes, heat and the other qualities of external objects can imprint various ideas on the brain through the mediation of the senses; •how hunger, thirst, and the other internal passions can also send their ideas there. And I explained which part of the brain must be ·identified with various supposed mental faculties — specifically, which part of the brain must be· taken to be the •‘common sense’, where these ideas are received; the •memory, which preserves them; and the •imagination, which can change them in various ways, form them into new ideas, and, by distributing the animal spirits to the muscles, make the parts of this body move in as many different ways, and as appropriately to the objects of the senses and the internal passions, as the parts of our bodies can move without being guided by the will. You won’t find that at all strange if •you know how many kinds of automata or moving machines the skill of man can construct with the use of very few parts, in comparison with the great multitude of bones, muscles, nerves, arteries, veins and all the other parts that are in the body of any animal, and if •this knowledge leads you to regard an animal body as a machine. Having been made by the hands of God, it is incomparably better organised — and capable of movements that are much more wonderful — than any that can be devised by man, ·but still it is just a machine·.

I worked especially hard to show that if any such ma- chines had the organs and outward shape of a monkey or of some other animal that doesn’t have reason, we couldn’t tell that they didn’t possess entirely the same nature as these animals; whereas if any such machines bore a resemblance to our bodies and imitated as many of our actions as was practically possible, we would still have two very sure signs that they were nevertheless not real men. (1)  The first is that they could never use words or other constructed signs, as we do to declare our thoughts to others. We can easily conceive of a machine so constructed that it utters words, and even utters words that correspond to bodily actions that will cause a change in its organs (touch it in one spot and it asks ‘What do you mean?’, touch it in another and it cries out ‘That hurts!’, and so on); but not that such a machine should produce different sequences of words so as to give an appropriately meaningful answer to whatever is said in its presence — which is something that the dullest of men can do. (2) Secondly, even though such machines might do some things as well as we do them, or perhaps even better, they would be bound to fail in others; and that would show us that they weren’t acting through understanding but only from the disposition of their organs. For whereas reason is a universal instrument that can be used in all kinds of situations, these organs need some particular disposition for each particular action; hence it is practically impossible for a machine to have enough different •organs to make •it act in all the contingencies of life in the way our •reason makes •us act.

These two factors also tell us how men differ from beasts [= ‘non-human animals’]. For it’s a remarkable fact no men (including even madmen) are so dull-witted or stupid that they can’t arrange different words together so as to form an utterance that makes their thoughts understood; whereas no other animal, however perfect and well-endowed it may be, can do anything like that. It’s not because they lack organs of speech; for we see that magpies and parrots can •utter words as we do yet can’t •speak as we do — i.e. utter words while showing that they are thinking what they are saying. Whereas men who are born deaf and dumb, and thus at least as lacking in speech-organs as the beasts are, usually invent their own signs to make themselves understood by those whom they live with, who have the opportunity to learn their language. This doesn’t show merely that the beasts have less reason than men; it shows that they don’t have reason at all. Here is why.

This indented passage is faithful to Descartes’s thought at this point, but it expands his words in a way that can’t easily be indicated by ·dots·.

Animals of a given species are unequal, as least to the extent that human beings are unequal; some of them, for instance, are much more easily trained than others. So we have the notion of beasts that are abler than or superior to their fellows. Now, obviously the ability to talk doesn’t require much reason; so if any of the beasts had reason at all, we would expect the superior members of some species — high-grade monkeys or parrots, for example — to speak as well as the stupidest child, or at least as well as a child with a defective brain. But none of them do; which shows that their souls are completely different in nature from ours, and don’t include any capacity to reason.

Don’t confuse speech with the natural movements that are evidence of passions and can be imitated by machines as well as by animals. And don’t think, as some of the ancients did, that the beasts speak a language that we don’t understand! For if that were true, then since they have many organs that are analogous to ours, they could make themselves understood by us as well as by their fellows. It is another remarkable fact that although many animals show more skill than we do in some of their actions, yet the same animals show no skill at all in plenty of others; so what they do better doesn’t prove that they have minds, for if it did, they would have better minds than any of us and would out-perform us in everything. It shows rather that they don’t have minds at all, and that it is nature that acts in them according to the disposition of their organs. Similarly, we with all our skill can’t count the hours and measure time as accurately as a clock consisting only of wheels and springs!

I went on to describe the rational soul, and showed that, unlike the other things of which I had spoken, it can’t be derived from the powers of matter, but must be specially created ·as a sheer addition to the human body·. The soul has been thought to be lodged in the human body like a helmsman in his ship, and this comparison may hold some of the way, specifically in the soul’s ability to move the body’s limbs; but I showed that the comparison doesn’t tell the whole story, and that the soul must be more closely united with the body ·than the helmsman is with his ship·, because if it is to make up a real man it must have not only the power to move the body but also feelings and appetites like ours. I went on at some length about the soul, because it is one of the most important topics. Second only to the error of those who deny God — which I think I have adequately refuted above — there is no error that leads weak minds further from the straight path of virtue than that of imagining that the souls of the beasts are of the same nature as ours, and hence that after this present life we have nothing to fear or to hope for, any more than flies and ants do. When we know how different the beasts are from us, we are better placed to understand the arguments proving that our soul is of a nature entirely independent of the body, and thus not liable to die with it. And since we can’t see any other causes that destroy the soul, we are naturally led to think that it is immortal.