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Nemo extensio in longum, latum et profundum, substantiae corporea naturam constituit (extension in length, breadth, and thickness constitutes the nature of corporeal substance). (Principles of Philosophy, Book I).This is Descartes' definition of matter. He goes on to elucidate it in this way: 'Everything else that can be ascribed to body presupposes extension.' The other qualities of matter, then, are less important and he tells us 'though substance is indeed known by some attribute, yet for each substance there is pre-eminently one property which constitutes its nature and essence and to which all the rest are referred'. The pre-eminent property of corporeal substance which constitutes its essence and nature is extension. The essence of matter then is extension, according to Descartes. As he puts it in his physical treatise 'Le Monde' , referring to matter he tells us 'I conceive its extension, or the property it has of occupying space, not at all as an accident, but as its true form and essence.' Extension is the essence of matter then. Whatever has spatial extension is matter and matter, by definition, is that which has spatial extension. Spatial extension, then, constitutes matter. What is spatial extension? It is length, breadth and thickness, the three straight lines that constitute Euclidean space. So that which has length, breadth and thickness is matter and all matter, on this definition, has length, breadth and thickness.
The matter of common sense has, however, many more properties than that merely of extension in space. It is coloured, soft or hard, brittle or elastic; it may be edible or inedible; it may emit a sound when struck or it may not; it is combustible or not; it has a variety of textures; it is bitter or sweet to taste, and so on. Why should spatial extension be selected to be the definitive quality of matter? What is the rationale behind their virtual identity of matter and extension? Why not some other quality of matter? Cartesian matter seems to lack variety. It does not tell us how to distinguish chalk from cheese. It does not seem enough to tell us that both chalk and cheese have length, breadth and thickness. We need to know a lot more than that. There is more to things than their geometrical aspect. It is not clear what Descartes hopes to achieve by this definition.
Descartes expounds his doctrine thus:
I observed that nothing at all belonged to the nature of essence of body except that it was a thing with length and breadth and depth, admitting of various shapes and various motions. I found also that its shapes and motions were only modes, which no power could make to exist apart from it; and on the other hand that colours, odours, savours and the rest of such things, were merely sensations existing in my thought and differing no less from bodies than pain differs from the shape and motion of the instrument which inflicts it.He then goes on to explain that he considered other properties of bodies to be modes of motion and so on. Gravity, for example, was for him reducible to motion and he tried to explain the various phenomena in terms of vortices of particles of varying sizes and velocities. He accepted the logical consequences of his identification of spatial extension and matter and denied the possibility of a vacuum in nature notwithstanding the work of his contemporaries Pascal and Torricelli. But I believe the idea of nature as a plenum has come back into fashion. (Newtonian space was distinct from matter and was envisaged as an empty container but the Theory of Relativity has united space and matter in Cartesian fashion.).
To return to the question of why Descartes identified matter and extension: The passage just cited gives us one answer. He did so because he believed the other qualities of matter to be either 'in my thought' or to be mere modes of extension. On this account shapes and motions are modes of extension and colour, odour, savour in my thought'. But this still does not tell us how Descartes reached this conclusion. He did so by subjecting matter to a process of Cartesian or systematic doubt. Seen in this way, colour, odour, taste and so on prove to be too volatile to constitute the essence of matter but extension, he believed, remained. In some passages (for example the famous wax passage in The Meditations) he uses the apparently argument that he cannot think of matter without also thinking of extension but this for him carried conviction because of his presupposition that 'clear and distinct ideas' - such as his idea of the identity of extension and matter - were self-validating and immune to doubt.
Again, Descartes distrusted sense-knowledge and consequently, apparent qualities of matter such as taste, colour and smell were dubious for him on this basis alone. He felt more sure of the apparently abstract intellectual quality of extension. He could form a 'clear idea' of it.
It is interesting to note that Descartes' great contemporary, Galileo, had a very similar approach to matter. Like Descartes and Bacon he was strongly opposed to the scholastic or Aristotelian tradition. Like Descartes, he distinguished between the real and the apparent qualities of matter - a distinction which goes back to the pre-Socratic atomists. A century later Newton was also to make the same point in his optics. For Galileo the real or objective qualities of matter are extension in space, figure, number and motion wherever colour, taste, smell, bitter or sweet 'are no more than mere names so far as the object in which we place them is concerned'. (Letter to Virginia Cesarini). Galileo could not begin his investigations into the mechanics of matter without some kind of definition of matter, an operational definition of matter. He needed to decide which properties of matter were essential and which could be neglected.
