Rene Descartes argued that the body is entirely distinct from the mind. According to Descartes, whereas the body is corporeal, the mind is non-corporeal. One of Descartes' arguments for this view, called Cartesian Dualism, is the divisibility argument. In this paper, first, I explain Descartes' divisibility argument. Then, I object that Descartes secretly relies on two notions of divisibility instead of one as he supposes. When the two notions are taken into account, Descartes' argument no longer works. Then I consider Descartes' reply and try to see how with these two notions of divisibility he could still argue for the immateriality of the mind. I find a different fault with this new account. I conclude with some reflections about the nature of arguments presented in this paper.
Descartes' divisibility argument depends on two premises. In his first premise, Descartes states that "the mind is utterly indivisible" (Descartes, pg 101). What Descartes means by "mind" is "pure intellection"(93) - the referent of the I in expression such as "I feel", "I think" and etc. In clarifying what this "I" is, Descartes distinguishes between mental activities that are not essential to mind and the mind. If he can conceive of his mind in the absence of an activity (such as imagination), that activity is considered to be distinct and not a constituent of the mind. For instance, imagination and sensing are not parts of the mind.
Moreover, I find in myself faculties for certain special modes of thinking, namely the faculties of imagining and sensing. I can clearly and distinctly understand myself in my entirety without these faculties, but not vice versa: I cannot understand them clearly and distinctly without me... Thus I perceive them to be distinguished from me...(96)
Mind is defined by the "modes of thinking" (different tasks mind carries out such willing and understanding) that are present in every conceivable scenario in which the thinker conceives of herself. Since it is inconceivable that "I" can refer (in the relevant sense) to an entity without any of these and mind is the referent of "I", these modes are indivisible constituents of mind. Consequently, mind cannot be broken down. Descartes' second premise is that all material things are divisible. Descartes reasons the fact that he can "in [his] thought... easily divide into parts"(101) any corporeal object is enough for this conclusion. Due to this, he doesn't worry about the possibility of very small, indivisible, physical atoms. After he has argued for these two premises, Descartes relies on Leibniz's Law of Indiscernibility. Leibniz's Law of Indescernibility states that if x and y are the same, then x and y are identical in all respects. Accordingly, physical matter and mind differ in one crucial respect: all physical matter is divisible, whereas mind is indivisible. From Leibniz's Law, it follows that mind and physical matter are not the same.
My main objection to this argument is that it doesn't make sense to apply the notion of divisibility to mind in the way we apply it to physical objects. When we apply the predicate "is divisible" to a physical object, what we mean is quite clear. Division, in the physical sense of the word, is an act that converts one object in space into two objects in space. We can't speak of division with the physical sense in the immaterial world, since immaterial things can't exist in space. Some predicates are restricted to certain domains and it doesn't make sense to use these outside of these domains. For instance, the predicate "is green" can only be applied to visual sensory data. It is nonsensical to apply it to sounds and say, for instance, "man's voice was green". Similarly, division in the physical sense can only be applied to physical things. When Descartes says "the mind is utterly indivisible", the notion of divisibility he has in mind is different from division in the physical sense. Division in the immaterial sense(i.e. as applied to mind) means, as I interpret Descartes, the removal of mind's various abilities such as imagination. These two senses of division share some similarities. For example, both senses of division require a whole being converted into parts (i.e. mind into its abilities and an object into other objects). Perhaps, these similarities and the fact that we call each process with the same name confuse us into thinking that they are the same. However, division in the physical sense is different from division in the immaterial sense in that the former applies to material objects in space while the latter applies to immaterial entities. Reinterpreted, the argument is no longer reliable. There can be two interpretations of division in Descartes' statement that "the mind is utterly indivisible". If we interpret the notion of division here in the physical sense, the premise is nonsensical. Here, I consider physical indivisibility as a property itself and not merely as the absence of a property, namely absence of physical divisibility. It is meaningless to ascribe a physical property to an immaterial object. For instance, Lucretius thought that there were very small, indivisible atoms that were the building blocks of every object. It makes sense to talk about such atoms because division in the physical sense can be applied to them. They are objects in space. Indivisibility in the physical sense can't be applied to an immaterial mind. If we interpret division here in the immaterial sense, then the argument is no longer valid, for Descartes needs to show minds have physical indivisibility that physical objects don't have. It doesn't follow from the fact that the mind is indivisible in the immaterial sense that it is also indivisible in the physical sense. We can also reinterpret the second premise and reason similarly. An argument such as "P1. Mind is indivisible in the immaterial sense P2. Matter is divisible in the immaterial sense. C. Matter and mind are distinct" won't make sense, it doesn't make sense to talk of matter being divided in the immaterial sense for the same reason.
In my opinion, the best reply would be to concede to all of the objection's conclusions and argue that even when we agree to these conclusions, the mind is still immaterial. Let's accept, for the sake of argument, that certain predicates are restricted to certain domains, as I have argued. The property of being indivisible in the physical sense applies only to the objects in the physical domain (again as I have argued). Descartes could argue that since it doesn't make sense to apply indivisibility in the physical sense (and other physical predicates) to mind, then mind is not in the domain of physical. Therefore, mind must be immaterial.
This argument relies on a mistaken notion of nonsensicality. It assumes that for any given predicate p and domain d it operates in, we can only construct nonsensical sentences with p outside d. This is wrong, for we can construct nonsensical sentences with p in d as well. Take the sentence, "The man divided (in the physical sense) the motion of the rabbit." The predicate "divided" here is physical and the motion of the rabbit is also in the physical domain - I think Descartes would agree with me that it is physical since he writes "I also acknowledge that there are certain other faculties, such as those of moving... that cannot be understood apart from some substance"(96). (Substance here is material, as opposed to immaterial mind.) Nevertheless, the sentence is nonsensical. It doesn't make sense to talk about dividing in the physical sense the motion of the rabbit. One might object that it is possible to divide the rabbit's motion - let's say into two motions, one from 0 to 1 sec and the other from 1 to 2 sec. However this isn't the sense of divisibility we are concerned with. According to the relevant sense of divisibility in the physical sense, I must have a tangible motion object like a table in space. When I divide the motion object, I must get two other objects in space. However, the very concept of a motion object like a chair or a table is meaningless. Similarly, even though it doesn't make sense to talk about dividing in the physical sense the mind, this still doesn't prove that mind is outside the domain of physical. Even though the mind may not be a physical object per se, it could be a physical event like motion. Thus, the mind could still be in the physical domain.
First, it was apparent that Descartes was relying on two different senses of divisibility. When analysis clarified the concept of divisibility, Descartes' argument was not compatible with two different senses as reinterpretations of his argument were all in vain. Even though Descartes' argument seemed intuitive at first, maybe an intuition pump in Daniel Dennett's wrods, further analysis dismantled its allure.
Works Cited:
Descartes, Rene. Trans. Donald A. Cress. Discourse on Method and Meditations on First Philosophy. Indianapolis:Hackett Publishing:1998