He defended his doctoral thesis at the University of Toulouse on the subject of Chinese congregations in French Indochina (published in 1941 by Sirey with a preface by Paul Couzinet).

He worked as a professor at the University of Toulouse from 1948 to 1954, then at the Faculty of Law of Caen from 1954 to 1966, finally at that of Paris from 1966 to 1976, where since 1952 he had been in charge of a course on the rights of the peoples of Indochina.

[1] In parallel to his teaching career, Định was politically active in the early fifties, where he was Minister of Foreign Affairs of Vietnam (1954) in the last phase of the regime of Emperor Bao Dai.

[1][3] It is therefore not a revolutionary scientific thought that is expressed in the restrictive framework of its license course included in the first edition of the Public International Law textbook, at the end of the ten years of presentation of this course to the University of Paris.

Its rejection of voluntarism as the basis of the obligatory nature of international law explains certain solutions deemed traditionalist, for example the opposability of existing general customs.