The foundation of Cartesian dualism is that there are two different types of substances: physical and mental.1 Physical substances have the fundamental property of extension while mental substances have the fundamental property of thought. Furthermore, they are entirely distinct from each other; a physical substance cannot think and a mental substance cannot have extension.2
Another key principle of Cartesian dualism is that humans have a body, in the physical state, and a mind, in the mental state, which interact with each other.3 Descartes compared this union of mind and body to a sailor and his ship in Meditation 6; like how a sailor steers his ship, we too have a thinking thing, existing independently of the body, which controls our body.4 Thus our mind can affect our bodies. Furthermore, Descartes reasoned that our bodies can affect our mind because when our bodies are damaged, our mind perceives this as pain.
However, this raises the issue of mind/body interaction; it seems impossible for a mental substance, with no physical properties, and a material object to affect each other.5
Binder, MD, Hirokawa, N & Windhorst, U 2009, Encyclopedia of Neuroscience, Springer Berlin Heidelberg, Berlin. 2
Lloyd, M 2013, Week 3 Descartes I, PowerPoint slides, University of Queensland, Brisbane.
McLeod, S 2007, Mind Body Debate, viewed 13th April 2013, .
Temple, C 2002, Meditations on First Philosophy, Philosophy Index, viewed 14 April 2013, . 5
Lloyd, M 2013, Week 4 Descartes II, PowerPoint slides, University of Queensland, Brisbane.
Elizabeth’s argument focuses in particular on the problem of mental causation and how the mind could control the body.
Descartes explained mental causation by saying that the pineal gland in our brain acted as a gateway through which the material body and the immaterial mind could communicate. When the soul wanted the body to move, it pushed the gland in a way that "drove the surrounding spirits towards the pores of the brain, which directed them…to the muscles".6 These “spirits” consequently caused movement.
However, Elizabeth asked "how the soul…(being only thinking substances) [could] determine the bodily spirits, in order to bring about voluntary actions".7 She reasoned there are only three cases that cause an object to move; a force being directly applied to the object, the object being hit by another object or some particular quality of the object's surface. Yet, a mind has no extension and cannot account for any of these cases. Even Descartes implied that the mind has extension when he explained how it pushed again the pineal gland to move the spirits.
The fundamental concept of Elizabeth's argument is that only physical things can affect other physical things. This can be proved through proof by contradiction, using the premise of the law of energy conservation which states that energy must be conserved in the physical world. If we assume the mind can affect the body, then it must activate the brain activity that brings about bodily movement. Furthermore, it must input energy to initiate this activity. However, the mind is non-physical and doesn’t have any energy to transfer to the brain.8 There would have to be a creation of energy for the brain activity to start. Hence,
Rene Descartes, The Passions of the Soul, trans. John Conttingham, Robert Stoothoff & Dougald Murdoch (Cambridge: CUP,...