Why does Descartes deny the existence of a vacuum? How does this conclusion follow from his identification of body as res extensa?
In order for me to explain why Descartes denied the existence of a vacuum, I will first briefly explain the arguments that led to this conclusion. Specifically, his definitions of clear and distinct perceptions, and his idea of body as res extensa, or ‘extended thing’. I will then compare Descartes’ ideas to Galileo Galilei’s, who believed the vacuum to exist; in contrast to Descartes, who did not. On the surface, these two positions seem to contradict one another. However, I will show this not to be the case. On the contrary, they are two interesting accounts which help us obtain a clearer picture of what the vacuum is. I will then finish with a brief look at a physics article which successfully merges the two positions that were once thought to juxtapose one another. Descartes’ approach to the vacuum is different, but the conflict he perceived is superficial. This is due to a difference in metaphysical decisions which led to a variation in how they describe their physics. Overlooking this, Descartes sees the concept of the vacuum as being potentially destructive to his own philosophical ideas, which possibly motivated his denial of its existence.
In Part One of Principles of Human Knowledge, several important claims are outlined that are the foundation of his rejection of the vacuum. Firstly, in the search for truth we must begin by doubting anything that is not clearly and distinctly perceived. A clear perception is when the attentive mind sees things as present and accessible; and a distinct perception is one that can be isolated from all other perceptions, containing only clarity . Descartes believed God is the source of our faculties of perception and knowledge, and that he does deceive. Therefore, if we only incline ourselves towards clear and distinct perceptions we can not be led astray . Following this, the idea of corporeal substance as extension is introduced. Out of all qualities that body possesses, extension is the only one that can be considered as the most clearly and distinctly perceived and its one principle attribute . All other qualities presuppose extension and should therefore be considered a mode (or modification of an extended thing). On this, he says: “shape is unintelligible except in an extended thing; and motion is unintelligible except as motion in extended space” . Anything other than its dimensionality is relative in nature, putting it into doubt. From here Descartes moves on to Part Two; his Principles of Material Things.
Expanding on the definition of body, examples of specific qualities are given such as weight, hardness, and colour; all of which Descartes says can be removed from the concept of corporeal substance . The concept can only be thought in terms of extension . Offering possible objections, he adds that when we are exposed to empty space, we incline to say that nothing is there. We are not likely to call empty space a body, even if we do perceive its extension . His response is that “empty” space and corporeal substance should not be seen as distinct things, but as equivalent to one another. The differences between them arise only from our customary ways of conceiving them: “For in reality, the extension in length, breadth and depth which constitutes a space is exactly the same as that which constitutes body” . Empty space is not something that has no attributes, as it is implied by words like ‘empty’ and ‘nothing’; it has dimension. He concludes that extension is something general; not limited only to objects but also to space. Having defined body as an ‘extended thing’ (res extensa) with reference to clear and distinct ideas, he makes his denial of the vacuum explicitly, describing it as an illogical concept:
The impossibility of a vacuum [..] is clear from the fact there is no difference between the extension of a space, or internal place, and the extension of a body. For a body’s being extended [..] warrants the conclusion that it is a substance, since it is a complete contradiction that a particular extension should belong to nothing; and the same conclusion must be drawn with respect to a space that is supposed to be a vacuum, namely that since there is extension in it, there must necessarily be substance in it as well.Descartes 
According to this, there is no room for the concept of absolute nothingness. Echoing the philosopher Parmenides’ claim that what-is-not necessarily can not be ; ‘nothing’ has no attributes, including extension. This raises a question about how to understand words that describe the void. Descartes attempts to explore this problem further.
