How far is Descartes' Argument from Indivisibility successful as a justification for the need of a metaphysical distinction between mind and body?
Descartes introduces the argument from indivisibility by positing what he discerns as the fundamental difference between the mind and body, stating that "...the body is by its very nature always divisible while the mind is utterly indivisible." (Descartes, 1641 6th Mediation). The distinction is arguably best understood as an application of Descartes' notion of extension, first outlined in Principles of Philosophy, Book I, as constituting the "...nature of corporeal substance..." (Descartes, 1644). For Descartes, if an object is extended (res extensor), it is a physical thing which can be quantitatively described, and thereby divided. Upon, what seems to be an almost prima facie consideration of the mind, Descartes reasons that he is "...unable to distinguish any parts within himself" in the way that the body seems to be able to be divided. Indeed, Descartes reasons that the mind isn't divisible in the way that the body is, postulating that if "...any..part of the the body is cut off nothing thereby has been taken away from the mind' ( Descartes, 1641 6th Mediation). If we accept Descartes' argument that all extended things are divisible, then it follows that that which is indivisible is non---material Thus, if we understand the mind to be indivisible (res cognates), then we a led to position of dualism.
However, the findings of modern neurophysiology, here, spring to mind as a glaring counterexample to Descartes' claim that the mind is indivisible. Countless studies seem to have shown the relation between mental states and, particular, physical brain regions (Feinberg 2001, pp. 2). One such example is found in the case of epileptic patients who underwent a corpus callosotomy (an operation severing the nerve fibres connecting the two hemispheres of the brain), the implications of which suggest that the mind actually seems to divide into two separate conscious awareness (Feinberg 2001, pp. 91). Cases such as this one seem to suggest that memories, thoughts, beliefs and other mental states are states of the physical brain. Ultimately, whether one accepts dualism or not, it seems that what happens in the mind is very closely related to what happens in the brain.
Nonetheless, perhaps merely considering the impact of neurophysiological changes upon the mind is to understand Descartes' argument rather crudely. If we take Descartes' exploration to be rooted in the qualitative experience of the mind, this is to say the phenomenological character of brain states,...