Descartes' distinction between the allegedly objective primary qualities of matter and its secondary largely subjective qualities is not new then and he said nothing about matter that was not said by Galileo. But it was Descartes not Galileo who invented matter because he put it in a total philosophy. He also tells us what matter is not - it is not 'res cogitans' or thinking stuff. He cuts the cake in two and gives us 'res cogitans' on the one hand and 'res extensa' on the other. As Capra puts it in The Tao of Physics
The birth of modern science was preceded and accompanied by a development of philosophical thought which led to an extreme formulation of the spirit/matter dualism. This formulation appeared in the seventeenth century in the philosophy of Rene Descartes who based his view of nature on a fundamental division into two separate and independent realms; that of mind (res cogitans), and that of matter (res extensa).There are assumptions here of course. Perhaps it is just not possible to talk about 'matter' without also talking about 'mind'. It is after all some mind that is doing the talking. Perhaps it is also not possible to talk about 'mind' without giving it a local habitation and a name - that is by mentioning matter. It was Descartes after all who first infected us with doubt so that he allowed, for example, only a doubtful inferential knowledge the existence of our own bodies. In the philosophy of Descartes the very existence of matter is called into question. According to Descartes, the existence of matter including our own bodies is not self-evident. He sees nothing incoherent in doubting the existence of one's own body. (But one could not doubt that 'mind' or thinking thing existed.) To talk, then, of matter in isolation from mind is contrary to the whole spirit of his philosophy which is rooted in epistemology. But putting aside our Cartesian doubts for the moment, it is interesting to see how Descartes' treatment of matter was received in the nervous climate of the Counter-Reformation. Descartes' Meditations appeared in 1641. In 1663, thirteen years after his death it was put on the Index of prohibited books by Rome. True the ecclesiastical authorities did mollify their condemnation somewhat by appending the 'donec corrigantur' (Until Corrected), but as the dissident theologian Hans Kung tells us 'this was asking a lot from a dead man', and Copleston tells us that Descartes was still on the index at the time of writing his history of philosophy some three hundred years later. Now Descartes had tried to get his last major work, The Principles of Philosophy, adopted as a textbook in Colleges and seminaries. Why then was Descartes, a man who always kept a nervous eye on the clergy, put on the Index? Copleston, the Jesuit, informs us, that it was because Descartes' doctrine of matter and substance was incompatible with the dogma of transubstantiation which was proclaimed at the Council of Trent some time before Descartes' birth. The Church, in the period from the Council of Trent until the recent Vatican II, some four hundred years, had relied heavily on the Aristotelian/scholastic doctrine of substance and accidents or matter and form in its formulation of the dogma of transubstantiation. Descartes who was educated at school at the Jesuit College of La Fleche was brought up on the Aristotelian/Thomistic doctrine of matter. He must have been aware when he rejected this scholastic doctrine of matter and substance that he was living dangerously. If he had any illusions the Jansenist theologian, Arnauld, was soon to disillusion him. Arnauld pointed out to Descartes that his definition of matter was theologically unacceptable. I quote Arnauld:
But the chief ground of offence to theologians that I anticipate is that, according to M. Descartes' doctrines, the teachings of the Church relative to the sacred mysteries of the Eucharist cannot remain unaffected and intact.It is worth considering Arnauld's letter in some detail. We have seen that Descartes identified material substance and extension, but Arnauld refers to extension as one of the accidents. This was the traditional scholastic view. Again Arnauld tells us, 'M. Descartes recognizes no sense qualities'. These were what we would call the secondary qualities and as we have seen Descartes describes them as being in my thought'. Arnauld goes on to say that Descartes, 'acknowledges only a formal distinction between these affections and substance'. I think Arnauld here actually understates the charge against Descartes. His definition gives only a token recognition of substance. It is only a manner of speaking. 'Substance' disappears and with it the whole substance/accidents distinction. Of course Descartes replied to this, but he seems to me to concede the case when he writes
For it is an article of our faith that the substance of the bread passes out of the bread of the Eucharist, and that only its accidents remain. Now these are extension, figure, colour, odour, savour and the other sensible qualities.
