During these past few days I have accustomed myself to leading my mind away from the senses; and I have taken careful note of the fact that there is very little about corporeal things that is truly perceived, whereas much more is known about the human mind, and still more about God. The result is that I now have no difficulty in turning my mind away from imaginable things and towards things which are objects of the intellect alone and are totally separate from matter. And indeed the idea I have of the human mind, in so far as it is a thinking thing, which is not extended in length, breadth or height and has no other bodily characteristics, is much more distinct than the idea of any corporeal thing. And when I consider the fact that I have doubts, or that I am a thing that is incomplete and dependent, then there arises in me a clear and distinct idea of a being who is independent and complete, that is, an idea of God. And from the mere fact that there is such an idea within me, or that I who possess this idea exist, I clearly infer that God also exists, and that every single moment of my entire existence depends on him. So clear is this conclusion that I am confident that the human intellect cannot know anything that is more evident or more certain. And now, from this contemplation of the true God, in whom all the treasures of wisdom and the sciences lie hidden, I think I can see a way forward to the knowledge of other things.

To begin with, I recognize that it is impossible that God should ever deceive me. For in every case of trickery or deception some imperfection is to be found; and although the ability to deceive appears to be an indication of cleverness or power, the will to deceive is undoubtedly evidence of malice or weakness, and so cannot apply to God.

[The cause of error]

You say that although you have no power to avoid error through having a clear perception of things, you can still avoid it by firmly resolving to adhere to the rule of not assenting to anything which you do not clearly perceive. But although you can always keep this rule carefully in mind, is it not still an imperfection not to perceive clearly matters which you need to decide upon, and hence to be perpetually liable to the risk of error?

You say that error resides in the mental operation itself in so far as it proceeds from you and is a kind of privation, but not in the faculty God gave you, nor in its operation in so far as it depends on him. But although the error does not immediately reside in the faculty God gave you, it does indirectly attach to it, since it was created with the kind of imperfection which makes error possible. Admittedly, as you say, you have no cause for complaint against God who, despite owing you nothing, bestowed on you the good gifts which you should thank him for. But there is still cause to wonder why he did not bestow more perfect gifts on you, given that he had the knowledge and the power and was not malevolent.

You go on to say that you have no cause to complain that God's concurrence is involved in your acts when you go wrong. For in so far as these acts depend on God, they are all true and good; and your ability to perform them means that there is, in a sense, more perfection in you than would be the case if you lacked this ability. You continue: ‘As for the privation involved – which is all that the essential definition of falsity and wrong consists in – this does not in any way require the concurrence of God, since it is not a thing and should not be referred to him.’ But although this is a subtle distinction it is not quite enough to resolve the problem.

[The rejection of previous beliefs]

[It is objected that] I am asking for something impossible in wanting us to give up every kind of preconceived opinion … But [my critic] has not realized that the term ‘preconceived opinion’ applies not to all the notions which are in our mind (which I admit it is impossible for us to get rid of) but only to all the opinions which we have continued to accept as a result of the previous judgements that we have made. And since making or not making a judgement is an act of will (as I have explained in the appropriate place) it is evident that it is something in our power. For, after all, in order to get rid of every kind of preconceived opinion, all we need to do is resolve not to affirm or deny anything that we have previously affirmed or denied until we have examined it afresh. But this does not entail that we cease to retain all the same notions in our memory. Nevertheless, I did say that there was some difficulty expelling from our belief everything we have previously accepted. One reason for this is that before we can decide to doubt, we need some reason for doubting; and that is why in my First Meditation I put forward the principal reasons for doubt. Another reason is that no matter how much we resolved not to assert or deny anything, we easily forget our resolution afterwards if we have not strongly impressed it on our memory; and this is why I suggested that we should think about it very carefully. [Appendix to Fifth Objections and Replies: CSM II 270]

Here I shall employ an everyday example to explain to my critic the rationale for my procedure, so as to prevent him misunderstanding it, or having the gall to pretend he does not understand it, in future. Suppose he had a basket full of apples and, being worried that some of the apples were rotten, wanted to take out the rotten ones to prevent the rot spreading. How would he proceed? Would he not begin by tipping the whole lot out of the basket?

[The real distinction between mind and body]

It is clear that the question he promised to answer still remains outstanding: how does it follow, from the fact that he is aware of nothing else belonging to his essence, that nothing else does in fact belong to it? I must confess that I am somewhat slow, but I have been unable to find anywhere in the Second Meditation an answer to this question. As far as I can gather, however, the author does attempt a proof of this claim in the Sixth Meditation, since he takes it to depend on his having clear knowledge of God, which he had not yet arrived at in the Second Meditation. This is how the proof goes:

We must pause a little here, for it seems to me that in these few words lies the crux of the whole difficulty.