With careful consideration, it can be shown that words like ‘empty’ do not necessarily represent the absence of bodies. For example, we can describe a bottle as empty when it is filled with air; or we can describe the fishpond as empty because there are no fish, though it is filled with water . Therefore words like empty, or nothing, do not necessarily represent absolute nothingness. Our wording can sometimes be deceiving. Descartes sees this as analogous to the vacuum, concluding that it is just as strange to imagine mountains without valleys as it is to think of nothingness as having extension. You can not say there is nothing between objects if there is distance between them. If there was nothing there, the objects would be touching . For these reasons, Descartes considers ‘empty’ space not as nothing at all, but as body. Equivalent to corporeal substance in that it has to be an extended thing, or res extensa.
Descartes’ denial of the vacuum was made problematic by Galileo Galilei, who believed they existed. However, their positions did not necessarily contradict, they just relied on different reference points. They both arrive at the concept of the vacuum from two different directions, giving each of them a unique perspective of the same thing. In the Dialogues Concerning Two New Sciences, Galileo describes an experiment involving two highly polished flat surfaces (like glass or metal). They become stuck together due to a force created by a ‘vacuum’ trapped between the two surfaces . On this he says:
The fact that the lower plate follows the upper one allows us to infer, not only that motion in a vacuum is not instantaneous, but also that, between the two plates, a vacuum really exists, at least for a very short time, sufficient to allow the surrounding medium to rush in and fill the vacuum; for if there were no vacuum there would be no need of any motion in the medium.Galileo 
It is clear from this that he believed the vacuum existed. The question raised herein is, do they both have the same concept in mind? The differences between Descartes and Galileo appear to arise from the different metaphysical decisions they make, leading them to different descriptions of their physics.
G.Nonnoi in his essay Against Emptiness, gives two important contributions to this observation. Firstly, that Descartes gave an unbounded value to the horror vacui , and failed to notice the process of transformation that the concept had undertaken with regards to its epistemology over time. Descartes had a historical view of the vacuum and assumes that Galileo’s use of word is equivalent to his own. Nonnoi explains this is not the case, as Galileo used the concept as a force rather than as a principle, greatly reducing its significance. Especially compared to its use by the scholiasts who had given the vacuum an overinflated status . Given its use as a force, Galileo did not necessarily consider it to be absolutely nothing and having no properties. Rather, as something with the absence of particles. Descartes does not appear to have taken this into consideration and did not see the normative value that Galileo had offered with his findings.
Secondly, The entire metaphysical foundation of Descartes’ mechanical system rests upon the contiguity of matter, and so the introduction of something as the absence of matter meant trouble for his principles . It would damage his description of the cosmos as a plenum, which his theories of motion also relied upon. In any case, it must have motivated his attempts to undermine his perceived opposition. Galileo on the other hand, simply chose a name for a force acting in space without particles. The fact that this space has extension is self evident, and not something that he raises as a problem. He is observing the differences between two opposites, places filled with matter and places without matter; recording his observations of the forces at play when they are exposed to each other. He is therefore, not concerned with what they have in common (extension) as this contributes nothing to his empirical advances. Descartes on the other hand, takes a completely different approach and instead observes the similarities. Both Descartes and Galileo posit themselves in different modes of thought to explain a similar concept. I will show how this leads to the contrast which Descartes later picks up on in his correspondence with the philosopher Mersenne .
When trying to understand something, it is possible to start from one of two positions; I will explore them both and then follow with a practical example to better illustrate where the problems stem from. A person could take up position A, which attempts to establish the differences between X and Y by putting them both in contrast with each other; pointing out what is unique between them. In doing so, position A has committed to seeing them as different, and the results will show them as separate. Position B on the other hand, focuses on the similarities between X and Y, using this to abstract concept Z. This new concept would be founded on their shared properties rather than their differences . The results from both positions will not be the same due to the two very different approaches, and it will appear to contradict if the differences in position are not considered.
