Third Meditation

The central claim of the Third Meditation is that the rational comprehension of reality depends on knowledge of God’s existence. To that end, the meditation contains two proofs of God’s existence. It also provides discussion of various metaphysical principles, how we can be certain of the truth of our ides, and a significant discussion of the nature of ideas.

The Structure of the Third Meditation

  • Review of Meditation 2 (7:34-5)
  • The ‘truth rule’ & vivid and clear perception (7:35)
  • Challenges to the truth rule (7:35-6)
  • The sources of our ideas & the resemblance thesis (7:36-40)
  • The ‘Natural light’ of Reason (7:38-9)
  • Ideas & Degrees of Reality (7:40)
  • The Causal principle (7:40-2)
  • First proof of God’s existence — from the idea of God (7:42-7)
  • Second proof of God’s existence — from preservation (7:47-51)
  • Innate ideas & the idea of God (7:51-2)
  • God is not a deceiver (7:52)


Descartes’s theory of ideas is central to his epistemology and his metaphysics. In the Third Meditation, Descartes uses his theory of ideas to both explain aspects of our cognition of ourselves and the external world, and to provide proof of the existence of God.

Objective & Formal Reality

Descartes distinguishes three different ‘levels’ or degrees of being. The most fundamental level of being is that of being a [[http:/][substance]]/. Substances and their powers are the fundamental units of explanation in Descartes’s metaphysics. According to Descartes substance is what is “fundamental” or what fundamentally exists. This is built into the very notion of substance for Descartes. He says,

By substance we can understand nothing other than a thing which exists in such a way as to depend on no other thing for its existence. And there is only one substance which can be understood to depend on no other thing whatsoever, namely God. (Principles I.51; AT 8A:24)

God, as infinite substance, exists totally independently of anything else. Finite substances such as ourselves, or any material being, exist independently from other substances, but not of God. In this way Descartes thinks of substance as fundamental, but also as bifurcated between infinite (God) and created finite substances. Descartes distinguishes two kinds of substance, material and mental.

Substance is defined by its principle attribute, which is the fundamental feature of a substance. In the case of material substance, this attribute is extension, in the case of mental substance, thought. Attributes are the second level or degree of being.

At the third and most derivative level of being we have modes, which are ways in which a substance may exist. So, for example, shape (e.g. being square or triangular) is a mode of extension (it is a way in which something may be extended), and extension is the principle or essential attribute of material substance. Ideas are modes of thought, which is the principle attribute of mental substance.

Near the beginning of the Third Meditation, Descartes distinguishes ideas from other modes of thought.

First, however, considerations of order appear to dictate that I now classify my thoughts into definite kinds, and ask which of them can properly be said to be the bearers of truth and falsity. Some of my thoughts are as it were the images of things, and it is only in these cases that the term ‘idea’ is strictly appropriate — for example, when I think of a man, or a chimera, or the sky, or an angel, or God. Other thoughts have various additional forms: thus when I will, or am afraid, or affirm, or deny, there is always a particular thing which I take as the object of my thought, but my thought includes something more than the likeness of that thing. Some thoughts in this category are called volitions or emotions, while others are called judgments. (7:36–7; EMT M.10)

Emotions, judgments, ideas, and acts of will are all modes of the mind. But ideas are distinguished from these other mental modes in virtue of being “as it were the images of things.” As we saw in the Wax Argument from the last meditation, Descartes does not think that all our ideas are sensory images of things. We can have purely intellectual ideas of objects, as Descartes argues we do in the case of the wax. But our sensory and intellectual ideas share a common feature, which makes them both ideas—viz., they present an object to the mind, either by resembling them (as is perhaps the case with sensory ideas), or by some other means. As he says in reply to both Hobbes and Gassendi, an idea is what is immediately present to or perceived by the mind (7:366, 393).

Descartes introduces two ways of considering an idea. The first is the idea merely as a mode of the mind. In this sense the ‘formal reality’ of the idea is that of a mental mode. But an idea may also be considered in virtue of what it makes present to the mind—what it, as it were, is an image of—this is to consider the ‘objective reality’ of the idea (7:40-1)

Reality, according to Descartes, comes in degrees or levels. We’ve seen this already in our discussion of substance, attribute, and mode above. The highest or most “perefect” level of reality is that of an infinite substance. The only such substance is God. So the highest level of reality is ascribed to God. The next highest level of reality is that of finite or created substance as defined by its principle attribute (i.e. mental or material) . Finally, at the lowest level of reality, we have modes or characteristics of substances other than (or derived from) their principle attribute. Each degree of reality is dependent on the prior. So modes depend on the attributes of substances, and finite substance depends on the creative activity of infinite substance. This is true not only of the formal reality of things but also the objective reality of our ideas of things.

