Macquarie University
PHIL360 Later Medieval Philosophy

Week 11: Medieval elements in Descartes

This is cassette 11, concerned with the medieval elements in Descartes Meditations.

Descartes (1596-1650) is generally regarded as the first of the modern philosophers. Indeed, until about 50 years ago most philosophers would have said that Descartes was the first significant philosopher since Aristotle. Descartes himself does not draw attention to his sources--not to conceal them (that would have been pointless, because to his contemporaries the continuities of his thought with the books they had all been brought up on would have been obvious), but so as to avoid getting embroiled in learned disputes about who had said what and what they had meant. Descartes wanted his readers to look at his arguments and judge them by using their own intelligence. He calls on his readers to empty their minds of prejudices, i.e. previous judgments, and take a fresh look. His writings had such an impact that philosophy after Descartes generally took his arguments and positions as a starting point or as a point of reference, and earlier writers were no longer read. (There was a social reason for this also: philosophical books were, after Descartes set the example, generally addressed in French or some other vernacular language to lay readers outside the universities, and most readers could not easily read the Latin authors Descartes had himself read when he was a student of the Jesuits.) So it came to be thought that Descartes had had no significant predecessors--that all of his ideas were original. Descartes was indeed an original thinker, but he used a good deal of medieval material. In this lecture I want to go through Descartes' chief work in philosophy, The Meditations, outlining the argument and drawing attention to the medieval material.

The Meditations was first published in 1641, in Latin; it was translated into French by someone else, and the French edition appeared some years later with Descartes' approval. Notice that the most authentic text is the Latin edition. Before publication Descartes arranged for copies of his little book to be circulated to some well-known philosophers and theologians in Paris. They wrote objections, and he wrote replies. The objections and replies formed volume 2 of the Meditations. The objectors included Thomas Hobbes, Antoine Arnauld and Pierre Gassendi.

Descartes' purpose

In the "Dedication" of the work (it is dedicated to the members of the faculty of theology in Paris), Descartes says why he wrote the book. I quote from the translation of Haldane and Ross:

I have always considered that the two questions respecting God and the Soul were the chief of those that ought to be demonstrated by philosophical rather than theological argument,

that is, the questions of the existence of God and the immortality of the soul. (Compare Thomas Aquinas in PHIL252 Readings, p. 105, where he describes such truths as "preambles to the articles [of faith], for faith presupposes natural knowledge"; and of course many other medieval thinkers tried to prove by philosophical argument the existence of God and the immortality of the soul--as Plato had tried to do, in the Laws, right at the beginning of this tradition of philosophical theology.) Descartes goes on to say that he will try to prove not only that God exists, but that his existence is clearer and more certain than the existence of anything other than ourselves--that we can be more sure that God exists than we can be that tables and chairs exist.

And as regards the soul,... some have even dared to say that human reasons have convinced us that it would perish with the body, and that faith alone could believe the contrary; nevertheless, inasmuch as the Lateran Council held under Leo X (in the eighth session) condemns these tenets, and as Leo expressly ordains Christian philosophers to refute their arguments... I have ventured in this treatise to undertake that task. (Haldane and Ross, trs., Philosophical Works of Descartes, vol. 1, p. 134).

To refute their arguments Descartes sets out to prove that there is a "real and true distinction between the human soul and the body" (p. 137). This is not enough to prove that the soul is immortal, but it removes a strong argument for the view that the soul must perish with the body. Descartes' objective is not to prove the immortality of the soul but to refute the argument that since there is no real distinction between soul and body the soul must perish when the body does.

So we gather from the "Dedication" that the purposes of the book are to prove

that God exists and
that God's existence is more certain than the existence of the bodies around us, and
that there is a real distinction between soul and body.

You might suspect that in a Dedication to the theological faculty Descartes would give a religious slant to his work, perhaps for political or public relations reasons, but in fact the book does argue the three points just listed.

