 According to the Relational Event Theory, sounds are events which involve both the source and the surrounding medium. They are relational rather than “monadic” events. (The distinction between monadic and relational events is not to be taken as cast in iron, since the latter can be reduced to the former by making the mereological sum of sources and surrounding medium the subject of sounds.) O’Callaghan (2002, 2007) has developed such a view at some length. He notes that the wave conception of sounds is not the only possible interpretation of Aristotle’s remarks about sounds in On the Soul. Aristotle writes that “everything that makes a sound does so by the impact of something against something else, across a space filled with air” (De Anima II.8 420b15). On O’Callaghan’s view, what Aristotle might have meant is that the sound itself is not a movement of the air, it is rather the event in which a vibrating object disturbs a surrounding medium and sets it “moving”. Waves in the medium are not the sounds themselves, but rather the effects of sounds. According to the Relational Event Theory, sounds are “disturbings” of a medium, hence depend existentially on a medium that is disturbed and that will transmit information to a listener. This account differs from the Located Event Theory insofar as the latter allows for sounds that exist in a vacuum, and thus distinguishes between a medium that hosts the vibration and a medium that transmits the vibration.