First of all, if the major premiss of this syllogism is to be true, it must be taken to apply not to any kind of knowledge of a thing, nor even to clear and distinct knowledge; it must apply solely to knowledge which is adequate.

I will now shut my eyes, stop my ears, and withdraw all my senses. I will eliminate from my thoughts all images of bodily things, or rather, since this is hardly possible, I will regard all such images as vacuous, false and worthless. I will converse with myself and scrutinize myself more deeply; and in this way I will attempt to achieve, little by little, a more intimate knowledge of myself. I am a thing that thinks: that is, a thing that doubts, affirms, denies, understands a few things, is ignorant of many things, is willing, is unwilling, and also which imagines and has sensory perceptions; for as I have noted before, even though the objects of my sensory experience and imagination may have no existence outside me, nonetheless the modes of thinking which I refer to as cases of sensory perception and imagination, in so far as they are simply modes of thinking, do exist within me – of that I am certain.

In this brief list I have gone through everything I truly know, or at least everything I have so far discovered that I know. Now I will cast around more carefully to see whether there may be other things within me which I have not yet noticed. I am certain that I am a thinking thing. Do I not therefore also know what is required for my being certain about anything? In this first item of knowledge there is simply a clear and distinct perception of what I am asserting; this would not be enough to make me certain of the truth of the matter if it could ever turn out that something which I perceived with such clarity and distinctness was false. So I now seem to be able to lay it down as a general rule that whatever I perceive very clearly and distinctly is true.

Yet I previously accepted as wholly certain and evident many things which I afterwards realized were doubtful. What were these? The earth, sky, stars, and everything else that I apprehended with the senses. But what was it about them that I perceived clearly? Just that the ideas, or thoughts, of such things appeared before my mind. Yet even now I am not denying that these ideas occur within me.

[Cogito ergo sum (‘I am thinking, therefore I exist’)]

I see that you conclude that this proposition, I am, I exist, is true whenever it is put forward by you or conceived in your mind. But I do not see that you needed all this apparatus, when on other grounds you were certain, and it was true, that you existed. You could have made the same inference from any one of your other actions, since it is known by the natural light that whatever acts exists. [Fifth Objections: CSM II 180]

When you say that I ‘could have made the same inference from any one of my other actions’ you are far from the truth, since I am not wholly certain of any of my actions, with the sole exception of thought (in using the word ‘certain’ I am referring to metaphysical certainty, which is the sole issue at this point). I may not, for example, make the inference Ί am walking, therefore I exist’, except in so far as the awareness of walking is a thought. The inference is certain only if applied to this awareness, and not to the movement of the body which sometimes – in the case of dreams – is not occurring at all, despite the fact that I seem to myself to be walking. Hence from the fact that I think I am walking I can very well infer the existence of a mind which has this thought, but not the existence of a body that walks. And the same applies in other cases. [Fifth Replies: CSM II 244]

When someone says ‘I am thinking, therefore I am, or I exist’, he does not deduce existence from thought by means of a syllogism, but recognizes it as something self-evident by a simple intuition of the mind. This is clear from the fact that if he were deducing it by means of a syllogism, he would have to have had previous knowledge of the major premiss ‘Everything which thinks is, or exists’; yet in fact he learns it from experiencing in his own case that it is impossible that he should think without existing.

‘I would not urge anyone to read this book except those who are able and willing to meditate seriously with me’, Descartes says to his readers in the Preface (p. 9, below), and he makes it clear that he means the Meditations not to be a treatise, a mere exposition of philosophical reasons and conclusions, but rather an exercise in thinking, presented as an encouragement and a guide to readers who will think philosophically themselves. Its thoughts, correspondingly, are presented as they might be conducted by its author – or rather, as though they were being conducted at the very moment at which you read them. Indeed, the ‘I’ who is having these thoughts may be yourself. Although we are conscious, in reading the Meditations, that they were written by a particular person, René Descartes, and at a particular time, about 1640, the ‘I’ that appears throughout them from the first sentence on does not specifically represent that person: it represents anyone who will step into the position it marks, the position of the thinker who is prepared to reconsider and recast his or her beliefs, as Descartes supposed we might, from the ground up.

