Found in the third volume of the Oxford Studies in Early Modern Philosophy (page 91-118), this essay by Prof. Richard A. Lee, Jr. (Philosophy, DePaul) is a very informative piece that traces the scholastic arguments of causa sui that would eventually lead to its application to God by Descartes - and Spinoza. But whereas Spinoza does not seem to have the need to elaborate much on his application of causa sui to his conception of God, Descartes on the other hand appears to have taken no small effort in trying to present his case to an audience of scholastic tradition that holds largely the opposite view. Here is the Introduction section of the essay (footnotes omitted):
The debate between Carterus and Arnauld on one side and Descartes on the other about whether God is causa sui has been seen as marking the distinction between scholastic and modern thought. This debate, which arises initially out of an issue that is not even central to Descartes's proof for the existence of God, has Descartes insisting that we must understand God as causa sui in the positive sense that God's essence is something like an efficient cause of God's existence, and Arnauld and Caterus, scholastic as they are, insisting that no philosopher holds such an incomprehensible position.
Caterus and Arnauld stand in a long tradition, stemming all the way back to Aristotle, that denies the possibility that something can be be the cause of itself. This tradition, as will be shown, has two main features. First, it denies the possibility that something can move itself, in the sense of either local motion, alteration, or augmentation. Second, it denies the specific notion that something can move itself as an efficient cause of itself because that would require a thing to be in potency and in actuality at the same time with regard to the same thing. However, beginning at least with Duns Scotus, scholastic philosophers did argue that God's being must be a se or ex se, i.e. from itself. While many would maintain, as does Caterus, that this is to be understood negatively in that God's existence needs no other cause besides the divine essence, the question that Descartes raises is, How do we give an account of that very fact? Descartes's insistence is that a thorough proof for the existence of God demands that one supply a reason or cause for God's existence, or a reason or cause why God needs no such cause. God is from Godself because God has an infinite power that entails that nothing could be repugnant to that existence.
In what follows, I would like to look at the classic scholastic rejection of causa sui. I will use Scotus and Aquinas as emblematic of these arguments, and I will trace their influence on Suárez's understanding of how God is a se. I am not merely interested in the rejection of the notion. Rather, I want to show how medieval thinkers, primarily Aquinas and Scotus, seem to require a concept very much like causa sui in order to bring together God's existence with God's infinity especially when treating the power of God that grounds God's independence. While I am primarily interested in showing that Scotistic origins for Descartes's ability to posit that God is causa sui, I will first set the stage by addressing Aquinas' arguments against the notion of causa sui within his proof for the existence of God in Summa contra gentiles. I want to pay attention to two features of these discussions: first, while medieval thinkers (and here Aquinas and Scotus are particularly instructive examples) roundly denied the cogency of the notion causa sui, they developed the means that Descartes will use in his positing of God as causa sui. Second, I want to show how those concepts that Descartes mobilizes in making sense of causa sui were developed not in proofs for the existence of God based on causation, but with reference to God's infinity, power, and perfection. What I hope to show is that Descartes is able to argue that God is causa sui, precisely because his argument turns on the notions of infinity and infinite perfection, and that these are thought by him to be related to power. This same kind of argument is found in Scotus ( and, to a certain extent in Suárez), where we also have a proof for the existence of God that is both ontological (proving the existence of something from its mere concept) and cosmological (proving the existence of a first cause). It is this kind of argument that allows Descartes to make sense of the concept of causa sui in a way that medieval thinkers could not.
Do read the whole thing.