And now it is possible for me to achieve full and certain knowledge of countless matters, both concerning God himself and other things whose nature is intellectual, and also concerning the whole of that corporeal nature which is the subject-matter of pure mathematics. M5 p49
The final Meditation weaves together several threads that have emerged over the work so far. The primary aim is to establish, at last, the existence of material objects. This leads Descartes to develop the real distinction between material and mental substances, and to clarify the relationship between his own mind and body.
The nature of physical objects was first addressed in the Second Meditation (with the piece of wax) and developed further in the Fifth Meditation. There, without needing to stipulate the existence of material objects, Descartes determined that the true nature of matter is a collection of primary qualities – extension, size, shape, position, number, motion and duration – knowable a priori as the ‘subject-matter of pure mathematics’ (p49, 55). These concepts are innate, and their clarity and distinctness indicate that they can be trusted.
Descartes now investigates another of his faculties: his imagination. He first distinguishes his imagination from his understanding or intellect. The distinction is a plausible one, and he uses a geometric example to illustrate it. When he thinks of a triangle, a pentagon or a chiliagon (a thousand-sided figure) he understands in each case the nature and properties of the shape; that is, he grasps the shapes mathematically. But only in the first two cases can he clearly imagine the shape, or see it with his mind’s eye. Furthermore, this act of picturing or imagining requires an ‘additional effort of mind’ (p51) – all of which suggests that the intellect and the imagination are different faculties.
The imagination does not seem to be essential to who Descartes is: ‘for if I lacked it’, he says, ‘I should undoubtedly remain the same individual as I now am’ (p51). The intellect, however, is essential to res cogitans: it is the faculty of cogitation itself. This leads Descartes to speculate that the imagination is somehow connected to the body:
So the difference between this mode of thinking and pure understanding may simply be this: when the mind understands, it in some way turns towards itself and inspects one of the ideas which are within it; but when it imagines, it turns towards the body and looks at something in the body which conforms to an idea understood by the mind or perceived by the senses. M6 p51
In the Second Meditation Descartes described the four faculties of thought: intellect, will, imagination and sensation. The intellect deals with ‘pure’ thought, a priori truths that are accessible to thought alone. The imagination, however, deals with the images of material things and finds its origin in sense perceptions. Imagination and sensation are thus ‘hybrid’ faculties which process the images of physical things, dealing in a posteriori truths derived from experience. This is what Descartes means when he distinguishes between imagining a triangle as a picture in his mind, and understanding the true nature of triangles intellectually.
This line of thinking leads Descartes to his proof of the existence of material objects. The key insight is that the imagination finds its origin in sense perceptions. The obvious question therefore is, where do our sensations come from? Descartes proceeds to answer this question by excluding all other possibilities, thus establishing the existence of the material world.
In its basic form, the argument for the existence of the material world is as follows:
The force of the argument depends upon Descartes’ reasons for excluding the alternative explanations: if neither Descartes nor God are the origin of his sensations, then a material world must exist.
Descartes has two reasons for discounting himself as the origin of his sensations. Firstly he notes that sensory perceptions are not subject to his will: he cannot choose to hear or not to hear a noise, to see what he wishes, to taste what he pleases. This is, of course, highly suggestive: but it is not conclusive. Dreams are likewise not subject to the will, but they originate inside us. However, in the First Meditation Descartes assumed that dreams, although false, ‘must have been fashioned in the likeness of things that are real’ (p13) – so it may be the case that dreams do ultimately originate from outside the mind. The second reason relies upon a version of the causal adequacy principle employed crucially in the Trademark Argument for the existence of God. Descartes, as he has shown, is essentially res cogitans, a non-extended thinking thing. His sensations, however, contain a degree of ‘objective’ extension: that is, they represent spatial and extended things. Since a cause must possess the same (or more) reality as its effect, and since Descartes is not an extended being, these extended sensations must have another source. This line of reasoning is subject to previous criticisms of the causal principle Descartes invokes, and is hardly persuasive. Nevertheless, Descartes concludes that he is not the origin of his sensations: or, to use the language of Meditation Three, such ideas are not invented and must therefore be adventitious or innate.
One way to satisfy the causal principle would be to posit God as the cause of sensations which represent extension (‘in which case it will contain eminently whatever is to be found in the ideas’ p55). But God, as we have seen, is not a deceiver:
I do not see how God could be understood to be anything but a deceiver if the ideas were transmitted from a source other than corporeal things. M6 p55
This is a characteristic argument, but hardly a convincing one. Descartes has already gone to great lengths to show that we are, in fact, constantly deceived in our perception of the world. We believe immediately and naturally that objects possess in themselves the secondary qualities like colour or warmth that we know are merely effects produced in the observer. Admittedly, these errors have been accounted for in the Fourth Meditation by showing how the intellect, properly deployed, can override such everyday notions. But if God were truly unable to deceive us, we might expect our senses to be more precisely tuned to the real nature of things. It is also worth noting that other philosophers, most notably George Berkeley, had no qualms about positing God as the immediate cause of our ideas, and considered this arrangement more efficient and plausible.
