  To illustrate this point Descartes appeals to divine omnipotence. He thinks that we cannot conceive an omnipotent being except as existing. Descartes’ illustration presupposes the traditional, medieval understanding of “necessary existence.” When speaking of this divine attribute, he sometimes uses the term “existence” simpliciter as shorthand. But in his more careful pronouncements he always insists on the phrase “necessary and eternal existence,” which resonates with tradition. Medieval, scholastic philosophers often spoke of God as the sole “necessary being,” by which they meant a being who depends only on himself for his existence. This is the notion of “aseity” or self-existence (a se esse). Since such a being does not depend on anything else for its existence, he has neither a beginning nor an end, but is eternal. Returning to the discussion in the First Replies, one can see how omnipotence is linked conceptually to necessary existence in this traditional sense. An omnipotent or all-powerful being does not depend ontologically on anything (for if it did then it would not be omnipotent). It exists by its own power: