This essay examines Duns Scotus’s celebrated modal argument for the existence of a first cause in the light of his most extensive discussion of modality: namely, the account of the senses of ‘potency’ in his questions on Aristotle’s Metaphysics, book IX, qq. 1–2. The author holds that it is possible to give two alternative reconstructions of Duns Scotus’s argument for the existence of a first cause depending on which of two alternative interpretations is given to the term ‘potency’. First, ‘potency’ can be taken as what is metaphysically possible. In this interpretation, the potential is co-extensive with ‘being’. Second, ‘potency’ can be taken to mean what is opposed to the actual. In this second interpretation, being in potency is a kind of non-being. The conclusion is that, contrary to what might first appear, it is the second interpretation of ‘potency’ that should be preferred if we want Duns Scotus’s argument for the existence of a first cause to work.