Marty asked:

Why doesn’t Descartes address the issue of unifying the body and mind?

Answer by Craig Skinner

He does. Only it’s less well-known and less-often emphasized than his separating them as two different substances, res extensa and res cogitans (Cartesian dualism). Having spent so much time and energy arguing that body and mind were distinct and independent substances, he spent much of his final 10 years insisting on their interdependence, which he described as a ‘real substantial union’. Of course he never convincingly explained how the ghost could interact with the machine, as it were, but then nobody else can, one of the flaws of dualism.

In 6th Meditation (1641) he says ‘I am not merely present in my body as a sailor is present in a ship, but… I am very closely joined and, as it were, intermingled with it, so that I and the body form a unit’ (here Descartes uses ‘I’ to mean his mind, soul or self).

In Objections against the Meditations and Replies (1641), he says ‘The mind is united in real and substantial manner to the body… if an angel were in a human body it would not have sensations as we do, but would simply perceive the motions which are caused by external objects, and in this way would differ from a genuine human being’. It seems to me that here Descartes acknowledges that a human being is a necessarily embodied creature.

However, the difficulty of the dualistic view is to explain how the mind can affect the body, and vice versa. This was not lost on one of Descartes’ most astute correspondents, Princess Elizabeth of Bohemia, niece of England’s King Charles I. How can a soul, being simply a thinking substance produce voluntary movements of the limbs, she asked, anticipating Ryle’s much later ghost-in-the-machine argument. Descartes had no satisfactory answer but replied (1643) that the mind-body union (human being) is a primitive notion (as with the mind and the body), and later added that the union is known only obscurely by the intellect but clearly by the senses. Here he seems to be saying: forget trying to analyze the union, it is enough that we feel it. We might say that the union has distinctive, irreducible properties in its own right. For example, hunger is not reducible either to making the intellectual judgment ‘I need food’ or to physiological events such as stomach contractions/ falling blood sugar. Thus Descartes says in his Principles of Philosophy (1644):

‘We also experience within ourselves certain other things which must not be referred either to the mind alone or to the body alone. These arise… from the close and intimate union of our mind with the body. This list includes… appetites… emotions… sensations.’

His last published work ‘Les passions de l’ame’ (Passions of the Soul, 1649) was a study of experiences unique to the mind-body union.

At one stage Descartes suggests the pineal gland as the site of mind-body interaction (falsely believing that this structure does not occur in animals). But this does nothing to explain how the interaction occurs — we are still left with thoughts mysteriously affecting the body’s ‘animal spirits’ and vice versa, or else God mediating all of it so that no genuine interaction is needed.

So you can see that Descartes does more than address the issue, he wrestles with it at length. Unfortunately he tends to be known only for his cogito, dualism and scepticism, and as a great mathematician.

Finally, it’s no walk in the park to explain how the mind affects the body on a monistic view, never mind a dualistic one: the question of how, indeed whether, a thought, feeling or conscious decision can affect a causally closed physical world; and this is intimately related to debates about free will/ determinism and the nature of causation.

A superb, short (57 pages) account of Descartes’ philosophy of mind is ‘The Great Philosophers: Descartes’ by John Cottingham (1997) Phoenix Paperbacks, obtainable for 01 pence plus postage (used copy) from Amazon.