Descartes theories regarding the distinction between the mind and body have been heavily criticised throughout, most notably by Gassendi and Elizabeth, who argued that for one thing to cause motion in another they must come into contact. However, for voluntarily bodily movements contact between mind and body would be impossible due to the mind’s non-extended nature. Furthermore, as recorded in Principles of Philosophy, contact must be made between two surfaces for any movement to occur, and whilst surface is a mode of the body it is not an aspect of the mind, therefore as both are completely different there is no logical explanation of bodily movement as a connection between mind and body. This criticism represents a very serious problem for Descartes, as it undermines his claims to have a distinct knowledge of the mind as it suggests that there is no reason as to why we should have any understanding of such a substance. Gassendi and Elizabeth go on to suggest that for us to have any understanding of the mind, that would require the mind to have extension and a surface which would allow it to cause bodily movements like Descartes suggests. However, this would imply that the only way for mind-body causal interaction to occur would be if the Descartes theory of both having mutually exclusive natures, would be false, the result of this being a complete undoing of Descartes theory of the mind and body being distinctively separate. Descartes responds to the mind and body problem by claiming that their criticism presupposes an explanation between the mind and body; ‘These questions presuppose amongst other things an explanation of the union between the soul and the body’ (Justin Skirry, 2017) Furthermore, that the theory itself stems from a false presupposition that two substances with opposing natures cannot communicate, Descartes argues that this is a ‘supposition that is false and cannot in any way be proved, namely that, if the soul and the body are two substances whose nature is different, this prevents them from being able to act on each other”.(The Philosophical Writings of Descartes, 1985). Furthermore, Descartes suggests in his Fourth Replies that insofar as the mind and body are complete substances, individually they only make up an incomplete being; ‘For there you said that the body and the soul, in relation to the whole human being, are incomplete substances; and it follows from their being incomplete that what they constitute is a being through itself” (Justin Skirry, 2017) Thus, for Descartes a complete human being is not through the motion of two substances casually interacting by means of contact, but rather that when both are working together they are working towards the same goal, which results in a whole and complete substantial human being. It is without doubt that for Descartes, to have knowledge and an understanding of the mind and body implies that they are distinct subjects; able to function independently yet work in unison to create a complete and whole human being. The mind or soul for Descartes, is a thinking, non-extended substance, which is indivisible by nature and makes up our essence as humans. Whereas the body, a non-thinking, divisible and extended substance is for Descartes a ‘kind of mechanism that is outfitted with and composed of bones, nerves, muscles, veins, blood, and skin in such a way that, even if no mind existed in it, the man’s body would still exhibit all the same motions that are in it now, except for those motions … from the mind”(Justin Skirry, 2008) Therefore, both are clearly very distinct and separate in nature, however through the pineal gland, they are able to communicate and work together, with a combination of the mind and body resulting in what makes us human beings.