This monograph studies the parable as a literary construct. It addresses the question why the ancients understood parables as mysterious speech. The study disputes the binary opposition of (clear) parables and (obscure) allegories. Defining allegory as an extended metaphor in narratory form, it argues that many parables are allegories. The parable is defined as narrative in form; tropical in mode of meaning; religious or ethical in genre; and rhetorical in purpose, intended to persuade. Requiring (sometimes challenging) interpretation, the parable can rightly be considered "mysterious." The monograph then discusses the parable in the context of Mark's theme of mystery.