To: Leonidas M.
From: Geoffrey Klempner
Subject: On atoms and the void
Date: 10 May 2002 13:25

Dear Leonidas,

Thank you for your fourth essay for the Ancient Philosophy program, in response to the question, 'All that exists are atoms and the void.' - What is an atom? What is the void?

The main problem here is to disentangle the physics of atoms from the metaphysics. On the one hand, the theory of atoms and the void is supposed to have been grounded in metaphysical principles. On the other hand, the theory is put forward as a hypothesis that accounts for our everyday experiences.

Regarding Aristotle's remark that 'Democritus believes that the nature of the eternal things is small substances...': on metaphysical grounds alone there is no reason to deduce that atoms should be any particular size in relation to us. As I point out in unit 12, the implication from the 'no more reason' argument is that atoms came in all sizes, small and large (12/227). So here is one place where experience is added to what we can deduce from metaphysics. No large atoms have ever been encountered, so any theory put forward must explain this in a way consistent with experience.

You quote Simplicius (KRS 557) saying that 'they believed that... atoms... are compact, as well that they do not have any void inside; because division is due to the void that exists within bodies'.

One might think that the concept of void necessarily involves the idea of distance, or separation. However, a simple thought experiment will show what is wrong with that idea. Two atoms placed in contact with one another would immediately become one indivisible atom! A key feature of the physics of atomism, that atoms can collide and rebound, would therefore be lost. The explanation of why atoms which come into contact with one another do not form an inseparable bond must be reconcilable with the metaphysics of atoms, i.e. with the logical argument for the atoms and void theory.

This is something I should have thought about more. What does it *mean* to say that two atoms are 'in contact'? If two atoms in contact are separated by 'void' and two atoms at a distance are also separated by 'void', how can we explain the use of the same term ('void') in the two cases? It seems that we are driven to posit two kinds of void: void(1) which is the sheer difference in being of two atoms in contact with one another, and void(2) which is the sheer absence of being over a given area.

You go on to say in parentheses, 'therefore they consider atoms indivisible not intellectually but naturally'. Atoms are divisible intellectually because, unlike Parmenides One which exhausts the whole of reality, atoms necessarily have shape and dimension. However, their indivisibility is not merely a physical fact, as would be the case for the modern equivalent of 'atoms'.

If the void is sheer absence of being, the result is not space as we know it, because space as we know it has physical properties, e.g. supporting gravitational and electro-magnetic fields. It is interesting to compare, as you do, the stark contrast of being/ the void with Parmenides' complimentary principles of light and dark in the 'Way of Opinion'. I wonder, though, whether Parmenides was not anticipating a similar idea, that 'dark' is the sheer absence of light.

I was intrigued by what you said at the end about how, 'in modern physics we can see also the same ancient problem: how can we describe something that has nothing to do with everyday experience, but still we must call it a particle? This is perhaps the root of the dualism particle-wave that the scientists have introduced to describe the behaviour of what they call "particles".'

In modern physics, the behaviour of the fundamental 'particles' is described precisely in mathematical terms, and the problem is relating this description to everyday experience. By contrast, the atomists clearly thought that there was nothing problematic about the idea of a *thing with a shape*. The properties of Democritean atoms, unlike modern 'wavicles', can be easily modelled with wooden or metal objects. This was, in fact, the way physics conceived of atoms up until the end of the nineteenth century. The problem with ancient atomism, which it shares with modern physics, is that of reconciling the description at the atomic level with the world of sights, sounds and smells.

All the best,

Geoffrey