We think that substance dualism separates the mind from the body in an irreconcilable way. This is the view that has been criticised by phenomenology – most staunchly by Heidegger. However, professor of philosophy at the University of Toronto Tarek Dika argues that this misses a crucial part of Descartes' philosophy, namely that the mind and body are fundamentally united. Although these two substances have different properties, we fundamentally have access to their union – from the way we feel pain to the way we move our bodies, dualism is much closer to phenomenology than we initially thought.

The standard interpretation of Heidegger’s criticism of Descartes runs as follows: Descartes, far from breaking with Aristotelian scholasticism, interprets the ego or “I” according to the very categories Aristotle employed to describe all beings: substance and property. Whether there is a change in my mind (from one thought to another) or a change in a stone (from one temperature to another), the underlying substance (the mind, the stone) abides, while the properties (the thought, the temperature) change. For Descartes, modifications in the mind are distinguishable from modifications in a stone only by the fact that they are conscious. For Heidegger, there are no modifications which I merely consciously “clock.” Descartes failed to ask whether categories like substance and property are adequate to beings such as ourselves (“Dasein,” in Heidegger’s technical terminology).

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What distinguishes Dasein from other beings is that its being is an issue for it; it doesn’t simply undergo changes in its properties, but must decide who and how to be in each case and in every circumstance, however mundane. If a stone changes temperature, then it has simply acquired a new property. If Dasein’s temperature changes, it hasn’t simply acquired a property, but rather experiences its existence through this change and has to decide who and how to be in relation to it. For example, if it is threatened by the fire, it flees to safety. Dasein experiences its changes existentially, and Dasein cannot be captured by categories designed to describe other beings. Descartes, Heidegger argues, “is ‘dependent’ upon medieval scholasticism and employs its terminology” to such a “profound extent” that “the way in which posterity has determined or failed to determine the ontological character of the res cogitans” according to “the ancient way of interpreting the being of entities” has not even been seen by him.

Secondly, Descartes’ mind-body dualism renders the “ego” or “I” worldless; it is certain of its own existence, but not of the existence of the world. This leads to what has come to be known as “external world skepticism.”