The modern Irish Éire evolved from the Old Irish word Ériu, which was the name of Ireland and of a Gaelic goddess. Ériu is generally believed to have been the matron goddess of Ireland, a goddess of sovereignty, or simply a goddess of the land. Ériu has been derived from reconstructed Archaic Irish *Īweriū, and further from the Proto-Celtic *Φīwerjon- (nominative singular Φīwerjū). This suggests descent from the Proto-Indo-European *piHwerjon- ("fertile land" or "abundant land"), from the adjective *piHwer- ("fat") – cognate with Ancient Greek píeira and Sanskrit pīvarī, ("fat, full, abounding"). The Archaic Irish form was borrowed into Ancient Greek. During his exploration of northwest Europe (circa 320 BCE), Pytheas of Massilia called the island  Ierne (written ). In his book Geographia (circa 150 CE), Claudius Ptolemaeus called the island  Iouernia (written ; ou represented /w/) and named a tribal group called the (,  or Iverni who lived in the southwest. This was borrowed into Latin as Hibernia.