Distinctness, Clarity, & the Truth Rule

One of the interpretations we considered in reading the Second Meditation was that Descartes aimed, in the Cogito, to extract a particular method for gaining knowledge. He suggests something along these lines at the beginning of the Third Meditation.

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. (7:35)

As Gary Hatfield argues, there is a fairly straightforward argument being made here ((Hatfield 2003), 144).

  1. I know with certainty that I am a thinking thing.
  2. This knowledge is based solely on a clear and distinct perception of its truth.
  3. Clear and distinct perception would not be sufficient to yield such knowledge if it were in any way fallible.
  4. ∴ Clear and distinct perception provides a sufficient ground for knowledge; whatever I so perceive is true.

The argument is valid. Its conclusion provides us with a general rule or principle for acquiring knowledge. This principle is often called the ‘truth rule’: whenever I am clearly and distinctly perceiving something, that which I perceive is true. The question is whether any sense can be made of the second premise, that the knowledge gained via the Cogito is due entirely to the clarity and vivacity of the relevant ideas. Is there a way to recognize clear and vivid ideas and to distinguish them from all other kinds of idea?

We’ll postpone until the Fifth Meditation the issue of whether there is a form of vicious circularity inherent in Descartes’s argument.

Ideas and Internalism about Content

Descartes’s notion of an idea suggests that our ideas could be just as they are and yet nothing beyond one’s mind could exist. In Descartes’s terms, the objective reality of one’s ideas (setting to the side issues concerning ideas of God) is something we can consider independently of the question as to whether anything exists which corresponds to those ideas. This notion of mental content is sometime called a ‘narrow’ or ‘inernalist’ conception of mental content, in contrast to a ‘wide’ or ‘externalist’ conception of content.

Descartes’s skeptical arguments and their relation to his theory of ideas give great force to the claim that mental content is narrow, and the basic elements of the narrow content view remain influential today. But is mental representation something that may be considered wholly independently from the subject’s environment? Or is mental representation ineluctably tied up with a subject’s environs so that the former cannot be intelligibly discussed independently of the latter?

Proof of God’s Existence

After giving his brief argument for the truth rule, Descartes goes on to talk of the sources of our ideas and the possibility that there is a deceitful God. Why is the issue of God’s existence and nature important? Descartes worries that so long as he may reasonably consider the possibility of a deceitful God, he cannot be sure of the truth of claims that otherwise seem obviously true, such as simple mathematical claims.

what about when I was considering something simple and straightforward in arithmetic or geometry, for example that two and three added together make five, and so on? Did I not see at least these things clearly enough to affirm their truth? Indeed, the only reason for my later judgment that they were open to doubt was that it occurred to me that perhaps some God could have given me a nature such that I was deceived even in matters which seemed most evident. And whenever my preconceived belief in the supreme power of God comes to mind, I cannot but admit that it would be easy for him, if he so desired, to bring it about that I go wrong even in those matters which I think I see utterly clearly with my mind’s eye. (7:36)

Here the ‘preconceived belief’ of an all powerful God threatens to derail the Meditator’s confidence in even the clearest of mathematical ideas. Descartes goes on to exclaim that when he put the issue of God out of his mind and just considers clear and vivid ideas, as in the Cogito reasoning or in mathematics, then he cannot be anything other than certain of their truth.

So in order that he may rid himself of (the admittedly slight) doubt about the certainty of such fundamental truths, Descartes explains that

any reason for doubt which depends simply on this supposition [that God is either deceitful or does not exist] is a very slight and, so to speak, metaphysical one. But in order to remove even this slight reason for doubt, as soon as the opportunity arises I must, examine whether there is a God, and, if there is, whether he can be a deceiver. For if I do not know this, It seems that I can never be quite certain about anything else. (7:36)

Descartes articulates two proofs in this Meditation for the existence of God. The first depends on the nature of the Meditator’s idea of God. The second depends on the the claimed dependence of finite beings on the creative powers of an infinite God.

The Sources of Our Ideas

Among my ideas, some appear to be innate, some to be adventitious, and others to have been invented by me. My understanding of what a thing is, what truth is, and what thought is, seems to derive simply from my own nature. But my hearing a noise, as I do now, or seeing the sun, or feeling the fire, comes from things which are located outside me, or so I have hitherto judged. Lastly, sirens, hippogriffs and the like are my own invention. (7:37–8)

Descartes distinguishes three sources for our ideas. Ideas may be ‘innate’, ‘adventitious’, or ‘invented’. Innate ideas are those which are had by the subject simply in virtue of having a mind. Adventitious ideas are those which seem to come unbidden from sources external to the subject. Invented ideas are fictions, generated by (in some sense) voluntary acts of imagination.