This ‘I’ is different, then, from the ‘I’ that occurs in the Replies to the Objections. (Extracts from both of these also appear in this volume; how they came to be written is explained by the translator in his Note on the text, p. xlix.) In the Replies, Descartes speaks straightforwardly for himself, and the ‘I’ represents the author of the Meditations. The ‘I’ in the Meditations themselves represents their narrator or protagonist, whom we may call ‘the thinker’. Of course the author has to take responsibility for the thinker's reflections. He takes responsibility both for the conduct of them and for their outcome, where that includes the beliefs to which we shall have been led if we are persuaded by the arguments, and also the improved states of mind that the author expects us to reach by following his work. But the author is not answerable for every notion entertained by the thinker and for every turn that the reflection takes on the way.

There are many matters which remain to be investigated concerning the attributes of God and the nature of myself, or my mind; and perhaps I shall take these up at another time. But now that I have seen what to do and what to avoid in order to reach the truth, the most pressing task seems to be to try to escape from the doubts into which I fell a few days ago, and see whether any certainty can be achieved regarding material objects.

But before I inquire whether any such things exist outside me, I must consider the ideas of these things, in so far as they exist in my thought, and see which of them are distinct, and which confused.

Quantity, for example, or ‘continuous’ quantity as the philosophers commonly call it, is something I distinctly imagine. That is, I distinctly imagine the extension of the quantity (or rather of the thing which is quantified) in length, breadth and depth. I also enumerate various parts of the thing, and to these parts I assign various sizes, shapes, positions and local motions; and to the motions I assign various durations.

Not only are all these things very well known and transparent to me when regarded in this general way, but in addition there are countless particular features regarding shapes, number, motion and so on, which I perceive when I give them my attention. And the truth of these matters is so open and so much in harmony with my nature, that on first discovering them it seems that I am not so much learning something new as remembering what I knew before; or it seems like noticing for the first time things which were long present within me although I had never turned my mental gaze on them before.

But I think the most important consideration at this point is that I find within me countless ideas of things which even though they may not exist anywhere outside me still cannot be called nothing; for although in a sense they can be thought of at will, they are not my invention but have their own true and immutable natures.

[Whether God's essence implies his existence]

You next attempt to demonstrate the existence of God, and the thrust of your argument is contained in the following passage:

When I concentrate, it is quite evident that existence can no more be separated from the essence of God than the fact that its three angles equal two right angles can be separated from the essence of a triangle, or than the idea of a mountain can be separated from the idea of a valley. Hence it is just as much of a contradiction to think of God (that is, a supremely perfect being) lacking existence (that is, lacking a perfection) as it is to think of a mountain without a valley.

But we must note here that the kind of comparison you make is not wholly fair.

It is quite all right for you to compare essence with essence, but instead of going on to compare existence with existence or a property with a property, you compare existence with a property. It seems that you should have said that omnipotence, for example, can no more be separated from the essence of God than the fact that its angles equal two right angles can be separated from the essence of a triangle. Or, at any rate, you should have said that the existence of God can no more be separated from his essence than the existence of a triangle can be separated from its essence. If you had done this, both your comparisons would have been satisfactory, and I would have granted you not only the first one but the second one as well. But you would not for all that have established that God necessarily exists, since a triangle does not necessarily exist either, even though its essence and existence cannot in actual fact be separated. Real separation is impossible no matter how much the mind may separate them or think of them apart from each other – as indeed it can even in the case of God's essence and existence.

Some years ago I was struck by the large number of falsehoods that I had accepted as true in my childhood, and by the highly doubtful nature of the whole edifice that I had subsequently based on them. I realized that it was necessary, once in the course of my life, to demolish everything completely and start again right from the foundations if I wanted to establish anything at all in the sciences that was stable and likely to last. But the task looked an enormous one, and I began to wait until I should reach a mature enough age to ensure that no subsequent time of life would be more suitable for tackling such inquiries. This led me to put the project off for so long that I would now be to blame if by pondering over it any further I wasted the time still left for carrying it out. So today I have expressly rid my mind of all worries and arranged for myself a clear stretch of free time. I am here quite alone, and at last I will devote myself sincerely and without reservation to the general demolition of my opinions.

But to accomplish this, it will not be necessary for me to show that all my opinions are false, which is something I could perhaps never manage. Reason now leads me to think that I should hold back my assent from opinions which are not completely certain and indubitable just as carefully as I do from those which are patently false. So, for the purpose of rejecting all my opinions, it will be enough if I find in each of them at least some reason for doubt. And to do this I will not need to run through them all individually, which would be an endless task. Once the foundations of a building are undermined, anything built on them collapses of its own accord; so I will go straight for the basic principles on which all my former beliefs rested.

Whatever I have up till now accepted as most true I have acquired either from the senses or through the senses. But from time to time I have found that the senses deceive, and it is prudent never to trust completely those who have deceived us even once.

