Θραϊκῶν δὲ τῶν πάντων οὐδένες πλείους εἰσὶ τῶν ἀνθρώπων ὅτι μὴ Κελτοὶ πρὸς ἄλλο ἔθνος ἓν ἀντεξετάζοντι, καὶ διὰ τοῦτο οὐδεὶς πω πρότερος Θρᾶϊκας Ῥωμαίων κατεστρέψατο ἀθρόους· Ῥωμαίοις δὲ Θράϊκη τε πᾶσά ἐστιν ὑποχείριος, καὶ Κελτῶν ὅσον μὲν ἀχρεῖον νομίζουσι διά τε ὑπερβάλλον ψῦχος καὶ γῆς φαυλότητα, ἑκουσίως παρῶπταί σφισι, τὰ δὲ ἀξιόκτητα ἔχουσι καὶ τούτων.
(Pausanias 1.9.5)
If you measure one nation against another no people among mankind are as numerous as all the Thracians, except for the Celts; this is why no one before the Romans ever subdued the whole of Thrace. But all Thrace is in the hands of Rome. Part of the Celtic country Rome considers useless because of extreme cold and poverty of soil, and deliberately overlooks it, but what the Celts had worth having belongs to Rome. (tr. Peter Levi)
Crambe Repetita 