Hi!
In my school Latin and Classics education, I was taught that the Caledonian chieftain Calgacus was an invention of Tacitus, either completely (ie, he never existed) or in part (his famous speech). What is the evidence Tacitus made him up?
His speech, from Agricola 30:
Whenever I consider the origin of this war and the necessities of our position, I have a sure confidence that this day, and this union of yours, will be the beginning of freedom to the whole of Britain. To all of us slavery is a thing unknown; there are no lands beyond us, and even the sea is not safe, menaced as we are by a Roman fleet. And thus in war and battle, in which the brave find glory, even the coward will find safety. Former contests, in which, with varying fortune, the Romans were resisted, still left in us a last hope of succour, inasmuch as being the most renowned nation of Britain, dwelling in the very heart of the country, and out of sight of the shores of the conquered, we could keep even our eyes unpolluted by the contagion of slavery. To us who dwell on the uttermost confines of the earth and of freedom, this remote sanctuary of Britain's glory has up to this time been a defence. Now, however, the furthest limits of Britain are thrown open, and the unknown always passes for the marvellous. But there are no tribes beyond us, nothing indeed but waves and rocks, and the yet more terrible Romans, from whose oppression escape is vainly sought by obedience and submission. Robbers of the world, having by their universal plunder exhausted the land, they rifle the deep. If the enemy be rich, they are rapacious; if he be poor, they lust for dominion; neither the east nor the west has been able to satisfy them. Alone among men they covet with equal eagerness poverty and riches. To robbery, slaughter, plunder, they give the lying name of empire; they make a solitude and call it peace
You have to understand the conventions of Greek and Roman historiography.
From Thucydides onwards, the speech delivered by the general or statesman became one of the premier methods in which ancient historians outlined their protagonists' motivation and character. But these speeches were not based on any kind of stenographic record or eye-witness account, (indeed, in many cases no such thing would exist or be available to the historian) but would be made up by the writer himself. As Thucydides put it:
"I have put into each speaker's mouth sentiments proper to the occasion, expressed as I thought he would be likely to express them."
We shouldn't forget either what the purpose of ancient history was: rather than merely understanding the past, it sought to teach lessons. Particularly, moral lessons. Tacitus stands as one of the great espousers of this tradition, and the speech you quote above is a premier example. Speeches were one of the best tools available to these writers to both add to the rhetoric impact of their work, and to clearly outline the moral lessons they sought to teach.
So, in short, we know the speech you quote was made up by Tacticus because every speech in every ancient history is made up by the historian, for the edification of the historian's audience.
(Edit: The passage below is based on me being dumb and mixing my British chieftains up. See further down the thread for the actual answer.)
As for Caratacus himself, though: I am not aware of any historians claiming he is fictional altogether. He is attested by both Tacitus and Dio, he was taken to Rome and was pardoned by Claudius. (So even if he didn't give the speech Tacitus wrote above, he may well have said something impressive and eloquent.) Grabbing a random book off my shelves, in this case Peter Salway's "Roman Britain" (1993), I see him being treated as a historical figure without any caveats or discussion.
Do you know what your teacher or lecturer based his assertion on?
(End offending passage.)
All that said, I do rather share your disappointment: that speech is one of the single most powerful pieces of ancient rhetoric, and a chilling statement on imperialism that seems equally relevant now as it did back then. It would have been nice to think of someone standing up to the Romans with these words as his rallying cry. On the other hand, it's almost equally interesting to consider that it was a Roman historian who wrote these words, and examine his highly complicated attitude towards his own people and their ambition.