Bryan asked:
Hey, I’ve been reading and researching a lot on Descartes and his views on the mind and the body. I’m have a bit of trouble differentiating how he feels about the mind and the body though. My questions are ‘Why does Descartes think you can never divide the mind?’ and ‘Why does he think you can ALWAYS divide the body?’ I would really appreciate if someone can clear these 2 things up regarding Descartes. Thanks!
Answer by Craig Skinner
Descartes wishes to convince us that there is a ‘real’ distinction between body and mind. Here, ‘real’ is a technical term of mediaeval philosophy meaning that body and mind are distinct substances. Furthermore, he thinks body and mind are not just distinct substances of the same kind (like two chairs for instance), but of different kinds having no properties in common (apart from those needed for existing, like duration). Thus the body is made of matter (substance extended in space, ‘res extensa’, which cant think), the mind is immaterial (thinking substance, ‘res cogitans’, which has no extension).
He has three arguments to support his position (none good).
1. Argument from doubt (Discourse on the Method; 2nd Meditation).
2. Argument from clear and distinct separation (6th Meditation).
3. Divisibility argument (6th Meditation).
I won’t deal here with 1. and 2. except to say that 1. is nicely rebutted by Arnauld in the 4th Set of Objections to the Meditations, using, aptly for Descartes, a geometric example.
As regards 3., the subject of your questions, Descartes is brief, and we can quote him pretty well in full:
‘…there is a great difference between mind and body, inasmuch as body is by nature always divisible, and the mind is entirely indivisible. …when I consider the mind, that is to say, myself inasmuch as I am only a thinking thing, I cannot distinguish…any parts…and although the whole mind seems to be united to the whole body, yet if a foot, or an arm, or some other part, is separated from my body, I am aware that nothing has been taken away from my mind. And the faculties of willing, feeling, conceiving, etc. cannot be…said to be its parts, for it is one and the same mind which employs itself in willing and in feeling and understanding.’
So, to your questions.
Why does he think you can ALWAYS divide the body?
He says it is ‘by nature always divisible’. Clearly he means its extended nature, which obviously allows cutting it at any point or points along its extension thereby dividing it. I think ‘always’ simply means that any and all extended bodies can be divided. We could speculate that by ‘always’ he implies that you can keep on dividing a part, and then a subpart, indefinitely, so long as the subsubpart has finite size, at least in imagination (eg dividing an atom into its left and right halves), if not in practice. I think we can accept that bodies are divisible. Whether indefinitely divisible maybe depends on whether space is quantized or continuous, but that’s another story that starts with Zeno and still runs.
Why does he think you can never divide the mind?
Clearly the mind, unlike the body, cant be divided in the physical sense. This is true whether we think,with Descartes, that the mind is an immaterial substance, or we think, as I do, along with many others, that the mind is the mental activity of the brain.
So, if the mind is to be divisible, this would have to be in the way that, say, 12 is divisible into 7 and 5, or philosophy divides into metaphysics, epistemology and ethics.
But Descartes thinks the mind is not divisible in any sense.
He has two reasons:
1. Although the whole mind seems united to the whole body, we can remove a bit of the body (a foot, say) without removing anything from the mind.
2. His mind feels whole to him, faculties being features of the whole mind rather than being mind parts.
Certainly a foot could be removed with little or no effect on the mind, but removing half a brain would remove a lot from the mind. Indeed, Descartes’ argument implies, absurdly, that the whole brain could be removed without removing anything from the mind.
As regards faculties being features of the whole mind, not parts, this may just be a verbal dispute. But there is no doubt that targeted brain damage can cause selective loss of a faculty, or even more strange changes to the mind, that I feel can count as showing mind as divisible.
Examples:
Hippocampal damage destroying existing long-term memories and the laying down of new ones.
Speech centre damage leading to lack of (understanding and/or producing) speech. Common in stroke.
Commisurectomy (cutting the connection between left and right brain halves, a treatment for intractable epilepsy) leading to each half having its own semi-autonomous self, truly a mind divided.
Schizophrenia where, among other features, emotion is cut off from cognition, again a mind divided in a different way.
In summary, Descartes’ arguments for mind/body real distinction (Cartesian dualism), including the Divisibility Argument, are implausible. And that’s without going into his (and every body else’s) failure to explain how mind and body could interact if they were distinct substances.
Ask a Philosopher 
As I see your argument, there seems to be a hole in it. Even though several mental diseases like for eg. Schizophrenia can cause mental change, it does not divide the “mind”. A Schizophrenic person still thinks and acts/reacts upon what he thinks is the reality. Mind may play games with your reality, but I do not understand how is mind still divisible ? I mean if mind is just thoughts, impressions and whatever you collect as a memory and the analysis thereof, how could you divide that ? You may damage that but not divide I suppose. Again, mind and brain might be considered 2 different systems or similar. This may open a whole new line of reasoning and rationale. Thank you for reading.