Descartes notes that in the case of adventitious ideas, we are inclined to take them to resemble their causes. But, Descartes also notes, there is no reason to believe that such resemblance holds. He cites three putative reasons for so believing, and seeks to undermine them. First, he notes that we simply have “a spontaneous impulse to believe” that there is a resemblance between our ideas and reality (7:38). But this he dismisses this as a reason because natural impulse is fallible (7:39). Second, he notes that such ideas simply come into the mind, and often quite involuntarily (e.g. opening one’s eyes in a lit room results in a lot of involuntary visual sensory ideas). But this involuntary character is no reason, he thinks, to believe that such ideas resemble anything objective, or that they even arise from anything beyond the subject. For example, dreams are involuntary in their ‘feel’ but they are wholly subjective. He then denies that such ideas must resemble their sources even if they do come from something beyond the subject.

finally, even if these ideas did come from things other than myself, it would not follow that they must resemble those things. Indeed, I think I have often discovered a great disparity in many cases. For example, there are two different ideas of the sun which I find within me. One of them, which is acquired as it were from the senses and which is a prime example of an idea which I reckon to come from an external source, makes the sun appear very small. The other idea is based on astronomical reasoning, that is, it is derived from certain notions which are innate in me (or else it is constructed by me in some other way), and this idea shows the sun to be several times larger than the earth. Obviously both these ideas cannot resemble the sun which exists outside me; and reason persuades me that the idea which seems to have emanated most directly from the sun itself has in fact no resemblance to it at all (7:39).

Hence, though we can distinguish different possible sources of our ideas, at this point (or so Descartes argues) the Meditator has no reason to think that any of her ideas come from anything other than herself, nor that they in any way resemble or otherwise reveal how things are without her mind, in the objective world.

Objective Reality & the Causal Principle

it is manifest by the natural light that there must be at least as much [reality] in the efficient and total cause as in the effect of that cause. For where, I ask, could the effect get its reality from, if not from the cause? And how could the cause give it to the effect unless it possessed it? It follows from this both that something cannot arise from nothing, and also that what is more perfect - that is, contains in itself more reality - cannot arise from what is less perfect. And this is transparently true not only in the case of effects which possess [what the school philosophers call] actual or formal reality, but also in the case of ideas, where one. is considering only [what they call] objective reality (7:40-1).

Something cannot come from nothing. This is a truth which the Meditator suggests is “obvious by the natural light”. This was a principle commonly accepted in Descartes’s time, and still remain a reasonable one. When put in terms of Descartes’s views on the hierarchy of reality, something with a lesser degree of reality cannot be the cause of something with a greater degree; the reality of the (efficient and total) cause must be equal to or exceed the reality of the effect. When applied to the objective reality of an idea, it means that the cause of the objective reality of an idea is one whose reality is at least as great as the reality of that which is ‘depicted’ by the idea. So, for example, when one has an idea as of a horse (a finite substance) then the objective reality of this idea (i.e. the representational content as of a horse) must have been caused by something that has at least the reality of a finite substance. This doesn’t mean that it has to have been caused by an actual horse. But, according to the principle, it cannot be the case that an idea whose objective content is that of a finite substance could be caused by a mere mode.

The First Proof – God and the Objective Reality of our Ideas

Descartes then straightforwardly applies this principle to the objective reality of the idea of God. This raises an immediate problem, for the idea of God is the idea of “a substance that is infinite, independent, supremely intelligent, supremely powerful, and which created both myself and everything else (if anything else there be) that exists” (7:45). According to the causal principle, a finite substance cannot have an idea whose objective reality is greater the reality of its cause. This means that neither the Meditator, nor any other finite substance (or their modes) could be the cause of the Meditator’s idea of God. In fact there is only one possible being who could be the cause of the Meditator’s idea—viz., God. Hence, given that the Meditator has such an idea, God must exist.

More formally, we can put the argument this way:

  1. The objective reality of an idea must have a cause equal to it or greater
  2. The objective reality of my idea of God is that of an omni-omni-omni substance
  3. No finite being has sufficient reality to cause the content (i.e. the objective reality) of my idea of God
  4. ∴ The cause of the objective reality of the idea must be such an omni-omni-omni being—i.e. God

Three Objections

Let’s consider three objections to Descartes’s argument that we both have an idea of God “innately”, and that this idea is accurate, in that it could only exist if God exists.

First, is the idea of God innate – i.e. is the idea of God “in” everyone? Why not think that our idea of God, like many other ideas, is acquired through experience of the world (e.g. through reading a religious text) or through our imagination? That the idea is acquired seems all the more plausible given that one might not have thought that God exists up until one reads the Meditations!

In reply, Descartes seems to think, not that everyone innately “has” the idea of God in that everyone is thinking of God all the time, but rather that any being capable of reasoning will come, eventually, on the idea of a creator who is absolutely good, knowledgeable, and benevolent. So the idea is innate rather than acquired in the sense that it may be conceived by any being in so far as they are rational.