Systematic doubt

To show that God's existence is more certain than the existence of tables and chairs would be a clever trick. How does he think he can do that? By first making the existence of bodies uncertain, and then showing that the goodness of God is our only assurance that the bodies that seem to exist really do exist; unless God exists and is good we can't be sure of the existence of bodies.

The book is divided into six "meditations". In the first Descartes assembles arguments mostly of the ancient academic and Pyrrhonian sceptics to bring into doubt as many things as he can. The main premiss underlying his procedure he formulates as follows:

But inasmuch as reason already persuades me that I ought no less carefully to withhold my assent from matters which are not entirely certain and indubitable than from those which appear to me manifestly to be false, if I am able to find in each one some reason to doubt, this will suffice to justify my rejecting the whole. (p. 145).

This is a variant of a principle first put forward by the ancient Stoics, that the wise man assents to and asserts only what cannot possibly be false. The ancient opponents of the Stoics seized on this principle and asked: how much is there that cannot possibly be false? Perhaps nothing, and then the wise man must withhold assent altogether, i.e. must be a sceptic. Their strongest argument for the answer that there is nothing that can't possibly be false was that no distinguishing mark can be assigned that will distinguish between true perceptions and seemingly true propositions that are in fact illusions; without such a mark or "criterion" it is always possible that what seems may not be. How can we be sure that we are not dreaming?

In the first Meditation Descartes uses these arguments, and also, at the climax of this Meditation, a new one that surely derives from the medieval discussion whether God can cause a deceptive intuitive cognition, or (if intuition is by definition not deceptive) a deceptive creditative cognition that we mistake for an intuition. Descartes says:

Nevertheless, I have long had fixed in my mind the belief that an all-powerful God existed by whom I have been created such as I am. But how do I know that he has not brought it to pass that there is no earth, no heaven, no extended body, no magnitude, no place, and that nevertheless I possess the perceptions of all these things and that they seem to me to exist just exactly as I now see them?

This is the hypothesis (like Ockham's "creditative cognition") that God may cause something in us from which we mistakenly believe that something exists when in fact it does not. At the end of the first Meditation Descartes writes:

I shall then suppose, not that God who is supremely good and the fountain of truth, but some evil genius [spirit] not less powerful than deceitful, has employed his whole energies in deceiving me; I shall consider that the heavens, the earth,... .and all other external things are nought but the illusions and dreams of which this genius has availed himself... I shall consider myself as having no hands, no eyes, no flesh, no blood, nor any senses, yet falsely believing myself to possess all these things... (p. 148).

But in the second Meditation he points out that, however hard I try to doubt all things, I cannot doubt my own existence:

But [the sceptical thought occurs] there is some deceiver or other... who ever employs his ingenuity in deceiving me. Then without doubt I exist also if he deceives me, and let him deceive me as much as he will, he can never cause me to be nothing so long as I think I am something. So... we must come to the definite conclusion that this proposition: I am, I exist, is necessarily true each time that I pronounce it, or that I mentally conceive it. (p. 150)

This is the famous cogito, ergo sum, "I think, therefore I am"; he does not use these words here, but they occur in the parallel place in his Discourse on Method, part 4 (Haldane and Ross, vol. I, p. 101).

So the attempt to doubt everything has shown

that the existence of bodies can be doubted, and
that the existence of my thinking self cannot be doubted.

Descartes has now found in the cogito a firm basis upon which his philosophy can be constructed. As he says in the Discourse:

Not that indeed I imitated the sceptics, who only doubt for the sake of doubting, and pretend to be always uncertain; for on the contrary, my design was only to provide myself with good grounds for assurance, and to reject the quicksand and mud in order to find the rock or clay (Discourse, p. 99).

So he has tried to doubt everything, so as to reject the quicksand and mud; and when he finds something he cannot doubt, his own existence as a thinking being, that is the rock on which he can build securely. We'll see what he builds shortly, but first some comment on his sources.

