In 1973, he proposed the glottalic theory regarding the reconstruction of the Proto-Indo-European consonant inventory, in parallel with the Georgian linguist Tamaz Gamkrelidze and the Russian linguist Vyacheslav Ivanov.
En 1973, propuxo a teoría glotálica que busca a reconstrución do sistema consonántico protoindoeuropeo, en paralelo co lingüista xeorxiano Tamaz Gamkrelidze e o ruso Vyacheslav V. Ivanov.