In this note, we present a new observation of relevance to Aquinas’s third way. Scholars have noted that Aquinas recognizes the existence of a multiplicity of necessary beings, but it has not been recognized that Aquinas’s views concerning the eternality of the world commit him to the epistemic possibility that the world itself is a necessary being. We explain how Aquinas is committed to this possibility and explore its bearing on the success or failure of the third way as a demonstration of God’s existence.