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.

Good thoughts as to the correspondence, Eikadistes. I haven't got much to go on yet but I think there's a bright line and that Epicurus would require that "weight" not imply that this particular cause of the motion of atoms is operating only because of some external force separate from matter and void. (In other words, given that the universe is infinite in size in all directiions and there's no "bottom," there's no force outside the atoms pulling them "down."

Regardless of anything else, our notion of gravity implies something giving off an attractive force at a distance. Maybe there's something in the magnetism discussion in Lucretius that could be used to explain what "weight" might mean without requiring "action at distance" (without touch being involved).

Yes I presume Goodwin is struggling with the same issue we are discussing. Epicurus is apparently using the word to designate a capacity for self-generated motion (as in the swerve) and in English "weight" does not give us that self-moving capacity. We need to figure out how to convey this better.

And solving the problem of terminology doesn't require that we wait for some kind of explicit physics rechnology explanation, any more than Epicurus waiting before taking about uncuttable atoms or the swerve. The issue is that the atoms possess within themselves the power of movement and don't wait on gods or outside forces of any kind to bestow it on them.

The sole external requirement is space to move within.

"Weight" carries modern connotations that obscure that original intent of independent capacity, and also makes it harder to see how the swerve is equally uncaused.

Good thoughts Bryan. I would not want to use a word not closely related to something actually used.

Do you have any thoughts on the Greek or Latin words used in any of the places you have seen this discussed? I don't gather that "weight" is a direct transliteration of either Greek or Latin.

I see βαρύς (barys), meaning heavy, but that goes to barometer or "barium" rather than our english word.
And I see Lucretius uses both pondus which gives us "ponderous," "pound," and related English words and also gravis (heavy)

Is it as Eikadistes quoted earlier perhaps better to use the term gravity if Lucretius is using gravitas? Did the Greeks have separate word for "gravitas"?

In english gravity to me implies a force, which is really more what we are looking for.

i am finding no academic articles, but I see citation to the following place (On Fate 24) where Cicero apparently used BOTH pondus and gravitas in the same sentence, arguably implying that he considered them to be separate things (perhaps having weight BECAUSE of the gravitational force within it):

Cicero, De Fato 11.23–24

De Fato 23: "si semper atomus gravitate ferretur naturali ac necessaria" — "if the atom were always carried along by natural and necessary gravitas"

De Fato 24 (repeated): "per inane moveatur gravitate et pondere" — "moved through the void by gravitas and pondus" — and then immediately: "ipsius individui hanc esse naturam, ut pondere et gravitate moveatur" — "it is the nature of the atom itself to be moved by pondus and gravitas"

[23] Hanc Epicurus rationem induxit ob eam rem, quod veritus est, ne, si semper atomus gravitate ferretur naturali ac necessaria, nihil liberum nobis esset, cum ita moveretur animus, ut atomorum motu cogeretur. 11. Id Democritus, auctor atomorum, accipere maluit, necessitate omnia fieri, quam a corporibus individuis naturalis motus avellere. Acutius Carneades, qui docebat posse Epicureos suam causam sine hac commenticia declinatione defendere. Nam cum docerent esse posse quendam animi motum voluntarium, id fuit defendi melius quam introducere declinationem, cuius praesertim [p. 261] causam reperire non possent; quo defenso facile Chrysippo possent resistere. Cum enim concessissent motum nullum esse sine causa, non concederent omnia, quae fierent, fieri causis antecedentibus; voluntatis enim nostrae non esse causas externas et antecedentis.

24] Communi igitur consuetudine sermonis abutimur, cum ita dicimus, velle aliquid quempiam aut nolle sine causa; ita enim dicimus “sine causa”, ut dicamus: sine externa et antecedente causa, non sine aliqua; ut, cum vas inane dicimus, non ita loquimur, ut physici, quibus inane esse nihil placet, sed ita, ut verbi causa sine aqua, sine vino, sine oleo vas esse dicamus, sic, cum sine causa animum dicimus moveri, sine antecedente et externa causa moveri, non omnino sine causa dicimus. De ipsa atomo dici potest, cum per inane moveatur gravitate et pondere, sine causa moveri, quia nulla causa accedat extrinsecus.

M. Tullius Cicero, De Fato, section 24

Martin what is the understanding of gravity today?

Is it held to exist independently as its own entity separate from all other things?

Is gravity itself "uncaused"?

Given that it appears to take a moon-sized or planet-sized body to generate much gravity, I am still thinking that it is something that is associated with "matter" even today (?)

Edit: Do you agree with this from Grok?

Does an atom generate gravity?

Yes, a single atom generates gravity. According to both Newtonian gravity and Einstein’s General Relativity (our best current theory), any object with mass (or energy) produces a gravitational field and curves spacetime. An atom has mass—mostly from its protons and neutrons in the nucleus, plus a tiny contribution from electrons and binding energies—so it sources gravity just like a planet or star does, only vastly weaker.

Excellent questions Patrikios. Thanks for asking them and it seems to me thought experiment is a useful way to approach the problem.

It's possible "modern physics" might have one answer, but even if it's different -- and I'm not sure it would be -- it would be helpful to think about that from Epicurus' perspective. Would the atom require interaction with another atom to move, or would it simply move due to the available space around it?