Strabo related, taking his text from Polybius, that "Pytheas asserts that he explored in person the whole northern region of Europe as far as the ends of the world." Strabo did not believe it but he explained what Pytheas meant by the ends of the world. "Thoulē", he said (now spelled Thule; Pliny the Elder uses "Tyle"; Vergil references "ultima Thule" in Georgic I, Line 30, where the "ultima" refer to the end of the world) is the most northerly of the British Isles. There the circle of the summer tropic is the same as the Arctic Circle (see below on Arctic Circle). Moreover, said Strabo, none of the other authors mention Thule, a fact which he used to discredit Pytheas, but which to moderns indicates Pytheas was the first explorer to arrive there and tell of it.