Descartes himself unknowingly stumbles into this problem, made clear by his extended criticisms of Galileo’s writing; seeing it as a threat to his arguments and being potentially destructive. His reasoning for this is stated again as: ‘It is as impossible to think of space as empty as it is to think of mountains without valleys’ . The problem is what elements should be included when using the word ‘empty’, and in part 2.17 of his Principles he gives several examples of its use to highlight this problem. When something is described as empty, certain aspects of the thing are intuitively excluded; the focus is on something particular. If the fishpond is being described as empty for example, the water in the pond is taken for granted. Not because the existence of the water is being denied, but because the focus is on the lack of fish. Similar to when advocates refer to the vacuum as a void; they are excluding the element of extension as something beyond their intended focus. It is included as an integral part of their observations. The absurdity of Descartes’ worries can be made evident by comparing it with the following examples:
- Person A – Looks at hot (X) and cold (Y) and empirically establishes them as two separate phenomena by observing their differences.
- Person B – Looks at hot (X) and cold (Y) and empirically establishes a concept of temperature or Kelvin (Z), based on observations of what they have in common.
Neither is wrong in their conclusion insofar as they recognise they are both using different methods to attain different results. Descartes has taken on position B, but rather than seeing the difference in approach, he sees a threat to his concept Z because he claims Y (the vacuum) does not exist. However, he later accounts for it with a different name; matiere subtile .
Nonnoi explains subtle material as something that “enters and occupies all the space which, for whatever reason, has become free of bodies. It is however, to all intents a mechanical agent; a body” . As mentioned before, the contiguity of matter was essential for Descartes’ mechanical world view and the foundation of his theory of motion. The introduction of subtle matter as something that permeated all empty space was a way to keep the vacuum classified as body. He had invested a lot of effort into establishing his theories from a foundation of clear and distinct perceptions, being sure only to take steps forward in his thought if the reasoning was sound. Descartes had to describe the vacuum as body and not as being absolutely empty. He had firmly determined in his metaphysics that body is defined by its extension, and coupled with his mechanical system the vacuum as absolutely nothing was unthinkable; he was left with no choice but to introduce the concept of aether .
Related to findings in modern Physics, Descartes was onto something; although he was not completely correct. In the opening statement of their essay, Is the Vacuum really empty?, Greiner and Hamilton write:
Ideas recently developed in theoretical physics have resulted in an expansion and thereby in a new understanding of the term “vacuum.” Here we will describe the deductions that lead to a revised picture of the vacuum as a physical object which can be modified under certain conditions.
Their essay confronts the issues I have mentioned above, and manage to establish the vacuum as a real, empirically observable phenomena. They have also established it as something that needs to be redefined and considered a “physical object” which echoes Descartes’ own thinking in his Principles of Material Things. This confirms that it was a mistake for Descartes to have perceived a conflict between his own thinking and Galileo’s findings, and that they were much more compatible than he first assumed. Both position A and position B were complimentary to one another; not destructive opposites. Further on in the essay, Greiner and Hamilton address defining the vacuum. They discuss how empty space is made up of energy fields which can not be pumped out of a space like particles can. Matter is also considered equivalent to energy . Therefore, you can not refer to matter-free space as empty. But we can still refer to it as a vacuum so long as its definition is clearly stated as having physical properties, or as Descartes preferred to refer to it, as a body; rather than an absolute void.
To conclude, I began by outlining Descartes’ argument on what a clear and distinct idea is, and why it is important in order to establish truth from which to build a comprehensible metaphysics. From here I introduced his reasoning on why he defines space as equivalent to corporeal substance, as they both share the principle qualities of length, breadth and depth. One must therefore conclude that as extension can not be said to be the property of nothing (as nothing has no qualities), then empty space must be considered body or res extensa. The concept of vacuum as an absolute void is an error in reasoning, which holds the potential to disrupt Descartes’ metaphysical theories, and which may have contributed as a motivation for his denial of the void. Once Descartes’ reasoning and arguments were established, I explored and contrasted his views to Galileo’s, proving that their ideas did not contradict and that in fact they could be seen as complimenting one another. Finally, a comparison was made of the ideas already expressed in research on modern physics which successfully establishes the merging of both Descartes and Galileo’s ideas. Leaving the vacuum as something which can be both empirically established as something without particles, and be considered as a physical object simultaneously.