Pretty sure I already tumbled this, but hey..
His definition of ordinal numbers (published when he was 20) is the one that is now universally adopted. His Ph.D. dissertation was about set theory too; his axiomatization has left a permanent mark on the subject. He kept up his interest in set theory and logic most of his life, even though he was shaken by K. Godel’s proof of the impossibility of proving that mathematics is consistent. He admired Godel and praised him in strong terms: “Kurt Godel’s achievement in modern logic is singular and monumental - indeed it is more than a monument, it is a landmark which will remain visible far in space and time. … The subject of logic has certainly completely changed its nature and possibilities with Godel’s achievement.” In a talk entitled “The Mathematician”, speaking, among other things, of Godel’s work, he said: “This happened in our lifetime, and I know myself how humiliatingly easily my own values regarding the absolute mathematical truth changed during this episode, and how they changed three times in succession!”