Some philosophers have undertaken to write philosophy in the form of fiction, including novels and short stories (see separate article on philosophical fiction). This is apparent early on in the literature of philosophy, where philosophers such as Plato wrote dialogues in which fictional or fictionalized characters discuss philosophical subjects; Socrates frequently appears as a protagonist in Plato's dialogues, and the dialogues are one of the prime sources of knowledge about Socrates' teaching, though at this remove it is sometimes hard to distinguish Socrates' actual positions from Plato's own. Numerous early Christian writers, including Augustine, Boethius, and Peter Abelard produced dialogues; several early modern philosophers, such as George Berkeley and David Hume, wrote occasionally in this genre.