Full Idea
When atoms are travelling straight down through empty space by their own weight, at quite indeterminate times and places they swerve ever so little from their course.
Gist of Idea
In downward motion, atoms occasionally swerve slightly for no reason
Source
Lucretius (On the Nature of the Universe [c.60 BCE], II.217)
Book Reference
Lucretius: 'On the Nature of the Universe', ed/tr. Latham,Ronald [Penguin 1951], p.66
A Reaction
Never a popular theory because it seems to breach the Principle of Sufficient Reason (Ideas 306 + 3646). This seems to be the beginning of a strong need for the concept of free will, and an underlying explanation. Most thinkers put mind outside nature.
Related Ideas
Idea 306 Nothing can come to be without a cause [Plato]
Idea 3646 There is always a reason why things are thus rather than otherwise [Leibniz]