An Account of Motion (15 Nov 2023; Descartes)

In this essay, I will explore Descartes’ account of motion in his Principles of Philosophy. First, I’ll introduce Descartes’ metaphysics, explaining the milieu it finds itself in, and compare it to the Ancient Greek approach. Next, I’ll outline Part One and Part Two of his principles, describing those of knowledge and the material world, as well as explicating various conceptual distinctions. Then, I’ll delve into how Descartes accounts for motion, explaining what role it plays in the world, God’s role in creating motion, and compare his account of motion to the account found in the Timaeus. For these sections, I’ll primarily reference Descartes’ Principles of Philosophy and borrow a few ideas from Plato’s Timaeus. Finally, I’ll describe the cosmological view presented in Principles; in other words, how we should see our place in the universe according to Descartes.

Descartes writes his metaphysics as the world is in the throes of the scientific revolution in the 17th century; the natural world, the cosmos, becomes an object of study for the sciences, losing its sense of cosmological significance. The idea that there is an intrinsic connection between the world, its subjects, and some sort of universal law begins to fade. While the Greeks approach philosophy by looking to the cosmos to discover moral laws, as is exemplified in the Timaeus with the emphasis placed on looking to the external world (particularly the celestial realm) as a model for imitation, Descartes uses reason to set up metaphysical axioms which cannot be doubted. To Descartes, we can discover these principles of knowledge from the non-extended realm, rather than from the extended world as the Greeks do. The senses are subservient and less known than the body according to Descartes and are subject to error, whereas to the Greeks, they are necessary (especially the eyes) for learning about the external world (in order to imitate it). The idea of perfection in the Timaeus is guided by the celestial sphere, but Descartes believes that perfection lies in perfect knowledge deduced from principles. These principles, or axioms, must be beyond doubt as well as foundational for all proceeding knowledge. His metaphysical axioms should be considered as roots in the tree of science, followed by a trunk composed of physics, and branches representing all other empirical sciences. The metaphysical axioms he establishes, Descartes believes, are necessary for all physical sciences.

Descartes’ principles of knowledge are outlined in part one of his Principles of Philosophy.  He discovers these principles through the method of doubt, a more scientific process than that of unconcealment found in Plato’s Timaeus, which aims to reveal things that show themselves as themselves. Descartes tears down everything he once believed to be true, anything that could be subjected to a shred of doubt, to establish clear axioms upon which all other knowledge can be built. The senses can be doubted not only because they are occasionally incorrect, but also because there “seem to be no marks by means of which we can with certainty distinguish being asleep from being awake” (Principles 6). Things seemingly as evident as mathematical demonstrations can be doubted not only because we make mistakes in these seemingly self-evident things, but also because we don’t know if the God who (allegedly) created us would prefer that we be deceived only occasionally rather than all the time (Principles 6). In the course of developing true knowledge, the first and most foundational axiom he establishes is that to doubt, something must be doing that doubting; thus, he posits, “I am thinking, therefore I exist” (Principles 7). This principle paves the way for the axioms of the existence of God, and thus the reconstruction of a world that can be mathematically and empirically measured.

Following his first axiom, he then moves to distinguish between soul and body, or a thinking thing and a corporeal thing. He writes, “If we… examine what we are, we see very clearly that neither extension nor shape nor local motion, nor anything of this kind which is attributable to a body, belongs to our nature, but that thought alone belongs to it. So our knowledge of our thought is prior to, and more certain than, our knowledge of any corporeal thing” (Principles 7). Not only does he create a dichotomy between the non-extended and the extended realm, but he also creates a dualism, positing that thought is not only prior to but also more certain than our knowledge of corporeal things. It is better known because we can find “more attributes in our mind than in anything else,” (Principles 8), which makes more certain knowledge of anything else impossible. In Descartes’ vocabulary, a ‘thought’ is “everything which we are aware of as happening within us, in so far as we have awareness” (Principles 8-9). This includes thinking, imagining, understanding, willing, and merely the awareness that we are sensing the corporeal world (but not a certainty of what is occurring in the corporeal world). The certainty of only an awareness of the sense of an outside world rather than an outside world in and of itself presents a solipsistic issue; beyond this point, Descartes cannot use the certainty of his own existence as a thinking thing alone to prove the existence of physics and the sciences. 

