His doctoral dissertation at Harvard University, Indo-European Origins of the Celtic Verb I. The Sigmatic Aorist (Dublin Institute for Advanced Studies, 1962), which deeply reflected the structuralist approach of Jerzy Kuryłowicz, opened a fresh era of creative work in Celtic comparative linguistics and the study of the verbal system of Indo-European languages.
A súa tese de doutoramento, Indo-European Origins of the Celtic Verb I. The Sigmatic Aorist (Dublin Institute for Advanced Studies, 1962), que reflectía as ideas estruturalista de Jerzy Kuryłowicz, foi a base dunha nova perspectiva na lingüística comparada das linguas célticas e ao estudo do sistema verbal das Linguas indoeuropeas.