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Philosophy Then

Mind Without Matter

Peter Adamson traces ancient arguments against materialism.

What would a philosopher say you’re using to understand these words as you read them? Well, as usual, it depends on which philosopher you ask. Some are ‘materialists’, and think that your mind is reducible to its physical substrate, and so might say ‘your brain’. Some are ‘dualists’ and think that the mind is distinct from the brain, being a completely different kind of thing, so might just say ‘your mind’.

Many, if not most, pre-modern philosophers were dualists. Here we may think of Plato and the many thinkers influenced by Platonism in later European and Islamic thought, or of dualist theories of self in classical India. But there were also materialists in these traditions too, like the Carvaka in India or the Epicureans and Stoics in ancient Greece. This means that the dualists needed to argue for their opinion.

One way they did so was through the ‘affinity argument’. It trades on a premise that was already accepted in Greek philosophy before Plato, namely that the knower must share a nature with whatever it knows, or, to put it more succinctly, that ‘like knows like’. Thus one might suppose that, in order to know things in the rest of the world, the soul (‘psyche’) or mind is made up of whichever element or elements constitute them. To escape materialism, we need add only the further premise that the things the soul knows are immaterial. The argument will go: the soul is like what it knows; what it knows is immaterial; therefore the soul is immaterial.

I wouldn’t suggest this argument is air-tight. For one thing, we should be told why the soul needs to be like its object in this exact respect. But in favor of the argument is that it appears in the work of no less a philosopher than Plato. He has Socrates use the affinity argument in his dialogue Phaedo, in which a series of arguments are brought forward to prove the immortality of soul before (spoiler alert!) Socrates takes poison and dies. In one of the arguments, Socrates posits a distinction between two kinds of things, the visible and invisible (79a). He then suggests that, since the soul investigates things of the latter kind – things that are ‘pure, ever existing, immortal and unchanging’ – it must be akin to them and therefore likewise immortal. By this means, Socrates is able to persuade his friends that when he dies his soul will not blow away like vapor.

Nor did this argument disappear in a puff of smoke after Plato used it. His followers in late antiquity were firmly convinced by the affinity argument, and on its basis deployed a distinction between at least two kinds of cognition, linked to two kinds of object. On the one hand we have sensation and related powers, which give us access to physical things. On the other hand we have immaterial minds, which can grasp the immaterial objects, such as the invisible ‘Forms’ introduced by Plato. This distinction generates the idea that some of our cognitive powers will be lost when we die. Once you have no eyes or nose, you won’t be able to see or smell anything. But you will still be able to think.

If we now move ahead several more centuries, we find the great Muslim philosopher Ibn Sina (Avicenna) updating the affinity argument to fit his own theory of knowledge, which owes more to Aristotle than to Plato. So instead of contrasting the visible to the invisible, he distinguishes between the particular and the universal. Lower powers of the soul deal with particulars: you might see, remember, or imagine Socrates, or other particular things. These powers are seated in bodily organs, like the eyes or the brain. By contrast, the highest, intellectual power of the soul deals with universals – as for instance when it understands the nature of humanity, or what belongs to all particular humans. Ibn Sina thinks that only humans can think universally. Animals might be able to grasp some features not available to sensation; his famous example is that a sheep can grasp hostility in a wolf. But a sheep cannot think at a universal level. It cannot, for instance, wonder resentfully why all wolves are so hostile to all sheep.

Ibn Sina thinks that he can now show the immateriality of the intellectual soul. If it were material, then it would ‘particularize’ the ideas it grasps, receiving them the way that color is received by a body, with one part of the body having one bit of color, another part another bit of color. Instead we know from experience that we are able to grasp universal ideas all at once and without any division of parts. And the intellect must be like its object – not by being universal, but by being undivided and immaterial.

Ibn Sina’s version of the affinity argument had a wide reception in later philosophy. We even find it in the Renaissance Platonist philosopher Marsilio Ficino, who ascribes it rather vaguely to ‘Arabic Platonists’. He agrees with Ibn Sina’s line of thought: if the mind were in a bodily organ it would be divided in that organ, so it could not grasp indivisible ideas.

In the Islamic world, though, the argument had a less favorable reception. It was pointed out that the hostility of a particular wolf is itself an undivided unity. So Ibn Sina’s argument would have the (presumably absurd) consequence that sheep have immaterial souls. It was also argued that an indivisible body could do the work required by Ibn Sina’s argument: if the soul were a single atom, then it could grasp a unitary idea without dividing it. These objections actually concede the premises of Ibn Sina’s proof. But they suggest that there might be other ways to secure affinity between the soul and its object than dualism.

Peter Adamson is the author of A History of Philosophy Without Any Gaps, Vols 1-5, available from OUP. They’re based on his popular History of Philosophy podcast.