To conquer the idea of certain knowledge of only his mind, Descartes turns to the existence of God for his next axiom: He writes, “The sense in which knowledge of all other things depends on the knowledge of God” (Principles 9). This is because the mind alone cannot say with certainty that even the most evident things are wrong, and thus the prerequisite for certain knowledge is an understanding of the author of such knowledge (God). In principle 14, he posits that God’s existence can be logically inferred from the fact that necessary existence is included in our concept of god; that contained within the idea of a supremely perfect being is proof of that external being’s existence, and we may find additional proof that he exists because the idea of a perfect being can only exist if the perfect being places that idea within us (Principle 11). Following this certainty, he claims not only that “We must believe everything which God has revealed” (Principles 14), but also that we should only inquire into the efficient causes of things and not their final causes (Principles 15-16), a principle which conforms to the milieu of the scientific revolution. Next, he claims that the first attribute of God is that he is truthful; that it would be preposterous to posit that he would deceive us or be the cause of errors in our experience (Principles 16). Thus, “everything that we clearly perceive is true” (Principles 16), dispelling the potential for god to be a deceiver. Following this conclusion, mathematical judgments should no longer be subject to doubt, as they are clear. Additionally, when our senses reveal to us something clear and distinct, as long as we distinguish it from what is confused and obscure we can recognize which are the aspects of those senses which may be regarded as true. Errors in mathematics or in things concerning the senses are purely our own, such that god may not be held accountable (Principles 18); we have free will to make judgments (Principles 6), and we error in our judgments only when we do not sufficiently perceive (Principles 18). He distinguishes the modes of thinking in principle 32: the perception of the intellect and the operation of the will. Sensory perception includes imagination and pure understanding, and “desire, aversion, assertion, denial, and doubt are various modes of willing” (Principles 17). Descartes then claims that we can be certain of our perceptions when they are clearly and distinctly perceived (Principles 23): That which is clear and distinctly perceived is both “present and accessible to the attentive mind,” but also distinct from all other perceptions (Principles 21-22). Next, he moves to define substance: It is “a thing which exists in such a way as to depend on no other thing for its existence” (Principles 24). A substance can be distinguished by one of its attributes, but each has its own main property that defines its essence. Length, breadth, and depth (L/B/D) are the principal property of extension, and thought is the principal property of thinking substance; thus, we arrive at these two distinct substances (Principles 26). Thought constitutes the nature of mind because it can be considered as nothing else, and the same is true for extension and nature of body (Principles 30-31).

The second part of Descartes’ principles seeks those of material things. While he is certain that we must believe that which god reveals to us, as he is not a deceiver, he thinks it necessary to investigate the arguments which prove the existence of the corporeal world, a preconceived opinion which he previously tore down. He posits that all sensations come to us from something distinct from the mind, that is, something extended. From sensory stimulation we have clear and distinct perceptions of the extended world, which is extended in “length, breadth, depth… which give rise to our sensations of colours, smells, pain, and so on” (Principles 40). God is not producing a clear and distinct perception of the extended world in our mind, as that would go against the premise that he is not a deceiver. Therefore, something exists that is extended in L/B/D, in other words, something corporeal (Principles 41). To get a clearer understanding of the corporeal world, of “body,” he explains that its nature “consists not in weight, hardness, colour, or the like, but simply in extension” (Principles 42). By this, Descartes means that all things which are extended simply consists of the three terms he initially posited in 41 (L/B/D), rather than how that body affects the senses. We can remove these sensations which we judge of a body, and the body will still be of the same L/B/D, existing as extension which we can perceive. Next, he distinguishes the concept of space and corporeal substance, which have virtually no difference: However, we conceive of the two in different ways. A corporeal substance, we perceive as having a certain extension and being something particular, like a stone; yet when a stone is moved from a resting place, we consider that the space the stone occupied is still the same as before, but replaced by something else (Principles 46). He distinguishes the terms “place” and “space,” saying that the former explicitly describes the position, and the latter describes the L/B/D (Principles 47-48).

