According to George Cœdès, the word ''Thai'' () means 'free man' in the Thai language, "differentiating the Thai from the natives encompassed in Thai society as serfs". A famous Thai scholar argued that Thai () simply means 'people' or 'human being', since his investigation shows that in some rural areas the word "Thai" was used instead of the usual Thai word ''khon'' () for people. According to Michel Ferlus, the ethnonyms Thai-Tai (or Thay-Tay) would have evolved from the etymon ''*k(ə)ri:'' 'human being' through the following chain: ''*kəri:'' > ''*kəli:'' > ''*kədi:/*kədaj'' > ''*di:/*daj'' > ''*dajA'' (Proto-Southwestern Tai) > ''tʰajA2'' (in Siamese and Lao) or > ''tajA2'' (in the other Southwestern and Central Tai languages classified by Li Fangkuei). Michel Ferlus's work is based on some simple rules of phonetic change observable in the Sinosphere and studied for the most part by William H. Baxter (1992).