 The Stoics want to place above this [the existent] yet another, more primary genus…. Some Stoics consider “something” the first genus, and I shall add the reason why they do. In nature, they say, some things exist, some do not exist. But nature includes even those which do not exist—things which enter the mind, such as centaurs, giants, and whatever else falsely formed by thought takes on some image despite lacking substance. (Seneca, Letters 58:13–15, quoted in Long &amp; Sedley 1987: 162; see also Caston 1999.)