He then goes on to explain the impossibility of atoms. He reasons that atoms, considered the smallest thing, must necessarily be extended, and therefore divisible, smaller than the smallest thing (Principles 51). Thus, not only is the extension of the universe indefinite (Principles 52), but also that there is only one type of matter that composes the earth and the heavens, and all other planets that may exist, as that which is extended is one, occupying all imaginable space (Principles 52). 

Given this sort of oneness associated with extension, the only way in which there may be diversity amongst extended forms is through motion. We can understand this motion as, “the transfer of… one body, from the vicinity of the other bodies which are in immediate contact with it, and which are regarded as being at rest, to the vicinity of other bodies” (Principles 53-54). The body is whatever is being transferred, which may be composed of many parts, each with different motions; it is very much a relative movement. Motion itself is in the moving body, and is not part of the body that brings about the motion, yet motion can be transferred. A body can be at rest while at the same time in motion in this way. The ultimate cause of motion, and therefore different forms is God, who preserves the same quantity of motion in the universe. The motion that he initially imparted to all matter in the universe remains constant, and it is he who maintains this constancy (Principles 61-62). The idea of motion in the universe as the cause of differentiation of forms leads to a mechanistic view of things, in that the universe is one big machine set in motion by the creator God.

Descartes also presents three laws of nature. His first law is that as far as each thing can, it will continue in the same state, and that which is in motion always continues to move. The only way that a thing can change (either in form or in the traditional sense of motion) is if some external cause acts upon it (Principles 62). The second law he presents states that all motion is rectilinear, or moving in a one-dimensional motion along a straight line. Thus, any body moving in a circle tends to move away from the center of that circle (Principles 63). His third law posits that if one body collides with another body stronger than the first, it loses no motion, but if it collides with a weaker body, it loses a quantity of motion equal to that which it imparts on the weaker body (Principles 64).

Descartes’ idea of motion is quite distinct from the Ancient Greek conception of motion. Both Plato and Descartes present ideas of spherical or circular motion (Principles 33) and both motions are the result of some sort of craftsman or God. However, Timaeus presents motion as an agent of divine order, rather than a mechanistic agent of distinctions. With motion, the intellect of the cosmic animal could rule over the body in the celestial realm, giving order to the physical cosmos (Timaeus 40A-40C). Human life, which by default is disordered, can bring itself into order imitating the divine motion of the cosmos. Additionally, Plato believes that motion plays a role in time, as that time itself is a moving likeness of eternity (Timaeus 38A); the circuits of motion of the celestial are an integral part of time itself, acting as markers to guard the numbers of time, enabling the animal of time to be “as similar as possible to the perfect and intelligible Animal in the imitation of its eternally enduring nature” (Timaeus 39D-39E). Descartes presents motion as a mechanistic concept, its primary role being the distinction of different forms in the material world. Time isn’t involved in Descartes’ conception of motion.

Descartes’ metaphysics severs the human connection to the cosmos presented in the Timaeus. The self, the thinking thing, the “I” exists outside of the world. The “I” to Descartes is a thinking thing, non-extended, that is merely connected to the extended world through a variety of sense perceptions. While he does defend the existence of the physical and extended world, the two substances remain distinct. We are thinking things, not corporeal things, as he claimed in his first principle. Additionally, our place in cosmos now lacks a moral component. We don’t need to imitate the celestial to bring order to ourselves; the natural world is now a machine that we sense rather than a living “cosmic animal.”