[Innate ideas]

When M. Descartes says that the ideas of God and of our souls are innate in us, I should like to know if the souls of people who are in a deep, dreamless sleep are thinking. If they are not, they do not have any ideas at the time. If follows that no idea is innate; for what is innate is always present. [Third Objections: CSM II 132]

When we say that an idea is innate in us, we do not mean that it is always there before us. This would mean that no idea is innate. We simply mean that we have within ourselves the faculty of summoning up the idea. [Third Replies: CSM II 132]

You next distinguish ideas (by which you mean thoughts in so far as they are like images) into three classes: innate, adventitious and made up. In the first class you put ‘your understanding of what a thing is, what truth is and what thought is’. In the second class you put ‘your hearing a noise, seeing the sun and feeling a fire’. And in the third class you put ‘your invented idea of sirens and hippogriffs’. You add that all your ideas may perhaps be adventitious or they may all be innate or all made up, since you have not as yet clearly perceived their origin. But in case some fallacy should creep in before you have managed to perceive the origin of your ideas, I should like to go further and note that all ideas seem to be adventitious – to proceed from things which exist outside the mind and come under one of our senses. The mind has the faculty (or rather is itself the faculty) of perceiving adventitious ideas – those which it receives through the senses and which are transmitted by things; these ideas, I say, are quite unadorned and distinct, and are received just exactly as they are. But in addition to this, the mind has the faculty of putting these ideas together and separating them in various ways, of enlarging them and diminishing them, of comparing them, and so on.

Descartes's Meditations on First Philosophy is, indisputably, one of the greatest philosophical classics of all time. The challenge it offers is in many ways definitive of the philosophical enterprise: to leave behind the comfortable world of inherited prejudice and preconceived opinion; to take nothing for granted in the determination to achieve secure and reliable knowledge. Descartes talks of ‘demolishing] everything completely and starting] again right from the foundations’, and for this purpose he famously uses doubt, stretched to its limits, as an instrument which self-destructs, impelling him forwards on the journey towards certainty and truth. These central themes are today part of every introductory course in the philosophy of knowledge: Descartes's masterpiece has achieved canonical status in that part of the philosophy syllabus we now call ‘epistemology’. Yet for Descartes himself these epistemic concerns were but one part of a much wider project: the construction of a grand, all-embracing system of philosophy which would encompass metaphysics, natural science, psychology and morals, connecting all the objects within the scope of human understanding. In the words of the famous metaphor which he deployed some six years after the publication of the Meditations, ‘the whole of philosophy is like a tree. The roots are metaphysics, the trunk is physics, and the branches … all the other sciences.’

Descartes spent much of his career occupied with what we would nowadays call theoretical physics: he devised a radical new theory of the nature of matter, defined simply as extension in three dimensions, and formulated a number of mathematical laws describing the results of collisions of moving particles of matter. He then proposed to apply these principles to a wide variety of subjects, from cosmology and astronomy to physiology and medicine; and towards the end of his life he planned to include a science of man, which would develop prescriptions for how to understand and control the workings of our bodies, and how to live fulfilled and worthwhile lives. Examining the course of Descartes's life, and the context in which the Meditations was written, helps us deepen our understanding of the metaphysical and epistemological themes of his most famous book by seeing how they fit into the broader philosophical system which he devoted his life to creating.

It remains for me to examine whether material things exist. And at least I now know they are capable of existing, in so far as they are the subject-matter of pure mathematics, since I perceive them clearly and distinctly. For there is no doubt that God is capable of creating everything that I am capable of perceiving in this manner; and I have never judged that something could not be made by him except on the grounds that there would be a contradiction in my perceiving it distinctly. The conclusion that material things exist is also suggested by the faculty of imagination, which I am aware of using when I turn my mind to material things. For when I give more attentive consideration to what imagination is, it seems to be nothing else but an application of the cognitive faculty to a body which is intimately present to it, and which therefore exists.

To make this clear, I will first examine the difference between imagination and pure understanding. When I imagine a triangle, for example, I do not merely understand that it is a figure bounded by three lines, but at the same time I also see the three lines with my mind's eye as if they were present before me; and this is what I call imagining. But if I want to think of a chiliagon, although I understand that it is a figure consisting of a thousand sides just as well as I understand the triangle to be a three-sided figure, I do not in the same way imagine the thousand sides or see them as if they were present before me. It is true that since I am in the habit of imagining something whenever I think of a corporeal thing, I may construct in my mind a confused representation of some figure; but it is clear that this is not a chiliagon. For it differs in no way from the representation I should form if I were thinking of a myriagon, or any figure with very many sides. Moreover, such a representation is useless for recognizing the properties which distinguish a chiliagon from other polygons.