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Descartes' Proof of the External World

Paul Sperring


In order to fully overcome the doubts of Meditation I Descartes had to show, in Meditation VI, that he could be certain of the existence of the material world, despite the dreaming and demon arguments. His proof that there is such a world involved the following two moves:

1.  Consider the alternative possible causes of our ideas of material objects and show that only material objects could actually be the cause.

2.  Consider the essential nature of material objects and show that only real existent bodies could possess such attributes.

In this essay, I shall examine both steps.

Part I: Material Objects Cause our Ideas of Material Objects

Possible Causes of My Ideas of Material Objects

Descartes considers four possible sources for my ideas of material objects:

I could be the cause of my ideas of the material world (much in the way that the solipsist sees things).
God could have provided me with the ideas directly (perhaps in the way that Berkeley thought that all material objects are merely ideas in our minds, and the mind of God).
Something other than me, but which possesses enough power to cause in me the ideas of material objects, but is distinct from the material objects and is not me or God (such as a deceiving demon or some such sceptical possibility).
Material objects.

One needs to pause here to introduce some technical notions to explain options (ii) and (iii) as understood by Descartes. In Meditation VI Descartes expresses the relationship between the cause of an idea and the idea itself in terms of the "formal" or "eminent" reality in the cause and the "objective" reality in the idea.[1] This terminology may be unfamiliar, and even confusing, to the modern reader, but the concepts are fairly straightforward. In essence a thing has objective reality, for Descartes, if it is a representation of something. Thus my idea of a horse is objectively real, and to say that the horse itself, as the cause of my idea, has formal reality, is to say that it has the apparent or represented properties that my idea has in actual fact (so the horse actually has legs, whereas the idea only has represented legs).[2]

Something has eminent reality if it contains within it the properties required to bring about the representation, although in some higher form than something that merely has the literal properties. If we consider God, then it seems right to suppose that God could cause my idea of a horse, and therefore God has certain properties which are at the root of my representation of the horse, but, of course, God obviously does not have the four-leggedness which forms part of my idea. So God has the eminent reality to bring about the objective reality in my idea.[3]

Descartes' job is to eliminate the first three possibilities and leave the fourth possibility alone as a source of our ideas of material objects. And if this is the only possible source then, obviously, if there are ideas of material objects (which seems undeniable) then there must be material objects. In short, the job is to show that the formal reality of tables is the true source of the objective reality of the ideas of tables. Let us begin with the self.

Can I Be the Cause of My Ideas of Material Objects?

I cannot create or avoid ideas of material objects at will. When I wish to have a fuller wallet, or the rock strikes my head, I may curse the way the world is fixed. I don't seem to be in charge of things in such cases. Secondly, my apparent sensory experiences are extremely vivid, in a way that my imagination cannot recreate. Both of these claims seem to point to the fact that I am but a passive observer of the material world, and do not actively construct the world of tables, persons, hedgehogs and bank balances.

But, one might ask here, why couldn't there be something in me that is providing the active element in perception? It is because, says Descartes, this so-called "active faculty" "presupposes no intellectual act on my part."[4] The idea here is that certain things exist only insofar as there is a mind already present ("presupposed"), whereas other things can exist independently of my mind. So my beliefs and judgements do "presuppose" an intellectual act on my part. That's to say, if I had no mind or understanding then I would have no beliefs, nor could I make any judgements. Beliefs and judgements are simply "modes of thought" and so require thinking substance in order to be. However, the cause of my sensory experiences is not itself a mode of thought, but whatever produces (i.e., the active part) the mode of thought. The only active part of my mind that could even conceivably be the cause of the sensory experiences is the will, but Descartes has established that these ideas arise independently of my will, so it follows that something other than me must be the cause of these ideas of sensible objects.

Might God Directly Cause My Ideas of Material Objects?

