QuestionsAre we free?The swerve: atoms deviate, so the will is free
Are we free?Free will

The swerve: atoms deviate, so the will is free

The clinamen (the swerve of the atoms)

Lucrèce c. 99 BC · c. 55 BCAtoms swerve slightly by chance (the clinamen): this is what makes the will free.
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What it claims

Atoms have, besides weight and blows, a very slight deviation (the clinamen), at no fixed place or time. It keeps the mind from being compelled from within like a conquered thing, and makes our free will possible.

Why

If atoms only fell and collided, everything would be fixed in advance; a slight swerve breaks the chain.

The same idea through history
Épicure300 BCEpicurus rejects the fate of the natural philosophers, but also refuses to make chance the origin of our actions: some things happen by necessity, others by chance, others through our own agency, and it is to the latter that praise and blame attach. The atomic swerve is attributed to him by Cicero and Lucretius; it is not in this letter.
Lucrèce55 BCLucretius shows that without a swerve atoms would fall like parallel rain and never collide: nature would create nothing. He then applies the same swerve to the free will.
In short

"The swerve: atoms deviate, so the will is free" is an argument by Lucrèce (De rerum natura II, lines 216-293 (c. 55 BC), 55 BC) on the question "Are we free?" (answer: free will). Its claim: Atoms have, besides weight and blows, a very slight deviation (the clinamen), at no fixed place or time. It keeps the mind from being compelled from within like a conquered thing, and makes our free will possible. According to AskPhilosophy's logic check (Z3 solver), the argument is valid only if one grants a premise the author leaves unstated (P4). AskPhilosophy presents 4 objections, 1 of them with a documented reply, and the original text behind each premise, checked word for word.

De rerum natura II, lines 216-293 (c. 55 BC)
Valid with an implicit premise

For the Epicureans, the world is made of atoms that fall through the void and collide. But if every motion always came from a previous motion, where would our free will come from, which drives us where pleasure calls? Lucretius answers: atoms swerve a tiny bit, at no fixed place or time. This swerve, the "clinamen", breaks the chain of fate and makes free will possible. It is the first known attempt to ground freedom in a chance event of nature itself, and it was attacked as early as antiquity.

P1We have a free will, "wrested from the fates": it makes us go forward where pleasure leads us, and turn aside from our course, not at a time or place fixed in advance.
P2It is each person's will that gives the start to motion, which then spreads through the limbs of the body.
P3Even when an outside force carries us along against our will, there is nevertheless something in our breast that can struggle against it and stand in its way.
P4 · implicit premiseThe mind and its motions are also made of atoms, subject to the same laws as everything else: there is no will outside nature. (Lucretius establishes this in book III. Cicero confirms that Epicurus feared the mind would be constrained by the motion of atoms: De fato, 23.)
P5If atoms had only weight and blows, every new motion would come from an old one, in a fixed order: cause would follow cause to infinity, and nothing would break the "covenants of fate". No motion would be a true beginning.
P6A will that gives the start cannot therefore be a mere link in that chain. We must admit in the atoms, besides weight and blows, another cause of motion, from which this power in us derives. For nothing comes from nothing.
P7This other cause can be a very small deviation: if it is, it does not contradict what we see. Heavy bodies never fall obliquely, but who can see that atoms never swerve from the straight line?
Therefore atoms have, besides weight and blows, a very slight deviation (the clinamen), at no fixed place or time. It keeps the mind from being compelled from within like a conquered thing, and makes our free will possible.

click a premise to reject it

If you accept every premise, logic forces you to accept the conclusion. So the debate is about the premises.
Checked by a logic solver: the premises cannot all be true while the conclusion is false.
Without the implicit premise: Without the hidden premise P4, one can accept everything and say: the will begins motions, but it is not made of atoms. Then the swerve is not needed. This is the position of those who, like Plato, see in the soul something other than an assembly of atoms.
The argument is valid. P1 and P2 say that the will begins motions, P4 that these motions are motions of atoms, P5 that without a swerve everything would be chained, P6 that a will that begins cannot be inside a chain. So there is a swerve, and P7 says it does not contradict experience. P3 reinforces P2 without being indispensable. The points of attack are P5 (is a swerve really needed for freedom?), P6 (is a beginning without a cause possible?) and above all the conclusion: does a swerve due to chance make us free?
Analogy. Our image, not Lucretius's. Imagine marbles rolling down a slope, each guided by the previous one: even if we ignore the start, everything is written. Lucretius adds that at the origin a tiny grain of sand can make a marble swerve, without any marble pushing it. Without that grain, no marble would begin anything.