Antoine Arnauld at the beginning of "Objections IV" writes:

The first thing that here occurs to me as worthy of remark is that our distinguished author should have taken as the foundation of the whole of his philosophy the doctrine laid down by St Augustine... In De Libero Arbitrio, book II, ch. 3, Alipius, when disputing with Evodius, setting about a proof of the existence of God, says: "Firstly, to start with the things that are most evident, I ask you whether you yourself exist? Or are you apprehensive lest answering this question you are in error, when in any case, if you did not exist you could never be in error?" (Haldane and Ross, vol. 2, p. 80).

In fact Augustine followed this line of thought in several places. For example, in De Trinitate XV.xii.21:

First, of what sort and how great is the knowledge that a man can attain... For to pass by those things that come into the mind from the bodily senses, among which so many are otherwise than they seem to be, that he who is over-much pressed down by their resemblance to truth, seems sane to himself, but really is not sane; whence it is that the Academic philosophy has so prevailed as to be still more wretchedly insane by doubting all things;--passing by, then, those things that come into the mind by the bodily senses, how large a proportion is left of things which we know in such manner as we know that we live? In regard to this, indeed, we are absolutely without any fear lest perchance we are being deceived by some resemblance of the truth; since it is certain, that he who is deceived, yet lives... The knowledge by which we know that we live is the most inward of all knowledge, of which even the Academic cannot insinuate: Perhaps you are asleep, and do not know it... Nor can the academic again say, in confutation of this knowledge: Perhaps you are mad, and do not know it:... but he who is mad is alive. Therefore he who says he knows he is alive, can neither be deceived nor lie. Let a thousand kinds, then, of deceitful objects of sight be presented to him who says, I know I am alive; yet he will fear none of them, for he who is deceived yet is alive.

And in the The City of God, book XI, c.26:

In the face of these truths, the quibbles of the skeptics lose their force. If they say; "What if you are mistaken?"--well if I am mistaken, I am. For, if one does not exist, he can by no means be mistaken. Therefore, I am, if I am mistaken. Because, therefore, I am, if I am mistaken, how can I be mistaken that I am, since it is certain that I am, if I am mistaken? and because, if I could be mistaken, I would have to be the one who is mistaken, therefore, I am most certainly not mistaken in knowing that I am. Nor, as a consequence, am I mistaken in knowing that I know. For, just as I know that I am, I also know that I know.

The arguments of the sceptics, and Augustine's replies, were of some interest to late medieval thinkers. For example, Henry of Ghent and Duns Scotus both discussed them; see Duns Scotus, Philosophical Writings, trs. Wolter, p. 96ff., and Adams William Ockham, vol. 1, pp. 552-571. The thinking of an Oxford writer just after Ockham was there, William Crathorn, parallels Descartes' argument. Crathorn rejects Ockham's rejection of species and criticises his doctrine of intuitive cognition. He points out that if God can cause or conserve vision of a non-existent object, we can never be certain of any object (Tachau, p. 268). Holding, himself, that the species are all that we know intuitively (here anticipating Hume), Grathorn also infers, like Hume, that we can never be certain that any external object actually exists. But we can be certain about ourselves:

For if one were to be in doubt concerning some proposition such as "I am", it would follow that he exists, because "I doubt that I am, therefore I am" follows, since he who is not does not doubt. Hence no one can doubt concerning this proposition, "I am". (Crathorn, quoted Tachau, p. 273)

The Criterion of Certainty

To return to Descartes. He has found that his own existence as a thinking being is indubitable, even while he doubts everything else, including the existence of his own body and other bodies. This gives him two things:

a criterion of certainty, i.e. an answer to the ancient sceptic demand for a mark by which to distinguish truth from error;
the beginnings of an argument for the distinction between soul and body.