But M. Descartes recognizes no sense-qualities, but only certain motions of the minute bodies that surround us, by means of which we perceive the different impressions to which we afterwards give the names of colour, savour, and odour. Hence, there remain figure, extension and mobility. But M. Descartes denies that those powers can be comprehended apart from the substance in which they inhere and that hence they cannot exist apart from it; and this is repeated in the reply to his critic.
Likewise he acknowledges only a formal distinction between these affections and substance, but a formal difference seems not to allow things so distinguished to be sundered from each other even by the Divine power.
I avow that I venture to hope that some day will come when the doctrine that postulates the existence of real accidents will be banished by theologians as being foreign to rational thought, incomprehensible, and causing uncertainty in the faith; and mine will be accepted in its place as being certain and indubitable.Descartes is here opposing the idea that there is even a conceptual possibility that substance and accidents could be separated and that consequently that 'real accidents' or independent accidents could exist. As we have seen Descartes defines substances in terms of their dominant accident or quality so that minds become 'thinking things' and matter extended things'.
Arnauld is here presenting the scholastic and also theological view of material substance. From the scholastic point of view Descartes' approach to matter is open to other objections. A scholastic would object that it is not enough to say that matter extends or is extension. Verbs presuppose a subject and consequently there must be some thing or some stuff which does the extending. They could further argue that this is the etymological meaning of 'material'. They would then define 'matter' as the indeterminate substratum which was mere potentiality or indeterminacy until united to a form in an actual thing. Matter in the hylomorphic theory of the scholastic was also regarded as opaque to the understanding or beneath the intelligence. It was also the principle of change and of individuality (in St. Thomas Aquinas).
Forms, on the contrary, were the principles of actuality, intelligibility and constancy. The essential properties were determined by the form whereas the accidental ones were determined by the matter. The forms, or essential forms, were the principle of intelligibility and to understand something was to be informed by it. These forms, as intentional forms, took possession of the mind and informed it.
Between 'forms' and 'mind', then, there is a certain conaturality so that in knowing, the informed mind is one with what is known. To quote the well-known neo-scholastic, Jacques Maritain: 'The knower, while all the time keeping its nature intact, becomes the known itself and is identified with it.' (Degrees of Knowledge, 136). It was in this way that the scholastics hoped to avoid the epistemological problem. They would, I think, have posed this problem for Descartes: what possible relationship of any sort can there by between your 'res cogitans' and your 'res extensa'? These are by definition totally disparate.
I quoted Capra's Tao of Physics earlier. Here is how the quotation continues:
The 'Cartesian' division allowed scientists to treat matter as dead and completely separate from themselves, and to see the material world as a multitude of different objects assembled into a huge machine. Such a mechanistic world view was held by Isaac Newton who constructed his mechanics on its basis and made it the foundation of classical physics. From the second half of the seventeenth to the end of the nineteenth century, the mechanistic Newtonian model of the universe dominated all scientific thought.Cartesian matter then is 'dead and completely separate from ourselves'. How then can we know it? The British empiricists, followers of Descartes in many things, defined matter in terms of sense-data' - that is quasi-mental elements and so side-stepped the whole issue. In fact the problem for them was to avoid idealism. But Descartes here, in his treatment of matter, seems to be assuming that matter is generically different from mind and yet can be confidently defined in complete isolation from mind. This is an assumption totally contrary to the spirit of systematic doubt, of epistemology. The scholastics built bridges between body and mind, and mind and matter, and so the dualism or polarity of Cartesianism. But Descartes derides the Hylomorphic matter/form doctrine. About substantial form he has this to say:
No one can doubt this who knows that these philosophical entities, which are unknown outside the schools, never crossed the minds of the prophets and apostles who composed the sacred scriptures at the dictation of the Holy Ghost. To prevent any ambiguity of expression, it must be observed that when we deny substantial forms, we mean by the expression a certain substance joined to matter, making up with it a merely corporeal whole, and which, no less than matter and even more than matter - since it is called an actuality and matter only a potentiality - is a true substance, or self-subsistent thing. Such a substance, or substantial form, present in purely corporeal things but distinct from matter, is nowhere, we think, mentioned in Holy Writ....And again in another letter:
