Is it true that the mind is indivisible, as Descartes claims?

Rene Descartes’ 1641 paper Meditations on First Philosophy claims that everything can be categorised as being composed of two mutually exclusive ontological substances: res extensa, substance extended into space that is incapable of thought, and res cogitans, substance unextended into space that is capable of thought. That is to say, Descartes claims that the mind, composed of res cogitans, is indivisible and exists independently of the res extensa composed brain and body (Robinson, 2016). In this essay, I will outline the reasons given by Descartes for the indivisibility of the mind, and then assess the validity of the claim in light of a modern understanding of neuroscience, to which I will ultimately argue that the mind is divisible - contrary to Descartes’ claims.

Descartes’ claim of mind indivisibility is predicated upon his prior claim that the body and mind are separate entities, which he demonstrates via a method of hyperbolic doubt. By discarding any and every belief that could possibly be false, either by means of honest misinterpretation by the senses or deliberate deception by an all-powerful and evil being, Descartes concludes that despite being able to doubt the existence of his own body and all that is around him, the fact that he is able to think means he cannot doubt his own existence, or else there would be no entity performing any introspection; hence cogito ergo sum (Newman 2019). Building upon this separation of mind and body, Descartes further separates the uniqueness of the mind by reasoning it to be composed of an ontologically different substance to the body, via what is now known as Leibniz’s law of the identity of indiscernibles: if entity X possess property P, and if entity Y lacks property P, then entities X and Y are not identical (Forrest 2010). In his sixth meditation, Descartes reasons that the ontological substance of the mind and body are different, as the body has the property of being divisible by means of amputation, but such a property is not exhibited by the mind, as one is unable to distinguish any individual part within themselves or self-divide thought. Therefore, in accordance to Leibniz’s law of the identity of indiscernibles, Descartes concludes that the mind and the brain cannot be of the same ontological substance, since they contradict in their basic property of divisibility (Calef, no date), thus giving rise to the distinct and independently existent res extensa, a dividable substance extended into space that is incapable of thought, and res cogitans, an undividable substance unextended into space that is capable of thought.

Descartes’ claim that the mind is indivisible faces objection on two possible fronts. The first seeks to disprove mind indivisibility by illustrating the untenability of the prerequisite claim that the body and mind are two separate and independent entities by raising the issue of interaction, thus removing the basis on which divisible res extensa and indivisible res cogitans stand upon. Also known as Elizabeth’s objection, the Princess of Bohemia points out that if the body and mind were truly distinct and independent entities, then by Descartes’ own definition, the mind would be formless (non-extended) and hence be unable to interact with the body (a substance extended in space) to produce voluntary movement (Shapiro, 2013). Descartes attempts to counter this contradiction by identifying the pineal gland as the point of interaction, theorising that voluntary movement occurs when spirits dwelling in the brain will the appropriate neurons into action (Lokhorst, 2013). However, this an untenable counterargument to make in light of its glaring conflictions with our modern understanding of neuroscience and the causal closure of the universe, and thus is sufficient reason consider Descartes’ claim of the mind being indivisible as false.

The second objection seeks to disprove mind indivisibility by empirically showing the diversity of the mind, in contrast to Descartes’ unified and holistic view of the mind. An example of the diversity of the mind is the discontinuity of consciousness from day to night – that is, during sleep. Upon the transition from light to dark, the mind has the possibility of dreaming, in which case the continuity of consciousness is preserved. However, more often than not, one goes to bed and somehow awakes refreshed with no recollection of the proceeding 8 hours. (Calef, no date). The Cartesian dualist may rebut that the unity of the mind is preserved since the division is temporal in nature and not spatial, but such an admission inherently concedes that there are two states in which a mind can exist in: one in which is it privy to conscious thought, and one in which it is not; thus, the mind is divisible.

A corpus callosotomy is a palliative procedure in which the corpus callosum, the link between the two hemispheres of the brain, is severed, and is an example of a spatial division of the mind. Patients that have undergone this procedure often exhibit callosal syndrome, in which their mind becomes fragmented. For example, if a patient is flashed a word on the side of their right eye only, they are able to correctly identify and enunciate the word flashed. However, if the patient is flashed a word on only the side of the left eye, they are unable to enunciate the word flashed, but if given a pencil and piece of paper, they are able to draw or write out the word, thus showing they had originally identified the correct word (Wolman 2012). Unfortunately for the Cartesian dualist, callosal syndrome demands a simple ultimatum: either the physically damaged res extensa brain has altered the formless res cogitans mind, which necessitates the acceptance of Elizabeth’s interactional objection, or accepting that the res cogitans mind is divisible through an apparent split in knowledge and function – either of which would force a denial of Descartes’ claim that the mind is indivisible.

The only refuge point then seems to a concession of the divisibility of the mind, and instead maintain that res cogitans-esque consciousness of a person is indivisible instead. This is still one point which neuroscience appears to support, as the res cogitans-esque consciousness does not seem to be compartmentalised within any particular region of the brain. Even in cases of patients with severe degenerative neurological conditions such as Alzheimer’s disease, a person’s possession of consciousness does not suddenly disappear with the degradation of the final neuron within a specific region of the brain. However, this alternative claim cannot be made under the Cartesian banner of dualism, since now there exists a slight distinction between a mind and the state of being conscious, regardless to whether consciousness is or is not a function and or property of the mind and or brain – in other words, we have transitioned to property dualism.

In summary, Descartes’ perspective of the mind and brain being separate and independent identities results in unfavourable contradictions with our current understanding of neuroscience. The central claim of Cartesian dualism, that the res cogitans mind is indivisible, faces contest in regard to its mechanism of interaction with the res extensa brain and body – a problem that is compounded by the temporal and spatial divisions illustrated by loss of consciousness and neurological disorders respectively. Overall, I find the above stated reason sufficient strong enough to deny Descartes’ claim that the mind is indivisible.


Calef, S. (n.d.). Dualism and Mind | Internet Encyclopedia of Philosophy. [online] Available at: [Accessed 8 Jan. 2020].

Forrest, P. (2010). The Identity of Indiscernibles (Stanford Encyclopedia of Philosophy). [online] Available at: [Accessed 8 Jan. 2020].

Lokhorst, G. (2013). Descartes and the Pineal Gland (Stanford Encyclopedia of Philosophy). [online] Available at: [Accessed 8 Jan. 2020].

Newman, L. (2019). Descartes’ Epistemology (Stanford Encyclopedia of Philosophy). [online] Available at: [Accessed 8 Jan. 2020].

Robinson, H. (2016). Dualism (Stanford Encyclopedia of Philosophy). [online] Available at: [Accessed 8 Jan. 2020].

Shapiro, L. (2013). Elisabeth, Princess of Bohemia (Stanford Encyclopedia of Philosophy). [online] Available at: [Accessed 8 Jan. 2020].

Wolman, D. (2012). The split brain: A tale of two halves. [online] Nature. Available at: [Accessed 8 Jan. 2020].

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