But the Daco-Thracian theory has been challenged since the 1960s by the Bulgarian linguist Vladimir I. Georgiev and his followers. Georgiev argues, on phonetic, lexical and toponymic grounds, that Thracian, Dacian and Phrygian were completely different languages, each a separate branch of IE, and that no Daco-Thraco-Phrygian or Daco-Thracian branches of IE ever existed. Georgiev argues that the distance between Dacian and Thracian was approximately the same as that between the Armenian and Persian languages, which are completely different languages. In elaborating the phonology of Dacian, Georgiev uses plant-names attested to in Dioscorides and Pseudo-Apuleius, ascertaining their literal meanings, and hence their etymology, using the Greek translations provided by those authors. The phonology of Dacian produced in this way is very different from that of Thracian; the vowel change IE *o > *a recurs and the k-sounds undergo the changes characteristic of the satem languages. For the phonology of Thracian, Georgiev uses the principle that an intelligible placename in a modern language is likely to be a translation of an ancient name.