Atomic swerve

The Atomic Swerve is a fundamental principle of Epicurean physics: it suggests that, as atoms travel down through the void that contains them, they swerve minimally from their course, lest they all remain isolated, and never meet to form the complexity of the universe.

This principle may have been the Epicurean counter-argument to the Democretian or Stoic view that everything moves in a pre-ordained, inexorable, inescapable chain of cause and effect. It is evident that Epicurus often had the Presocratics in mind when arguing against the "natural philosophers". But the absence of any mention of the swerve in any of the fully extant works of Epicurus may be an indication that it was formulated very late in his life, perhaps in response to a Stoic challenge.

The main ethical conclusion that Epicurus drew from the atomic swerve is the existence and operation of human free will (Ref: On Choices and Avoidances), whereas the Stoics emphasized compliance with fate -- by definition immutable by human will and choice.

Yet human choice need not, nor can it be defended on grounds of atomic motion.

It is also plausible that Epicurus may have argued by analogy (i.e. atomic swerve is to matter, as choice is to human action) and not necessarily believed that atomic swerve somehow caused human free will.