The criterion is clarity: it is because it is so very clear that I must exist if I doubt or am deceived or think at all that I cannot doubt the truth of the judgment that I exist:

I am certain that I am a thing which thinks; but do I not then likewise know what is requisite to render me certain of a truth? Certainly in this first knowledge there is nothing that assures me of its truth, excepting the clear and distinct perception of that which I state, which would not indeed suffice to assure me that what I say is true, if it could ever happen that a thing which I conceived so clearly and distinctly would be false; and accordingly it seems to me that already I can establish as a general rule that all things which I perceive very clearly and very distinctly are true. (Haldane and Ross, p. 158)

The distinction of soul and body

If I can be certain that I exist while doubting the existence of my body, then there is a distinction between myself and my body. Compare Duns Scotus's argument in Hyman and Walsh, p. 562: "Every intellect certain about one concept and doubtful about different ones has a concept of that of which it is certain which is different from the concepts about which it is doubtful." Cf. second-last para. on p. 577. But according to Scotus this is enough to prove only a formal distinction. The author of the first set of objections to Descartes' argument, Caterus, raises this point.

It appears that the distinction between soul and body, if real, is proved by the fact that they can be conceived as distinct and as isolated from each other. Here I leave my opponent to contend with Scotus, who says that insofar as one thing can be conceived as distinct and separate from another, the adequate distinction to draw between them is what he calls a formal and objective one, which is in termediate between a real distinction and a distinction of reason... it does not follow that because God's justice can be conceived apart from his pity, they can also exist apart. (Haldane and Ross, vol. 2, p. 8).

Descartes answers (vol. 2, p. 22) that Scotus's formal distinction, or as he calls it "modal" distinction, applies only to incomplete entities (i.e. to accidents or modes, not to substances); if we can clearly and distinctly think of A and B as complete and independent substances then the distinction must be real, not merely modal. In Meditation VI, where he completes the argument for the real distinction of soul from body, he writes:

Because I know that all things which I apprehend clearly and distinctly can be created by God as I apprehend them, it suffices that I am able to apprehend one thing apart from another clearly and distinctly [and we must add: as an independently existing being] in order to be certain that the one is different from the other, since they may be made to exist in separation at least by the omnipotence of God.

This is exactly Duns Scotus's criterion of a real distinction--possibility of separate existence at least by miracle, by the power of God. It is also Ockham's criterion. Of course Ockham rejects Scotus's formal distinction, but as far as the real distinction is concerned all three of these authors agree: if there is nothing self-contradictory in supposing separate existence, at least by miracle, then the two things are really distinct. When he re-wrote his Meditations in text book form as The Principles of Philosophy, Descartes devoted a section to distinctions, Principles of Philosophy, part I, principles 60-63. In principle 60 he says:

Distinction is of three sorts, viz. real, modal and of reason. The real is properly speaking found between two or more substances; and we can conclude that two substances are really distinct one from the other from the sole fact that we can conceive the one clearly and distinctly without the other. For in accordance with the knowledge which we have of God, we are certain that he can carry into effect all of which we have a distinct idea... Because each of us in conscious that he thinks, and that in thinking he can shut off from himself [i.e. deny to be identical with himself] all other substance... we may conclude that each of us... is really distinct from every... corporal substance. And even if we suppose that God had united a body to a soul so closely that it was impossible to bring them together more closely, and made a single thing out of the two, they would yet remain really distinct... because however closely God connected them he could not set aside the power which he possessed of separating them or conserving them one apart from the other, and those things which God can separate, or conceive in separation, are really distinct. (Haldane and Ross vol. 1 pp. 243-4).

