"That," said Firenze calmly, "is human nonsense.....Trivial hurts, tiny human accidents....We watch the skies for the great tides of evil or change that are sometimes marked there. It may take ten years to be sure of what we are seeing."

....It was the most unusual lesson Harry had ever attended...Firenze told them to look for certain shapes and symbols in the pungent fumes, but he seemed perfectly unconcerned that not one of them could see any of the signs he described, telling them that humans were hardly ever good at this, that it took centaurs years and years to become competent, and finished by telling them that it was foolish to put too much faith in such things anyway, because even centaurs sometimes read them wrongly....His priority did not seem to be to teach them what he knew, but rather to impress upon them that nothing, not even centaurs' knowledge, was foolproof.

J.K. Rowling. HARRY POTTER AND THE ORDER OF THE PHOENIX.