And, as we have seen, Epicurus wrote a popular, but now lost, letter against the philosopher Stilpo -- and one of the things Stilpo was most known for was the position that only what is in the mind really matters to the wise man.
Seneca (fl. 35 CE), Letters to Lucilius, 9.18:
"[the wise man] will confine all good within himself and say what that Stilpo said – Stilpo whom a letter of Epicurus attacks -- for this man, with his country captured, his children lost, and his wife gone, yet he came out from the general destruction, alone, and yet happy: to Demetrius who was asking – who had the surname Poliorcetes from the destruction of cities – whether he had lost anything, [Stilpo] said 'all my goods are with me!' Behold a brave and vigorous man! He conquered the very victory of his enemy. 'I have lost nothing' [Stilpo] said. he made that man [i.e., Demetrius] doubt whether he had truly won, 'everything that is mine is with me – Justice, Virtue, Wisdom, and this very thing: to consider nothing good that can be taken away.'"