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Descartes and His Arguments for Substance Dualism: Informative Essay

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Descartes’ arguments for substance dualism seek to show that there are two distinctly different kinds of substance – body and mind. He seeks to show that they perform different functions and are constituted of entirely different things. Two arguments he uses in an attempt to do this are the argument from indubitability and the real distinction argument. This essay will explain these arguments for substance dualism and argue that they are not widely convincing as accepting them is hinged upon his flawed trademark argument.

Descartes’ argument from indubitability focuses on showing the distinction between mind and body through the degree to which we can doubt that either exist. He states that whilst we could doubt that we really have a body, through the very fact that we are able to question this we cannot doubt that we have a mind. It is important to understand Descartes’ concept of doubt in which he holds that we could be forced to doubt our own perception through three things: our own senses deceiving us, being asleep, or being deceived by an evil demon. Descartes sees that the first two do not cause an issue for the argument from indubitability, since in order for our own senses to deceive us, we must at least have a mind for these senses to perceive and a mind to be in a state of sleep and worry about this. Having a body however could be brought into doubt upon these conditions, as simply contemplating a body does not suggest its existence, and thus our senses could be deceiving us into having the apparent experience of a body which may be untrue. Sleep would also cast doubt over the existence of a body as a body is not necessarily needed to sleep, whereas a mind is. Therefore, according to Descartes, indubitability shows that there is a clear gap in the doubt that can be cast on having a mind, and a body and this gap shows that they are indeed separate substances.

However, Descartes does recognize that his criteria for doubt of deception by an evil demon are not ruled out in this argument, and therefore we could indeed have been deceived into the idea that the mind and body are separate substances. Whilst in a modern, less superstitious context, it is unlikely that we would be concerned about deception via an evil demon, it seems credible to be concerned that our entire experience could be a deception from an outside force. This is seen in modern philosophical discussions such as the brain in a vat hypothesis, in which it is contemplated whether we are simply a matter stimulated by technology, whose thoughts and experiences are not independent thoughts but the output of a machine, and therefore we could doubt that we have a mind. As such, Descartes’ argument had the potential here to have great modern philosophical value if this line of reasoning was explored in this secular context. However, in his real distinction argument, he turns to rule out this doubt using his trademark argument and as such, loses this appeal by relying upon the leap of faith of defining God into existence, something that is largely unappealing to the modern secular audience considering the mind-body problem.

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In the real distinction argument, Descartes reasons that as we clearly perceive mind and body as entirely separate substances, and God created all, including our perception, they could be separate substances. As they can exist independently, they remain separate substances. Whilst the previous argument suggested that we could doubt the body existing at all, Descartes sees that this is not the case as we have ideas of material things, such as the body that we did not willfully produce. As we did not produce them ourselves, they must come from an outside source. As God would not deceive us with the mere illusion of material things such as bodies, the idea of them must come from bodies really existing. This argument relies upon the assertion that God is the ultimate creator, not a deceiver, as this rule out the doubt of an evil demon deceiving us, since God’s omnipotence would surpass this power. To accept this, one must accept that the Abrahamic God exists, to accept this existence and perfection.

Descartes provided his trademark argument to prove God’s perfect existence. This argument hinges upon the idea that a cause must have at least as much reality as its effect, a widely accepted idea at Descartes’ time of writing. Descartes argues that as we have the idea of God as a perfect being or the innate trademark of God within us, this idea must have come from somewhere, as a trademark requires a creator to plant it. This trademark could not have come from outside of God, since God himself as the cause of the idea holds the most real of it. Coupled with the clear idea that God is perfect, he must exist and not as a deceiver, as this would undermine his perfection. For Descartes, this meant that the idea of God must have come from God, and therefore, a perfect God exists. Whilst this is not his ontological argument, it is very similar in relying upon a priori reasoning and the belief that an idea of something is a basis for arguing its existence. This has proven a flaw in Descartes’ argument for God, and once this argument is rejected, his real distinction argument collapses too.

Descartes’ trademark argument mistakenly assumes that we all have the idea of God as perfect in the same Abrahamic sense that he does. Whilst this was Descartes’ experience, it seems to be a flawed foundation upon which to base an argument today, as this point itself requires empirical proof that actually goes against the argument of the universal idea of God. Evidence shows that this idea is not the case as 30% of England and Wales report having no religion (Office for National Statistics, 2011). As such, it clearly is not true that we all have the idea of a perfect God existing, as many have no concept of a God, whilst others may have a pluralistic concept or one of a flawed God. Resultingly, Descartes’ trademark argument appears incorrect and unable to prove God’s existence, meaning that a gap is left in the real distinction argument as we are unable to show that our perception of separation is not deceived. The argument becomes entirely ineffective.

In conclusion, Descartes’ arguments for substance dualism of indubitability and real distinction have the potential in casting doubt over the certainty of our existence and substances, however, become too vulnerable when relying upon the belief in God through Descartes’ trademark argument. As such, the existence of God is rejected, and the real distinction argument collapses. This leaves only the argument from indubitability and the quandary it presents of whether our perception is deceived by an outside force such as a demon. Presenting this quandary seems the only convincing element of Descartes’ arguments for substance dualism, particularly if taken down a secular route.


  1. Cottingham, J. (ed.) (2017) Descartes: Meditations on First Philosophy: With Selections from the Objections and Replies. 2nd ed. Cambridge: Cambridge University Press (Cambridge Texts in the History of Philosophy). doi: 10.10179781107416277.
  2. Gere, C. 2004. The Brain in a Vat. Studies in History and Philosophy of Biological and Biomedical Sciences (35), 219-225.
  3. Office for National Statistics. 2020. Exploring Religion in England and Wales.
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