"Though the dungeon, the scourge, and the executioner be absent, the guilty mind can apply the goad and scorch with blows"

This quote by Lucretius suggests that even without physical penalty or consequences, a guilty conscience can be just as excruciating as being in a dungeon, getting a scourging, or dealing with an executioner. The guilt and regret felt by an individual can be a form of self-punishment that is simply as mentally unpleasant as physical punishment.