God is no deceiver, claims Descartes, since supremely perfect, and deception is always a consequence of imperfection.[5] It follows, therefore, that he would not have provided me with the ideas of material objects directly without also giving me some faculty for recognising their origin (i.e. some sort of extra sensory capacity to pick up on God's role in the production of these ideas). Further, God appears to have given me an extremely strong propensity to believe that material objects cause my ideas of them - if they didn't then this would be an instance of deception.[6] To illustrate:

Suppose as a small child I trod, barefoot, on an upturned drawing pin. Now, as I hopped about hooting and screeching what was I naturally led to believe? Was it that God had placed in my mind the ideas of things like feet and drawing pins directly, and accompanied this with the idea of pain? Or was it that I had just encountered a real, extended object, which had attached itself to my real, extended foot, and it was because of this coupling that I felt pain?

It seems impossible that the former could be the case unless we take God to be a deceiver.

This point about our natural propensities (or inclinations) is important. Whatever is natural is God-given - by "nature" or the "natural" Descartes means God himself, the laws of nature and also our own, human, nature. Now, with respect to my own nature, if I have a set of faculties which constantly lead me into error no matter how carefully I use them then it seems that this would ultimately redound to the discredit of the creator. If I think that there are material objects, with certain properties, existing, in certain respects, independently of me and just as they appear to, and I am wrong, then this looks obviously bad for the creator of me. To make use of one of Descartes' analogies (found elsewhere in Meditation VI [7]), if a clock is constructed in such a way so that it never tells the right time then we rightly blame the clockmaker for having produced such a poor clock.[8] If we suppose ourselves to be truthfully identifying mind-independent, extended objects when we have ordinary sensory experiences then we jolly well better be right at least some of the time if God is to be considered a good "clockmaker".

However, God needn't be impugned here if it were the case that we had some sort of faculty for recognising that sensory experiences are not of the mind-independent, extended items we take them to be of. Perhaps if they appeared with some sort of glow, reflecting their divine origin one might say, and triggering in us some epistemic revelation, then we wouldn't ever suppose them to be anything other than placed directly in the mind by God. Since this is not how things go, ordinarily, then it scarcely ever occurs to us think anything other than "here is an item, existing in a mind independent realm of extended items." [9] [10]

Might a Deceiving Demon Be the Cause of My Ideas of Material Objects?

For the same reasons, mutatis mutandis, we can reject some demon hypothesis since I have been given no capacity to recognise such deception. So the blame would lie with God in setting the world up in such a way that my most careful considerations and natural inclinations led me up the garden path, so to speak.

So far then, we have been given reasons to reject options (i)-(iii) as sources for our ideas of material objects. Since (i)-(iv) are the only possible sources for our ideas, then if option (iv), that material objects cause our ideas of material objects, is the only option left to us, then it must be the case that material objects exist.

Knowing that there are material objects, however, doesn't tell us what they are like (as Descartes points out earlier in the Meditations, questions of existence and essence are always separable regarding things other than God). Can we say, with any confidence, that material objects are in fact as they appear to be?

What Descartes attempts to show is that material objects do indeed have the features that we take them to have - at least in certain respects.

Part II: Material Objects Have (Some Of) the Properties They Appear to Have

Having established that material objects exist Descartes points out that "they may not all exist in a way that exactly corresponds with my sensory grasp of them."[11] It is important to establish, however, in which respects these objects resemble our ideas of them, otherwise we might continue to think false thoughts about what the external world is really like. So if an idea of some material object appears to us to be of something with properties X, Y and Z, then we want to know which of these apparent properties are actually found in the object itself. To illustrate, supposing my idea of the horse to be of something that is in motion, chestnut brown, and extended, we can ask whether any of these three properties belong to the horse itself.

So, in general I have certain ideas of material objects as possessing extension, motion, location in space, and so on. I also have ideas of material objects as being coloured, textured, odorous, tasty, noisy, and so on.

