Dionysius claimed to already be an expert on philosophy and therefore turned out to be a bad pupil, Plato claims. Plato claims that Dionysius was an impostor, for he had written a metaphysical treatise that he claimed was superior to Plato's lectures. Plato can claim that Dionysius was an impostor because the truth about metaphysics cannot be expressed in writing and all those who know the truth know this; hence, if Dionysius thought he had expressed the truth about metaphysics in writing, he did not know the truth.