Existence and uniqueness are standard questions in cases where definite descriptions are used. In his Proslogion Anselm of Canterbury uses definite and non-definite descriptions of God: He is “id/aliquid quo maius cogitari non potest” (and similar). While Anselm’s proof for the existence of God is widely discussed, including its relations to those famous descriptions, this is not the case for the question of uniqueness. Is there at most one perfect being or might there be more than one? ‘Of course there can at most be one’, one might want to answer – but what reasons for this claim can be found in the Proslogion? In this celebrated work of philosophical theology, Anselm does not prove uniqueness explicitly. But one may try a proof “in the spirit of the Proslogion”.