Consider a second objection. Assuming that we do all have an idea of God, why think that idea has the same content for everyone, and in particular that the content is one of an omni-omni-omni being?

Descartes’s reply here relies heavily on a principle we can call the priority of the infinite: According to Descartes, we must have an idea of God as an infinite being because our ideas of finite things presuppose an idea of infinity. He says, “my perception of the infinite, that is God, is in some way prior to my perception of the finite, that is myself” (7:45)

Descartes illustrates this principle by means of the claim that an awareness of a lack or absence (e.g. a hole) presupposes an awareness of perfection or presence (e.g. the doughnut which has the hole). Another relevant example might be one involving awareness of a spatial boundary. Awareness of the boundary presupposes an awareness of the space extending beyond the boundary. This may be iterated, such that awareness of any particular space seems to presuppose an awareness of an infinitely extended space of which this local space is merely a limitation (later, in the 18th century, Immanuel Kant makes much of this argument in his discussion of space). The relevant point here is that Descartes considers us as all having ideas of finite things, and this things that are lacking or deficient in some manner. But according to the principle of the priority of the infinite, we can only have an idea of such a lack, deficiency, or imperfection, if we have an idea of something that is more perfect or non-deficient with which to compare it. Ultimately, Descartes thinks our ideas of finite things must depend then on an idea of an absolute perfect, totally non-deficient being, and that being is God.

Consider a third objection. Does the idea of God really require an infinite cause? Couldn’t the objective reality of the idea of God be acquired through finite causes? For example, why couldn’t the idea of an infinite substance simply be constructed from the idea of a finite substance, whose limits are then abstracted away to generate the idea of God?

Here again, Descartes argues that we cannot generate the idea of God via a construction out of finite materials, since the finite presumes a grasp of the infinite. Descartes thus appeals again to the the priority of the infinite in our grasp of the finite content of any idea.

Nevertheless, one might object that at best, Descartes’s argument yields only the conclusion that an infinite substance must be the cause of our idea of God. But why think that this infinite substance is God rather than something else?

It is not clear that Descartes, at least at this point in the Meditations, has a reply to this objection. It may be that he gives such a reply only in his proof for God’s existence as offered in the Fifth Meditation. However, he does make several references here to God’s “perfection”, which might suggest that the issue isn’t simply one of infinite power, but of infinite perfection, and that any being possessing such perfection must be God.

The Second Proof – God’s preservation of finite beings

The Meditator then asks “whether I myself, who have this idea [of God], could exist if no [more perfect] being existed” (7:48). He then offers a proof by process of elimination. Four causes of his existence are considered, viz., self-causation, his parents, another less perfect being, and finally, God. Descartes rules out the first since any being that could create itself from nothing would be God.

Descartes then rules out the other options by appeal to a “preservation” principle. He says,

a lifespan can be divided into countless parts, each completely independent of the others, so that it does not follow from the fact that I existed a little while ago that I must exist now, unless there is some cause which as it were creates me afresh at this moment – that is, which preserves me. For it is quite clear to anyone who attentively considers the nature of time that the same power and action are needed to preserve anything at each individual moment of its duration as would be required to create that thing anew if it were not yet in existence. Hence the distinction between preservation and creation is only a conceptual one, and this is one of the things that are evident by the natural light (7:49)

Descartes makes several points here. The first claim is that a substance is temporally extended in the sense that its existence can be divided into discrete moments, each of which is independent of the others. The second point is Descartes’s claim about preservation being equivalent to creation. He endorses the following principle.

Preservation Principle:
the same power or action is needed to preserve a substance from moment to moment as is needed to create that substance at a moment

Since the Meditator is at least a finite thinking substance, something is needed to sustain the Meditator’s existence as a finite being. This cannot come from the Meditator’s finite creator in the sense of their parents or any other finite substance, since such beings would lack the power necessary to preserve the Meditator’s existence from moment to moment. So it must come from an infinite substance – God. More formally, the argument looks like this:

  1. Any substance that exists exists at discrete moments that are independent of one another
  2. The existence of a substance at one moment does not give it the power to exist at any other moment
  3. Existence from one moment to the next requires an act equivalent to creation (preservation = creation)
  4. No finite entity can create itself
  5. ∴ No finite entity can preserve itself from one moment to the next
  6. At least one finite substance exists from one moment to the next
  7. ∴ There must be some entity capable of creation & preservation—God

The argument appears sound, but that isn’t to say that it is unobjectionable. Obviously, one of the most controversial claims here is that there is no fundamental difference between preservation and creation. Further controversial assumptions include the claim that time may be divided into discrete and independent moments in the manner that Descartes suggests, and that a substance which exists at a moment needs something other than itself to continue to exist at the next moment.



Hatfield, Gary. 2003. Routledge Philosophy GuideBook to Descartes and the Meditations. London: Routledge.

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