His priority did not seem to be to teach them what he knew, but rather to impress upon them that nothing, not even... knowledge, was foolproof.

Never," said Hagrid irritably, "try an' get a straight answer out of a centaur. Ruddy stargazers. Not interested in anythin' closer'n the moon.

I don’t believe him,” said Hermione in a very unsteady voice, the moment they were out of earshot of Hagrid. “I don’t believe him. I really don’t believe him. . . .”
“Calm down,” said Harry.
“Calm down!” she said feverishly. “A giant! A giant in the forest! And we’re supposed to give him English lessons! Always assuming, of course, we can get past the herd of murderous centaurs on the way in and out! I — don’t — believe — him!

Always the innocent are the first victims, so it has been for ages past, so it is now.

I mean, you could claim that anything's real if the only basis for believing in it is that nobody's proved it doesn't exist!

Stretching one arm behind him, the man passed his hand over the horse's coat, his own skin transformed, or skin which had transformed into him. ("The Centaur")