The famous theologian Thomas Aquinas reported that he learned from Ibn Sīnā that everything that is in a genus must have a distinction between its essence and its existence. Despite the essence-existence distinction being one of Ibn Sīnā’s most well-known doctrines, no one has identified where Aquinas might have found this doctrine in Ibn Sīnā. Indeed, there is little agreement among scholars concerning where Ibn Sīnā proves this doctrine and to what this distinction amounts. Therefore, in an effort to increase our knowledge of Ibn Sīnā’s essence-existence distinction, its nature, its place in his metaphysical system, and the various ways Ibn Sīnā tries to prove it, I analyze three notable arguments for the distinction in Ibn Sīnā’s Dāneshnāme-ye ʿAlā’ī. I argue that these arguments should be included in the scholarly catalogue of Ibn Sīnā’s arguments for the essence-existence distinction, that they are the source of Aquinas’s Genus Argument for the essence-existence distinction, and that they reveal that Ibn Sīnā’s essence-existence distinction follows from his understanding of the ten categories of being.