The possibility that God can cause X and Y to exist separately is also Scotus's criterion of real distinction. In Meditation 6 Descartes goes on to say that the distinction between the soul and its faculties is a distinction of modes from substance (p. 190). In all of this Descartes is close to Duns Scotus. (See Gilson, Jean Duns Scot, ch. 7). According to Scotus the immortality of the soul cannot be proved. However, thinking does not involve a bodily organ (though he does not say that this is shown by the fact that the concept of thinking is clear and distinct without reference to a bodily organ, which is Descartes' argument)--and because thinking does not involve the body it is possible that the soul survives the body and the two are really distinct; which is all Descartes is trying to prove. Like Descartes, Scotus says that the soul is really distinct i.e. a distinct thing, from the body, though these two things are brought into close union. And according to Scotus the distinction between the soul's faculties is formal, not real, which is also Descartes' position; and Descartes' doctrine of distinctions is close to Scotus's , relying for proof of real distinction on God's power to cause one to exist without the other.

God's Existence

In going to Meditation 6 to see the completion of Descartes' argument for the real distinction of soul and body we have departed from his order; we have been talking about God's power before he has been shown to exist. Let's go back to Meditation 3, to Descartes' argument for God's existence. At that stage of the argument all the doubts raised in Meditation 2 remain unresolved: the only thing in the universe whose existence is free from doubt is my own existence as a thinking thing; in particular, the existence of all bodies, including mine, is in doubt. So it will not be possible for Descartes to argue for God's existence as Thomas Aquinas did from the fact that with our senses we perceive that things are in motion.

The starting point of Descartes' argument is the fact that he finds in his mind the idea of a most perfect being. As "modes" (or as medieval writers would say, accidents) of the thinking substance all thoughts are equal in their degree of reality and are sufficiently accounted for by finite causes, such as the mind itself. But in respect of the objective reality that it contains, the idea of God contains objectively such a high degree of reality that it can be caused only by God. God therefore exists. See Meditation 3 in Haldane and Ross, pages 162-3, 165.

Several things require further comment here. The first is the notion of objective reality. This is the esse obiectivum that we met with in Scotus and Ockham; see Ockham, Philosophical Writings, tr. Boehner, pp. 41-3, where esse obiectivum is translated "being as a thought object". Descartes asks us to order beings in the mind in ranks corresponding to different degrees of being outside the mind. For example, if spiritual beings rank above material beings, then the idea of a spiritual being contains objective reality of a higher rank than what is contained in the idea of a material being. Scotus and Ockham and other medieval authors do not seem to have thought of objective reality as something contained in concepts and did not draw up any rank order: this seems to be Descartes' modification of the medieval notion. Now Descartes says

that a cause must have at least as high a degree of being as its effect, and
that the sort of being appropriate to a cause is real being (not objective being).

Medieval writers would have agreed with both points. He also says that the cause of an idea containing a certain degree of objective reality must contain at least the corresponding degree of formal reality. This seems to be a new principle, and it is the basis of his first argument for God's existence. Only God can have a high enough degree of real being to correspond to the degree of objective reality contained in the idea of God. Since we have this idea, God must really exist.

For Descartes' formulation of the crucial premiss, see Meditation 3 (Haldane and Ross, top of p. 163), the appendix to Replies to Objections II, "Arguments drawn up... in geometrical fashion", axiom 5 (Haldane and Ross, vol. 2, p. 56), and Principles of Philosophy, part 1, ppl. 17 (Haldane and Ross, vol. 1, pp. 225-6).

His argument for God's existence is a good illustration of the relationship between Descartes' philosophy and medieval thought: he takes medieval materials, adds something new, and constructs a new argument, in this case for a traditional conclusion.

The argument we've just looked at is the first of three arguments Descartes offers for God's existence. Like Thomas Aquinas and Scotus he bases this argument on the causal principle, but the effect for which the cause is found is different--it is the existence in one's mind of a certain idea.

Another argument for God's existence

Later in Meditation III he offers another argument. Descartes asks: From what source do I derive my existence? If it were from myself, then I would not lack any of the perfections I can imagine. If it is from another, but not God, then the same question can be asked: From what source does that other derive its being? Either from God, or from another; and so on, but not ad infinitum. (See Haldane and Ross vol. 1, pp. 167-9). This is recognisable as a variant of the medieval regress arguments, as in Thomas Aquinas and Scotus, where a vital premiss is that the regress cannot be infinite.

