He has tried to reconstruct the alleged Persian-Kurdish-Baluchi linguistic unity presumably in the central parts of Iran. According to Mackenzie's theory, the Persians (or Proto-Persians) lived in the province of Fars in the southwest (proceeding from the assumption that the Achaemenids spoke Persian), the Baluchis (Proto-Baluchis) lived in the central areas of Western Iran, and the Kurds (Proto-Kurds), in the wording of G. Windfuhr (1975: 459), lived either in northwestern Luristan or in the province of Isfahan. Kurdish is divided into three groups, where dialects from different groups are not mutually intelligible without acquired bilingualism. In historical evolution terms, Kurmanji is less modified than Sorani and Pehlewani in both phonetic and morphological structure. The Sorani group has been influenced by among other things its closer cultural proximity to the other languages spoken by Kurds in the region including the Gorani language in parts of Iranian Kurdistan and Iraqi Kurdistan. The Kermanshahi group has been influenced by among other things its closer cultural proximity to Persian. Philip G. Kreyenbroek, an expert writing in 1992, says:  According to "Encyclopaedia of Islam", although Kurdish is not a unified language, its many dialects are interrelated and at the same time distinguishable from other Western Iranian languages. The same source classifies different Kurdish dialects as two main groups, northern and central. The reality is that the average Kurmanji speaker does not find it easy to communicate with the inhabitants of Sulaymaniyah or Halabja. Some linguistic scholars assert that the term "Kurdish" has been applied extrinsically in describing the language the Kurds speak, whereas some ethnic Kurds have used the word term to simply describe their ethnicity and refer to their language as "Kurmanji", "Sorani", "Hewrami", "Kermanshahi", "Kalhori" or whatever other dialect or language they speak. Some historians have noted that it is only recently that the Kurds who speak the Sorani dialect have begun referring to their language as "Kurdî", in addition to their identity, which is translated to simply mean Kurdish. Mokriani dialect of Central Kurdish is widely spoken in Mokrian. Piranshahr and Mahabad are two principal cities of the Mokrian dialect area.