Around 600 BC, the inhabitants of Massilia (Marseille, France), a Greek colony, referred to the people of Provence using the terminology ‘Keltoi’. Later, the Romans would ‘latinise’ the name: ‘Celtae’ (Celts). They also used this name for all populations north of Provence (including the Alps).
Herodotus refers to them in his book Histories.
‘This latter river has its source in the country of the Celts near the city Pyrene, and runs through the middle of Europe, dividing it into two portions. The Celts (Ed. Greek Keltoi) live beyond the Pillars of Hercules, and border on the Cynesians, who dwell at the extreme west of Europe. Thus the Ister flows through the whole of Europe before it finally empties itself into the Euxine at Istria, one of the colonies of the Milesians.’
The Ister is an ancient name for the Danube. The Cynesians were a population of the Iberian Peninsula. Herodotus does not equate them with the Celts. Pyrene is a city unknown to archaeology. Herodotus specifies that the Danube ‘begins’ near this city. Either he is referring to its source in the Black Forest (Germany), or to a seaside city on the Black Sea where the river flows (on the present territory of Bulgaria).
The Romans did not equate Belgians and Germen with Celts. When confronted with the Belgian populations of Britain, they would not equate the other peoples of the island with Celts either. This confusion may date from the Germanic invasions of Britain. Faced with a multitude of local populations, the Germans probably adopted a generic term for them.
No inscriptions from before the Roman colonisation of Gaul and the island of Britain mention Celts. During the ‘Gallic’ war, the Romans distinguished three Gauls: Belgium, Celtic and Aquitaine. Who were these Keltoi of Provence? What is the origin of the Gauls of Roman Celtic? What is the link between the Keltoi and the Gauls?
Linguistics is of little help as the root ‘kelt’ is unknown to Indo-European languages and the Celtic peoples had an oral, not written, tradition.