The aim of the article is twofold: (i) to document how what the author labels the "Principle of Reference" - viz. the claim that that which is not cannot be referred to - inspires both actualist and possibilist philosophical conceptions in the analytic tradition as well as in scholasticism, and (ii) to show how Duns Scotus's rejection of the Principle allows us to see that there are two distinct and logically independent meanings of the actualism-possibilism distinction: viz. metaphysical actualism/...possibilism, and semantic actualism/possibilism. By way of an appendix, the author offers some critical remarks on recent Czecho-Slovak debates about the ontological status of non-existents.