Descartes deals with two objections. One is that perhaps my existence (or the existence of the others that are not God) does not derive from anything--"I need not seek for any author of my existence". That won't do, he says:

For all the course of my life may be divided into a infinite number of parts, none of which is in any way dependent on the other; and thus from the fact that I was in existence a short time ago it does not follow that I must be in existence now, unless some cause at this instant, so to speak, produces me anew, that is to say, conserves me. (Haldane and Ross, vol. 1, p. 168).

Conservation is a familiar medieval notion, e.g. from Ockham: God creates and conserves. The idea that time consists of disconnected instants would have been rejected by medieval Aristotelians, following Aristotle's treatment of the continuum (in answer to Zeno's paradoxes and other puzzles). For a medieval precedent see PHIL252 Supplement, p. 59, an extract from Gilson's History of Christian Philosophy in the Middle Ages (p. 185). There you'll find that some Muslim thinkers also held that the universe exists from instant to instant so that God must create it anew at every instant.

The other objection Descartes deals with is that perhaps several causes concur in producing me, and perhaps I get the idea of some perfections from one cause and of others from another. Descartes replies, "On the contrary, the unity, the simplicity or the inseparability of all things which are in God is one of the principal perfections which I conceive to be in him" (p. 169). Absolute simplicity is of course a basic tenet of the neo-Platonic and medieval conception of God.

Descartes' variant of Anselm's argument

Descartes offers a third argument for God's existence, in Meditation 5, Haldane and Ross, p. 180: Just as I find in myself the ideas of various geometrical figures, from which I can deduce properties of those figures have (if they exist), similarly I find in myself an idea of God from which I can deduce that one of God's properties is to exist; existence is a perfection, so the most perfect nature must have existence.

This is reminiscent of Anselm's argument. Descartes answers the objection, first made by Gaunilon, that perhaps this idea of a most perfect being is merely a fiction. His answer is that when an idea is a fiction we can add bits to it and take them away at will, whereas this idea has an internal coherence and is not subject to our will, as is shown by the fact that we can demonstrate properties of it:

I discern in many ways that this idea is not something fictitious, and depending solely on my thought, but that it is the image of a true and immutable nature; first of all, because I cannot conceive anything but God himself to whose essence existence necessarily pertains; in the second place, because it is not possible for me to conceive two or more Gods in this same position; and, granted that there is one such God who now exists, I see clearly that it is necessary that he should have existed from all eternity, and that he must exist eternally; and finally, because I know an infinity of other properties of God, none of which I can either diminish or change. (Haldane and Ross, vol. 1, pp. 182-3; compare the top of p. 180).

There is an interesting restatement of the argument in the Reply to the first set of objections (Haldane and Ross, vol. 2, pp. 19-22), which clarifies the claim that the idea of God as necessarily existing is a true nature and not a fiction because it has its own coherence. Suppose we invented the idea of a necessarily existing body, and argued that since this idea contains necessary existence, such a body necessarily exists. Descartes says that this argument would not work, because my idea of this body is a fiction:

I can well enough recognise that that idea has been put together by my mind uniting together all corporeal perfections, and that existence does not arise out of its other corporeal perfections... I see in it no force by means of which it may produce or preserve itself. But now, if we ask not about a body but about a thing (of whatever sort this thing may turn out to be) which has all those perfections which can exist together [note the echo here of Scotus's "compossibiles"], whether existence must be included in the number of these perfections... we shall be able clearly and distinctly to perceive... that possible existence is at least predicable of it... Further, because we cannot think of God's existence as being possible, without at the same time, and by taking heed of his immeasurable power, acknowledging that he can exist by his own might, we hence conclude that he really exists and has existed from all eternity; for the light of nature makes it most plain that what can exist by its own power always exists. And thus we shall understand that necessary existence is comprised in the idea of a being of the highest power, not by any intellectual fiction, but because it belongs to the true and immutable nature of that being to exist. (Haldane and Ross, vol. 2, p. 21).

