Explain and analyse Descartes' modal argument for dualism.

In the modal argument, Descartes attempts to support his theory of substance dualism by arguing that the only thing we can be absolutely certain of is our own mental existence. This argument is best captured in his famous demon example, which posits that the only thing an omnipotent demon could fail to mislead me about is the fact that I exist as a thinking being, because even the act of doubting that I exist is proof of my consciousness. However, Descartes argues that this only constitutes proof of my mental existence and not my physical existence, because it establishes me only as a thinking being, not as one which extends spatially. This thought experiment brings into clear focus the idea that it is possible to imagine a state in which one’s mind exists separately to – or entirely without – one’s body, and it is usually argued that if such a concept is conceivable, then the mental and the physical must be two distinct types of thing. The argument can also be phrased in a similar way by applying Leibniz’s law of identity: if mental states have the property of certainty and physical ones do not, then mind and matter must be two entirely separate categories of substance. Philosophers such as Ryle dispute Descartes’ modal argument by suggesting that in his definitions of mind and body, Descartes has made a fundamental 'category mistake'; that is, a mistake which involves assigning qualities to the wrong category of things. For instance, a tourist who, having visited all the colleges and faculty buildings in Oxford, complained that they had not seen the University of Oxford would be an example of someone who has committed a category error. The University of Oxford is the abstract idea linking all of these physical buildings together, and the phrase ‘The University of Oxford’ is thus in fact identical with these buildings. In a similar way, it can be argued that Descartes makes a category mistake when he distinguishes between the mind and the body, or between mental states and brain states. Physicalists would argue that dualists are mistaken in supposing that mental states are different to brain states, and that in fact the two things are identical: a particular pattern of neurons firing is exactly the same as a certain thought or feeling. Therefore, it does not make sense to conceive of mind and body as two different entities (because all mental states are necessarily physical ones) and hence Descartes' modal argument fails to adequately support his theory of substance dualism.(this answer assumes knowledge of physicalism and dualism because the length was so short, but in a real lesson/essay I'd distinguish them first)