The debate concerning whether Aquinas changed his view on the perfection of the separated soul’s natural cognition, i.e., the cognition not elevated by God’s supernatural grace, remains to be settled. Anton Pegis has argued that according to Aquinas’s earlier view, a separated soul ‘will function perfectly as a separate substance’. However, according to Aquinas’s mature view, it ‘does not have the intellectual power to function properly as a separate substance’, so Aquinas changed his view. In contrast, John Wippel has denied that Pegis showed any radical change in Aquinas’s works on this issue. In this paper, I intend to advance this debate by arguing that, in his Commentary on Sentences IV (1252–56) d. 49, q. 1, a. 4, qc1, Aquinas expounded a view on the perfection of the separated soul’s natural cognition that is incompatible with his view in Summa Theologiae I (1265–68), q. 89, a. 1, co. I will also identify two markedly different views on the implication of the human soul-body union for the perfection of the separated soul’s cognition in these two texts. I contend that the incompatibility between the views in these two texts suggests that Aquinas changed his view.