Therefore, Descartes concludes, his ideas of material objects must originate from some extended substance outside himself that is not God. In other words, the material world must exist.
Once he has established the existence of a second substance, res extensa, Descartes is keen to distinguish its nature from that of mind or res cogitans. This distinction – the ‘real distinction’ mentioned in the subtitle – has already been sketched in the Second Meditation, but it is established further through two arguments – from Conceivability and from Divisibility.
... it remains one of the remarkable paradoxes of Cartesian philosophy that in order to construct a reliable and informative account of the physical world we are urged first to turn away from it. Cottingham 149
By establishing the existence of the material world and clarifying its nature, Descartes brings an end to the scepticism that prompted his enquiry in the beginning. The world has been restored as a possible object of knowledge; but with some crucial differences. Descartes’ new world is geometric, quantifiable, scientific; and the type of knowledge available to him is primarily abstract and rational. Such a world – such knowledge – can be perceived clearly and distinctly, and its true nature is guaranteed by God’s goodness.
Wilfrid Sellars famously drew attention to the difference between the ‘manifest’ image of the world (how we experience the world as persons within it) and the ‘scientific’ image of the world. The physicist Arthur Eddington famously described two tables in his Gifford Lectures: one table solid and familiar, the other ‘mostly emptiness... numerous electric charges rushing about with great speed.’ In its broadest sense, this distinction should be familiar to all of us. Science describes an objective world of elementary particles and fundamental forces, of mass-energy equivalence and quantum entanglement. There is no place in this picture for value and meaning and subjectivity, and yet these constantly inform our everyday lives. The manifest image of the world engages our practical reason: it is the map we use to navigate the world day by day. But, in some profound sense, we feel that the scientific image is more true.
A similar distinction informs the Meditations. Descartes draws a clear line between the common-sense account of the world and the scientific or geometric account. The gap between these two perspectives is precisely the reason that sceptical doubt is possible. The distinction is realised in other ways, too: for Descartes, the two accounts derive from different faculties, namely the senses and the intellect respectively. The senses inform us (misleadingly) of the secondary qualities that seemingly exist in the world; the intellect corrects that misapprehension and instead describes the primary qualities which are ‘the subject-matter of pure mathematics’ (p55).
The ideas that we have of things in the world are therefore accurate, in so far as they contain clear and distinct perceptions of primary qualities; but also (usefully) misleading. The everyday manifest image of the world, our ‘natural’ manner of forming beliefs, is not without value: ‘I should not doubt that there is some truth in this’, Descartes concedes:
For the proper purpose of the sensory perceptions given me by nature is simply to inform the mind of what is beneficial or harmful for the composite of which the mind is a part; and to that extent they are sufficiently clear and distinct. But I misuse them by treating them as reliable touchstones for immediate judgements about the essential nature of the bodies located outside us; yet this is an area where they provide only very obscure information. M6 57f
Descartes subscribes to a form of representative realism: there are real, independent objects in the world which cause in us representations or ‘ideas’ that (more or less) resemble those objects. When these representations fail us, we can use our intellect as a corrective. Of course, this combination of observation and reason is the basis of the scientific method, and Descartes has therefore fulfilled his intention from the First Meditation to show exactly how reliable, scientific knowledge is possible. With the foundations of empirical science in place, the project of pure enquiry can be completed.
Descartes has addressed the main elements of his sceptical doubt. He has resolved the Argument from Illusion by accounting for the mistakes of the senses and showing how they can be corrected by the intellect. The Deceiving God argument has been refuted by his proof of a wholly good, non-deceiving God. He ends the Meditations, therefore, by returning to the Argument from Dreaming. With his newly established trust in his capacity to avoid error, he now dismisses the argument as unconvincing:
... when I distinctly see where things come from and where and when they come to me, and when I can connect my perceptions of them with the whole of the rest of my life without a break, then I am quite certain that when I encounter these things I am not asleep but awake. M6 p62
The idea that there is an objective standard which reality meets, but dreams do not, was not available to Descartes at the start of the Meditations. With the divine guarantee underwriting his clear and distinct ideas, however, he is now able to deploy that standard and recognise the difference between waking and dreaming.