Full Idea
The swerve of Epicurus takes place without a cause; it does not take place in consequence of being struck by another atom, since how can that take place if they are indivisible bodies travelling perpendicularly in straight lines by the force of gravity?
Gist of Idea
Atoms don't swerve by being struck, because they move in parallel, so the swerve is uncaused
Source
comment on Epicurus (fragments/reports [c.289 BCE]) by M. Tullius Cicero - On Fate ('De fato') 10.22
Book Reference
Cicero: 'On Fate, Stoic Paradoxes, Oratory', ed/tr. Rackham,H. [Harvard Loeb 1942], p.219
A Reaction
The swerve is the most ad hoc proposal in the history of theoretical physics. This is interesting for spelling out that the travel in vertical parallels. What's that all about, then?