This argument is perhaps an echo of Scotus's argument that if a most perfect conceivable being is possible then it must actually exist--Scotus means because there is no other way for it to be possible, but Descartes argues that it must exist because its infinite power enables it to produce itself. So power is here the key attribute which binds the idea of this being with a coherent, non-fictitious, true and immutable nature, such that we can affirm of it whatever is contained in its concept, including, notably, actual existence.

The existence of bodies

None of these arguments for God's existence depends in any way on the actual existence of bodies, only on the ideas I find in my mind. Bodies were left under a cloud of doubt back in the first Meditation. It is time now to bring them back. The idea of God, whose existence has now been proved, is Anselm's idea, that of a supremely perfect being.

From this it is manifest that he cannot be a deceiver, since the light of nature teaches us that fraud and deception necessarily proceed from some defect. (End of Meditation 3, Haldane and Ross, vol. 1, p. 171).

You may have noticed that Descartes has a great fund of principles taught by the light of nature, like the medieval writers' principles per se nota, principles evident in themselves. Recall that one of the earliest things Descartes drew from his "I think, therefore I exist" is a criterion of truth: whatever I conceive clearly and distinctly must be true. So here it is clear, self-evident, that the most perfect being cannot be a deceiver. So why, then, does he create me with such a nature that I am sometimes deceived? Descartes' answer, developed in Meditation 4, is, in brief, that God has given me free will of universal extent so as to fit me for the perfect happiness of heaven, but he has given me an understanding of more limited compass. This means that I can wilfully affirm things that I do not understand--or, on the other hand, I can choose not to affirm things I don't see clearly and distinctly to be true. In the first case I fall into error, and the error is not God's fault but mine--he has not deceived me, I've been precipitate. In matters that are clear and distinct, where I can't help affirming, God's goodness guarantees truth; in other matters I ought to suspend judgment. The connection of this is not with medieval thought but with the ancient Stoics, who taught that it is always within our power to withhold assent from propositions that may possibly be false.

So how much of what I formerly believed about bodies was based on clear and distinct perception, and how much was affirmation without understanding? In Meditation 6 he argues that the mathematical properties of bodies are clear and distinct, but not their colour, taste, smell and so on, the qualities that have been called since the 17th century "secondary" qualities. I can, therefore, without falling into error affirm what the mathematical natural sciences establish about bodies, but I should not affirm anything more. God's veracity guarantees that what, given my nature, I can't help believing because of its clarity and distinctness will be true, but God does not guarantee the truth of things I voluntarily affirm beyond what is clear and distinct.

Insofar as I do have knowledge or justified beliefs about bodies, its certainty depends on the veracity of God: because he is perfect he is not a deceiver; if he had made me incapable of withholding assent from falsehood he would have been a deceiver; what I cannot help believing must therefore be true. Thus Descartes redeems his promise to show that God's existence is more certain than the existence of bodies: what enables us to be certain of anything about bodies is that God is perfect, and that he has created our powers of knowledge. It is only for this reason that we know we can't be deceived in what we can't help believing.

Descartes' argument in the Meditations didn't completely satisfy too many people, but his book had a great impact, so that, for the rest of the 17th century and beyond, Descartes' philosophy was the stimulus to most other philosophical activity, at least to most of what became part of the modern philosophical tradition. It was from Descartes that many later philosophers learned all they ever learned of medieval philosophy. Some, notably Leibniz, continued to read the medieval authors directly, but this practice dwindled toward the end of the 17th century, especially when it became the established practice for philosophers to write in French or English for non-university readers. By the end of the century the works of Descartes and other moderns were sometimes adopted as university textbooks, and during the 18th century direct knowledge of medieval thought almost died out.

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