The huge mistake Descartes made – and for which we are all still paying – is to conflate mass with extension. We know today that mass is not extension – we think of mass and extension as two different things, and that matter has both properties. They overlap so seamlessly – everything with mass has extension, and the only things we really can sense has mass – that we can forgive Descartes for missing that there are things in this world that have extension but not mass.
Of course, we need not forgive him too much – for one thing he was more or less the inventor of the concept of empty space. He created the Cartesian coordinate system (hence the name), by which we mathematically understand that there are three dimensions – eighth grade’s famous x, y and z axes (length, breadth, and height) – and knew: you can map some object in space, or you can leave that space empty. And that empty space still has extension. It just has no mass – there is no matter in it.
It simply did not occur to Descartes that there might be some substance in this space. He assumed a vacuum – a nothingness – a total emptiness and absence. As a result he assumed that all material In the sixth of his six famous meditations, with the subtitle “On the Real Distinction Between the Mind and Body of Man,” Renee Descartes famously divided the universe in half (not including an all pervading God). One half was extended in space – it had three dimensions. He called this the res extensa – the extended or corporeal substance. The second half was the mind. He called it the res cogitans – the thinking substance – and it was (obviously) not extended.
Henry Allison, discussing Kant’s critics in Kant’s Transcendental Idealism, points out that one of the knocks against Kant has long been that in order for him to be right he might have to claim that the mind is extended in space:
Simply put, the claim [that claiming space and time are in our heads makes him an inconsistent Berkeley, ala HA Prichard and Galen Strawson] is that Kant’s subjectivistic starting point forces him to choose between the following equally unpalatable alternatives: either (1) he must maintain that things only seem to us to be spatial (or temporal), a doctrine which entails that our consciousness of a world of objects extended and located in space is somehow illusory; or (2) he must claim that appearances, that is to say, representations, really are spatial, a doctrine which is absurd because it requires us to regard mental items as extended and as located in space.
But what if mental items are extended (spread out in three dimensions) and located in space?
The masslessness of the mind has always been implied to walk hand-in-hand with its non-extension in space. At least in everything I’ve ever read. In the same way I always said “New York, New York” I was taught to say “massless, formless” when describing the mind. So used did I become to this link that I never stopped to consider the implication: that because to have mass is to have extension (which is true enough), to have extension is mass. It was though I was being taught that because all men are human; all humans are men – that’s the logic we’ve always been offered. But what if to be human is not necessarily to be a man? What if women exist and are human? Which is to say, what if to be extended is not necessarily to have mass?
Of course this is Freshman physics – just Freshman physics that nobody talks about in the philosophy of mind. To to be extended is not necessarily to have mass. The electromagnetic field is extended – it pervades space – and the electromagnetic field lacks, obviously, mass. So if the mind is one and the same as the electromagnetic field, and the contents of the mind – the things we think – are the ever-shifting potentials of that field – then of course the mind is extended. Which would render this critique of Kant the very opposite of absurd. It would convert it into support.