Now, as Descartes showed with his arguments for dualism, no idea itself could be extended. A thought cannot have a weight, nor be in motion, nor be put in a cooking pot with some onions. So, if our ideas of material things are about things which are actually movable, locatable, cookable, and so on, then there must in fact be objects such that they can be in motion, pinpointed in space and cooked. If there were not objects with these properties, and the properties simply belonged to ideas, then there wouldn't in fact be anything that the concept "extended" picked out (since, as we've seen, no idea could itself fill space), and therefore would be an empty notion. Therefore, if these notions of extension, motion, change in position, are meaningful at all, it must be in virtue of there being things which are extended, moving, now here now there. As Descartes says:

[I] recognize that there are [...] faculties (like those of changing position, of taking on various shapes, and so on) which [...] cannot be understood apart from some substance for them to inhere in, and hence cannot exist without it. But it is clear that these other faculties, if they exist, must be in a corporeal or extended substance and not an intellectual one; for the clear and distinct conception of them includes extension, but does not include any intellectual act whatsoever.[12]

So once we know that there are material objects (and the previous section set out Descartes' proof for that) then it must follow that such "faculties" must really belong to material objects, since they simply could not belong to intellectual items in any real sense. Our representations of material items as being extended really do have as their source extended material items (or to revert to Descartes' terminology: the objective reality of the idea arises as a result of the formal reality of corporeal nature). These physical properties are the so called mathematical properties (which Locke termed "primary properties") which really exist in the objects, and are the proper object of scientific study. But what of the other properties which our ideas of material objects possess, such as the chestnut brownness of the horse? Does this sort of thing have to belong to the horse, that's to say, considered as something independent of me? Descartes thinks not. None of the "sensible properties" need be thought to exist independently of my mind (they all "presuppose" some intellectual substance, and can be understood as mere modes of thought, unlike the mathematical properties). So, although the objects that I perceive to be outside of my mind, do not have all of the properties that my ideas attribute to them, "at least they possess all the properties which I clearly and distinctly understand, that is, all those which, viewed in general terms, are comprised with the subject-matter of pure mathematics."[13]

Therefore, the cautious meditator need no longer fear that she will fall into error with respect to the external world. Firstly, she now knows that there is one, and secondly she knows which properties it is appropriate to attribute to it, considered in itself, and which to herself.

Paul Sperring
Brighton College


[1]  AT VII, 79; CSM II, 55 (all references to Descartes' text come from The Philosophical Writings of Descartes, translated by Cottingham, Stoothoff and Murdoch (CSM Vol. no, Page no.), which is translated from the Oeuvres de Descartes, edited by Adam and Tannery (AT Vol. no, Page no.)).  [back] 

[2]  We can, of course, distinguish between the idea, considered as the carrier of the representational content, and also the idea considered as the content itself - much in the same way that when we consider a painting we can think of it as canvas and paint, and also as a a painting of something (say, a horse).  [back] 

[3]  Readers of the Meditations will be familiar with these notions from Descartes' causal (or "Trademark") argument for God's existence found in Meditation III; there Descartes attempted to show that only God could have the formal or eminent reality required to cause the objective reality of our idea of God.  [back] 

[4]  AT VII, 79; CSM II, 55  [back] 

[5]  "[God] cannot be a deceiver, since it is manifest from the natural light that all fraud and deception depend on some defect." (AT VII, 52; CSM II, 35) Again, this comes from Meditation III. This premise bears an awful lot of weight in Descartes' argument for the proof of the external world.  [back] 

[6]  AT VII, 80; CSM II, 55  [back] 

[7]  AT VII, 84-85; CSM II, 58-59.  [back] 

[8]  Supposing it to be in motion and always either too fast or slow, and not supposing it to have stopped where it might be thought to be correct twice a day.  [back] 

[9]  Not, of course, that we usually have any such thoughts. We probably don't ever think "here is a table", unless we are in a philosophy class, we just take it that there is a table in front of us when we have a sensory experience of it.  [back] 

[10]  And so the Berkeleyan idealist is going to have to give an account of why God fixes things in the way that he does that makes it seem so evident to us that objects exist "out there" filling space, that avoids charging God with radical deceit.  [back] 

[11]  AT VII, 80; CSM II, 55  [back] 

[12]  AT VII, 79-80; CSM II, 54-55  [back] 

[13]  AT VII, 80; CSM II, 55  [back]