They were introduced by philosophers solely to account for the proper actions of natural things, of which they were supposed to be the principles and bases, as was said in an earlier thesis. But no natural action can be explained by these substantial forms, since their defenders admit that they are occult and that they do not understand them themselves. If they say that some action proceeds from a substantial form it is as if they said that it proceeds from something they do not understand; which explains nothing.'They explain nothing' - this is the essence of Descartes case against substantial forms. They are non-observables. All the quiddities, essences, substantial forms, entelechies, perfective principles and the rest of scholastic matter were banished by Descartes, the great simplifier. Hundreds of years of intense debate were wiped out by Descartes. Scholasticism had proved to be sterile and a fresh start was required. His definition ignores substantial forms: 'extension in length, breadth, and thickness constitutes the nature of corporeal substance'. The definition is phenomenal:- matter has been denuded of its 'within' as Teilhard de Chardin would call it. It is a geometer' s definition of matter, without colour, taste or warmth and it is pragmatic and operational and free from any distracting metaphysical undertones. It is simple. It is intelligible. Matter is described in Euclidean terms. It includes both animate and inanimate nature as Descartes was an anti-vitalist who believed nature could be understood in mechanistic terms.
Descartes' definition is also universal and unlike Aristotle he recognised no distinction between the matter of the 'sub-lunary sphere' and the 'incorruptible matter of the eternal stars'. For Descartes, both terrestrial and extra-terrestrial matter was mere 'res extensa' to be approached in the same way. The definition encourages confidence by its simplicity. Its apparent banality is beneficial because it encourages us to get on with actual empirical investigation of matter.
It will be seen also that we have here a purely geometrical, and a Euclidean geometrical, concept of matter. Length, breadth and thickness, properties ascertainable with a ruler or measuring rod, constitute the nature of matter. Matter is what we measure. What about the non-measurable, metaphysical properties of matter? On this definition there are none. There is no essence behind the appearances, no substance underlying the accidents, no form. Matter has been denuded of every property except that of measurability. We have here a classic example of positivistic reductionism, of the application of Ockham's razor. Matter is what we do with matter when we measure it. Matter has no mysteries, no ontological depth, matter is what is measurable. This is a purely operational definition of matter. When we perform the operation of measurement, we have matter. This operational definition of matter is exactly what the new physics needed. The non-mathematical properties of matter have been ruled out of order by Descartes, the creator of co-ordinate geometry.
Now its importance for physics, I maintain, was to give it the right philosophical environment in which it could flourish. His definition of matter contains the tacit assumption that from hence forward metaphysicians were to stay away from matter as only those properties of matter were to be accepted as real which were amenable to mathematical treatment. The success of the programme of physics was underwritten by a definition of matter which rigorously excluded all problems that could be an embarrassment for it. Cartesian matter has been vigorously purified of all distracting metaphysical overtones and rendered entirely suitable for physics. Matter is measurable extension in space and all questions of form, essence, substance can be disregarded. This antiseptic definition of matter was exactly what the new and infection prone science of physics needed. There was no fear of metaphysical contamination. For this definition alone Descartes deserves his place as one of the patron saints of the scientific revolution.
A special debt is acknowledged in relation to the the following works:
Capra, Fritjof. The Tao of Physics. Fontana/Collins, 1975.
Copleston, Frederick. A History of Philosophy. Vol. 4, Chapter 5. New York, 1963. (On Descartes and the dogma of transubstantiation).
Descartes, Rene. Philosophical Letters (translated by Anthony Kenny). Oxford, 1970.
Philosophical Works of Descartes. (translated by Elizabeth Haldane). Cambridge, 1931.
Donner, Martin (ed.). The Intellectual Tradition of the West, Vol. 2, Illinois, 1968. (Cf. Chapter on Galileo.)
Heidegger, Martin. Being and Time. Oxford, 1973. (Cf. 123 on the definition of the world as res extensa).
Kung, Hans. Does God Exist? Collins, 1980.