Last night in our zoom the question came up as to whether Epicurus held that atoms differ in weight. This is what AI has to say - take it for what its worth and let's discuss whether they differ in weight or not.
Atomic Weight (βάρος) in Epicurean Philosophy
Does Epicurus say atoms have different weights?
Yes — but with important qualifications. Epicurus lists weight alongside size and shape as one of the three intrinsic properties of atoms. However, the relationship between weight and the other properties is philosophically murky in ways that size is not.
The key passage is Letter to Herodotus §54: atoms differ in shape, size, and weight (βάρος). This seems straightforward. But complications arise immediately.
The Upper and Lower Bound Problem for Weight
Lower bound — every atom has some weight: This is unambiguous. Epicurus breaks decisively from Democritus here. Democritus did not attribute weight to atoms as an intrinsic property — weight for him was a relational or emergent phenomenon arising from atomic collisions and vortex motion. Epicurus insists weight is primitive and intrinsic: every atom, no matter how small, has weight, and weight is what drives the fundamental downward motion through the void (the clinamen aside). There is no weightless atom. So the lower bound is: at least some minimal quantum of weight, corresponding presumably to the smallest atom.
Upper bound — here it gets complicated: Unlike size, where Epicurus gives a clear upper bound (atoms must remain sub-perceptible), he does not give an equally crisp upper bound for weight in the Letter to Herodotus. The constraint that seems to apply is:
- Weight must correlate at least roughly with size (a larger atom, having more minimal parts, should be heavier).
- Since atomic size is bounded above by sub-perceptibility, atomic weight is indirectly bounded — an atom cannot be so heavy that it produces perceptible gravitational effects in isolation.
- But Epicurus never states this explicitly for weight the way he does for size.
The Lucretius Complication
This is where things get genuinely difficult. Lucretius, in DRN 2.225–242 and 2.333–380, appears to deny that atoms differ in weight at all — or at least to deny that heavier atoms fall faster than lighter ones. His argument for the clinamen (the atomic swerve) depends on all atoms falling at the same speed in the void regardless of weight. This is the famous passage: in empty space, a feather and a lead ball fall at the same rate — only resistance (plagae) from surrounding matter creates apparent differences in falling speed.
This produces an apparent contradiction:
- Epicurus says atoms have different weights (βάρος varies).
- Lucretius says all atoms fall at the same speed.
How scholars resolve this:
Sedley (Lucretius and the Transformation of Greek Wisdom, 1998) argues these are not actually contradictory. Atoms have different quantities of weight, but in the infinite void with no resistance, the rate of fall is uniform regardless — because there is no medium to create differential drag. Weight differences matter only when atoms are in collision with other atoms, contributing to the mass and downward tendency of compound bodies. The weight of an atom is real but its effect on velocity is masked by the homogeneity of the void.
Bailey and others have argued more skeptically that Lucretius may simply be softening or misrepresenting Epicurus on this point.
Does Weight Scale with Size?
This is another contested question. The natural assumption is that a larger atom (more minimal parts) is also heavier. Epicurus seems to assume this, but he never makes it explicit. The problem is that shape also varies independently of size — a very intricately shaped atom might have more minimal parts than a compact spherical atom of the same overall dimensions. Weight, size, and shape are listed as independent variables in §54, which suggests they can vary independently of each other — a philosophically awkward result if weight is just a function of volume.
Summary of Constraints
| Size (μέγεθος) | Weight (βάρος) | |
|---|---|---|
| Lower bound | ≥ 1 minimal part (ἐλάχιστον) | Every atom has some weight — no weightless atoms |
| Upper bound | Must remain sub-perceptible | Implicitly bounded by size; no explicit ceiling stated |
| Source of constraint | Minimal parts theory + empirical observation | Empirical (Democritus' weightless atoms contradicted by experience of falling bodies) |
| Varies independently of other properties? | Yes (from shape) | Disputed — may scale with size or vary independently |
| Key controversy | None — fairly settled | Whether weight affects fall speed; Epicurus vs. Lucretius apparent tension |
The sources available to us are less developed than treatment of size. The minimal parts doctrine gives size a rigorous lower bound with clear mathematical structure. Weight by contrast seems to be asserted as a primitive intrinsic property largely to explain why atoms move at all — and the upper bound problem is left more implicit than resolved.
