 The Herculaneum library contains much work of the Epicurean Philodemus (1st c. BCE). Philodemus wrote extensively, including on the history of philosophy, ethics, music, poetry, rhetoric and the emotions. He wrote a treatise on the theory of signs: because they are empiricists, believing that all knowledge comes from our sense experience, later Epicureans were concerned about the basis for our knowledge of imperceptibles like the atoms, and engaged in an extensive debate with the Stoics about the grounds for inferences to imperceptible entities.