Colloquium presented at The Cave Wall, a student-run philosophy club at Stellenbosch University, covering the modal ontological argument. Given the diverse interdisciplinary audience, I do not assume familiarity with formal logic. Abstract: In Prologion III, Anselm presents a famous ontological argument for the existence of God. The argument assumes possibility of the existence of a being whose very existence is necessary, and from this possibility alone concludes the existence of such a being. On first reading, this argument may appear abstract and unconvincing - in particular, one may doubt whether the conclusion indeed follows from the premiss. Worse still, it is equally difficult to convincingly rebut this argument without first making very precise several hidden assumptions about the modalities of “necessity” and “possibility” upon which the argument depends. In this talk, I introduce one framework in which such alethic modal reasoning may be formalised, namely the S5 modal logic. I then formally prove Anselm’s argument in S5, and discuss the implications for the forcefulness of the argument. The main reference for this talk is Philosophical and Mathematical Logic by De Swart (2018).