Objections

4

Each objection is a conversation: click to see who replies and where the debate stands.

Cicero 44 BC · attacks the conclusion

Cicero, who sets out the doctrine, notes that this swerve is a motion without a cause: Epicurus is forced to admit it in fact, if not in words. A motion that appears outside weight and impact, with nothing to provoke it, therefore explains nothing.

no reply
No reply possible: objection raised in -44, after the author's death (-55).
The debate goes on (our summary) Lucretius died before the De fato: we have no reply from him. An Epicurean would say that the nature of the atom is enough to explain its swerve. Further on (De fato, 46-47), Cicero asks more questions (why so small a swerve? why only some atoms?) and concludes: "optare hoc quidem est, non disputare" (that is wishing, not arguing). The debate has stayed alive to this day. (our summary)
Carneades, reported by Cicero 44 BC · attacks premise 5

Carneades, more subtly, taught that the Epicureans could defend their case without this invented swerve. Since they hold that there is a voluntary motion of the mind, it was better to defend that than to introduce a swerve whose cause they cannot find.

no reply
No reply possible: objection raised in -44, after the author's death (-55).
The debate goes on (our summary) This objection is stronger than the first, because it proposes another route: the will would itself be a cause, with no need for a swerve. Cicero adds elsewhere (De fato, 24) that to say a motion of the mind happens "without a cause" means that it happens without an external and antecedent cause, not without any cause. This is the path later followed by defenders of "agent causation". It remains to be seen whether a will that comes from no prior cause is not also a beginning without a cause. (our summary)
David Hume 1739 · attacks the conclusion

Hume holds the liberty that removes causes to be the same thing as chance. But an action that comes from nothing lasting in the person's character does not imprint itself on her. It brings her neither honour if good nor infamy if evil: the person is not responsible for it.

no reply
No reply possible: objection raised in 1739, after the author's death (-55).
The debate goes on (our summary) Hume does not reply to Lucretius: this is the classic objection of all who see in chance the opposite of freedom. Epicurus himself writes that chance is inconstant, whereas our own actions are free. Lucretius says the swerve only serves to keep the mind from having an inner necessity that compels it (II, 289-293): he does not say it makes our acts. Many nevertheless judge that this moves the problem rather than solving it. (our summary)
The "luck" arguments (van Inwagen, Mele, Haji), summarised by O'Connor and Franklin 1983 · attacks the conclusion

A second family of arguments against free will holds that nondeterministic theories of freedom entail either that the agent lacks control over his choices or that the choices cannot be adequately explained. They are called the "Mind", "Rollback" or "Luck" arguments. This is Hume's objection revived against modern indeterminism.

Lucrèce replies
Defenders of nondeterministic theories (Clarke, O'Connor) 2005
They often reply that control is not exercised before the choice, but at the time of the choice.
The debate goes on (our summary) Quantum physics has made indeterminism respectable: it is now widely admitted that nondeterministic causation is possible, but interpretations of quantum mechanics remain debated, and nothing shows that random events in the brain are the ones that matter. Above all, an act triggered by a random quantum event would raise the same question as the clinamen: is it free, or merely due to chance? Defenders answer that freedom lies in the control exercised at the time of the choice, critics that this control remains mysterious. (our summary)
What today's voices say
Criticize it
Daniel DennettHe wants to 'confront that tradition and say it was a mistake': linking free will to indeterminism, from Democritus and the swerve of atoms to quantum theories.