 Lucretius argues that is evident that animals can and do act freely. However, if atoms did not sometimes randomly swerve to the side, animals would not be able to act freely. Therefore, atoms sometimes randomly swerve to the side. In this argument, Lucretius moves from something he thinks we can observe (that animals act freely), and on its basis infers something that we cannot directly perceive (that atoms swerve slightly to the side). Free actions are ones where the body follows the mind’s desires (DRN 2.268), and free actions have an “internal source” in a literal sense for Lucretius: they are produced by the animal’s mind, which is a bodily organ in its chest.