In Part 1, Article 53 of his Principles of Philosophy, Descartes argues that extension is the primary attribute that constitutes the nature of body, i.e., corporeal substance. But he continues later in Part 2, Article 11 that there is no real difference between space and corporeal substance. Why? Because that which we normally conceive as space is also principally constituted by extension. His argument goes as follows. Imagine some corporeal body such as a stone. Now imagine leaving out everything we know to be non-essential to the nature of a stone: exclude its hardness, exclude its color, exclude its heaviness, exclude cold and heat and all other qualities, either because they are not thought of as being in the stone, or because if they change, the stone is not on that account reckoned to have lost its bodily nature (227, Cottingham edition). Once this imaginary exercise is completed Descartes tells us that we will see that nothing remains in the idea of the rock except that it is something extended in length, breadth and depth. Thus space is constituted by the same primary attribute as the stone (body) itself.Now it seems to me that this fact does not exclude the idea that a vacuum could still exist in nature. But Descartes thinks otherwise; in fact, he thinks it is an utter contradiction to hold that a vacuum exists in nature. Descartes claims this conclusion is clear from the fact that there is no difference between the extension of space, or internal place, and the extension of a body (229-30, emphasis mine). How can this conclusion be so clear though? Descartes reasons, since space is constituted by extension then it must be a substance for it is a contradiction to believe that a particular extension belongs to nothing. So, since space has extension, it must have substance in it and thus no vacuum can exist there.
To my modern ears this claim sounds rather bizarre. But as with much of Descartes s writings, once his line of argument is properly followed its conclusion is hard to argue against, at least on purely philosophical grounds. But, I believe, there are some criticisms that can be pointed out in the above thought experiment and its attendant explanation that should make us question Descartes s conclusion about the non-existence of vacuums in nature.In Descartes s thought experiment we were asked to imagine that we could abstract all of the non-essential qualities of some stone. Then we were supposed to see that the only thing left of the stone was its extension: length, breadth, and depth. When trying to do this experiment within my own mind, I tend to see the result as some sort of non-descript figure akin to one contained in typical geometry textbooks. For instance, suppose that our stone in question happens to be cubic (or at least roughly cubic barring microscopic attention, i.e., cubic to the naked eye). Next, we remove all of the non-essential qualities from our cubic stone as Descartes suggests. The mental picture I seem to be left with looks something like a cube in a geometry textbook. Now such a figure is supposed to represent an object that is purely extensional; but in actuality, this is obviously not the case. A purely extensional object is made up of simple geometric lines, which are nothing more than mere successions of points. Being such, actual geometric lines cannot be accurately portrayed on paper or a computer screen. Thus, in essence, all geometric representations are idealizations given for pedagogical ease and explanation. Being an astute mathematician, I m sure that Descartes was more than aware of this point.
But it seems to me that in the same way we cannot accurately represent a purely extensional object with pen or computer, neither can we clearly and distinctly perceive an object as purely extensional within our minds. What is the nature of this perceiving if not analogous to seeing? Pushing the analogy of seeing with Descartes may be unfair since he may not have had such a literal picture in mind. But it seems very hard to conceive of a body apart from making a mental picture as such; and Descartes s own language lends towards this idea: we will see that nothing remains in the idea of the stone except that is something extended (227, emphasis mine). My point is simply that I find it questionable whether such a thought experiment is really conceivable, especially when this analogy with sight is found wanting.But even if we grant that this thought experiment is conceivable, in some sense, it seems to me that there is a further criticism: Descartes could be charged with equivocating. In taking away the non-essential qualities of an object, it seems one could claim that we are taking away the actual or real extension of an object as well. The idea here is that there would be no extension left once we abstracted all of the supposed non-essential qualities from an object. True, one may posit that an abstract extension is left where the object was; but this is merely a mind-dependent construction. So, on this construal, a stone and the idea of a stone may both have extension (even the exact same dimensions) in common. An actual stone has real extension, and the idea of a stone has merely abstract extension. But the Cartesian might still say, even if space is constituted by abstract extension, it must still be a substance, for according to Descartes it is a contradiction to believe that a particular extension belongs to nothing. But if the Cartesian thinks this is the case, then it seems that space must be a substance more like mind, or at least more like mind than body since abstract extension is mind-dependent on the proposed delineation. I m sure the Cartesian will find this unacceptable since for Descartes thought is the primary attribute of mind and it seems that space does not think. But suppose that we granted, for the sake of argument, that space is constituted by the same primary attribute (viz., the same sort of extension) as body; it still doesn t necessarily follow that space is the same kind of substance as body. Why can t there be a third sort of substance (besides body and mind) in the Cartesian universe? Besides Descartes s stipulations, I don t see why this could not be the case.
In sum, I question whether Descartes s thought experiment is really conceivable. Even if it is, I question whether body and space are constituted by the same (sort of) primary attribute. But even if they are, I question whether this fact makes them the same sort of substance. Nothing I have said specifically argues against Descartes s conclusion that an absolute vacuum cannot exist in nature. In particular I have given reasons for why I think the premises upon which this conclusion are based are suspect, thus making his overall argument unsound. As previously stated, Descartes s jarring conclusion is surprisingly hard to argue against, at least on purely philosophical grounds.
