Is Motion One Of The Three Eternal Properties of Atoms? I.E. Are The Three Properties Shape, Size, and MOTION?

This topic was also discussed in a recent zoom meeting. The answer appears debatable, and I see that Sedley's article "Epicurean Anti-Reductionism" has this to say (see especially footnote 29):

Here is the translation from Demetrius Lacon cited on page 306:

This topic arose in today's Zoom meeting and it needs further research and expansion. I am also going to modify the title of this thread to make the issue more clear.

I feel sure there are academic articles directly on point beyond what I've cited above.

Ok I asked Claude to summarize David Sedley's position. Apparently the main article is not publicly available but I will include the cite below:

The key Sedley text regarding the Epicurean relationship between weight and motion turns out to be his own entry in the Routledge Encyclopedia of Philosophy (1998), where he lays this out with unusual precision. Here is what he says:

Weight as the first of three causes of motion

There are three causes of an atom's motion. The first is its own weight, interpreted as an inherent tendency to move downwards. The second is collisions with other atoms, which can deflect an originally downward motion along any number of new rectilinear trajectories, thus generating the patterns of motion of which compounds are born. The third cause of atomic motion is the swerve, whereby an atom may shift from its rectilinear trajectory onto an adjacent one — a displacement sideways by a distance of one minimum. Routledge

So Sedley's answer is precise and structural: weight is not merely a property atoms happen to have — it is the first cause of motion itself. It is the explanatory ground of atomic movement before anything else enters the picture.

Weight does not control speed

Then comes the crucial complication Sedley also states directly: atoms all move at equal speed, since in a vacuum, unlike air, there is no resistance from the medium to slow down the lighter ones more than the heavier ones. Routledge This is Sedley citing Letter to Herodotus 61. So weight is the cause of motion but plays no role whatsoever in determining how fast atoms move — all atoms move equally fast regardless of their weight or size.

The implication

Taken together, Sedley's position gives you the clearest possible statement of why "weight" in Epicurean physics is so philosophically strange. Weight is the source of motion — the intrinsic directional tendency that makes atoms move at all — but it does not differentiate fall-rate, does not produce attraction between atoms, and does not operate the way ordinary physical weight does. The atom's property of weight was designed to account for motion as a whole in Epicurus' physics St-andrews — it is the answer to Aristotle's objection that Democritus had no explanation for why atoms move in the first place.

This is exactly why "motion" seems like it could substitute for "weight" — because in Sedley's reading, Epicurean weight just is the cause of natural motion, and the two concepts are inseparable. But Sedley himself maintains "weight" as the right term because the Greek is clearly βάρος, and because weight is the cause of motion, not motion itself. Substituting "motion" would collapse the distinction between the property and what the property produces — which matters for understanding how the swerve then modifies things.

The Routledge Encyclopedia entry by David Sedley

4. Motion

Surprisingly, atoms never stop moving, even within a compound object, since the medium through which they move is void, which can offer them no resistance. More surprisingly, for the same reason they move at a vastly greater speed than any familiar motion through an obstructive medium such as air; even than sunlight, which is seen to spread from horizon to horizon virtually instantaneously (Lucretius II 142–64). More surprisingly still, they all move at equal speed, since in a vacuum, unlike air, there is no resistance from the medium to slow down the lighter ones more than the heavier ones (Letter to Herodotus 61). In stating all these claims, Epicurus is accepting paradoxical consequences of the hypothesis that void exists, consequences which Aristotle had drawn (Physics IV) in the belief that they were sufficiently absurd to discredit the hypothesis. Moreover, the equal speed of atoms was confirmed by another objection Aristotle thought he had found to atomism (Physics VI 2): if there is a minimal magnitude, there can be no differences of speed, because then in the time the faster object took to travel one minimum the slower one would, impossibly, have to travel less than one minimum. Epicurus welcomed this argument, along with the conclusion Aristotle thought absurd, because his theories of void and minima now offered two independent grounds for the same conclusion, that atoms move at equal speed.

The apparent lack of fit between these findings about atoms and the variable speed of macroscopic motions is explained as follows (Letter to Herodotus 62). Even in a compound object the individual atoms are perpetually moving, but in tight and regular cyclical patterns which make the complex as a whole stable. Phenomenal differences of speed, say between two runners, represent merely the aggregate motions of the atoms in each over an observed period of time.

There are three causes of an atom’s motion. The first is its own weight, interpreted as an inherent tendency to move downwards (see §8). The second is collisions with other atoms, which can deflect an originally downward motion along any number of new rectilinear trajectories, thus generating the patterns of motion of which compounds are born.

The third cause of atomic motion is the ‘swerve’ (parenklisis), whereby an atom may shift from its rectilinear trajectory onto an adjacent one – a displacement sideways by a distance of one minimum (there being no smaller distance). This happens ‘at no fixed place or time’, meaning that the occurrence of a swerve is causally undetermined. The theory, derided by Epicurus’ opponents but now recognized as comparable in its implications to modern quantum indeterminism, looks like a drastic solution requiring a drastic problem. Two such problems are recorded (Lucretius II 216–93). First, since all atomic motion starts out as vertical and equal in speed, without a swerve no collisions would ever have started, and hence no world could have been formed. It may be doubted whether this was a sufficiently pressing problem to motivate an abandonment of universal causality: given the infinite past history of the universe,Epicurus had no need to posit a very first collision; in which case every collision could have been explained as the effect of previous ones. The second problem seems to have been the real motivation of the swerve: if all atomic motion is causally determined, free will becomes impossible (see §12).

https://www.rep.routledge.com/articles/thematic/epicureanism/v-1/sections/motion

So the reason this topic came up today is that Patrikios brought up the question of whether atoms move because of some external force applied to them. And this directly relates to the motion of bodies such as magnets, discussed in Book 6 of Lucretius.

If I am reading this correctly, then it would not be appropriate to say that the the three eternal properties of atoms are size, shape, and motion. It is correct to say "size, shape, and weight."

However Epicurus intended "weight" to be understood as the internal cause of motion without need of any external force (which we might think of as gravity) acting on it. Thus Epicurus was holding that no external force is required to cause atoms to move, as they have moved eternally.

If someone has a better way to state that please post.

Thanks Joshua. So clearly the word being used is "weight." It seems possible however that that does not end the inquiry because it's potentially not clear what is meant by weight.

Today (i gather) we are using weight as something that is attracted differentially by gravity (?)

Epicurus apparently was not using that paradigm (and would not, given that what we think of gravity would be a force outside the atom) ??

What Seldey seems to be saying is that Epicurus is using the term to mean a potential to move when space allows it, which itself is the cause of motion without interaction with anything outside it.

Am I reading that right? If so then a straight use of "weight" in our modern context might be confusing the issue just as it is confusing to think that Epicurus meant "atom" in the same way we do.