Seneca, Letters to Lucilius
9.20-21 (tr. Richard M. Gummere):
He [Epicurus, fragment 474 Usener] says: "Whoever does not regard what he has as most ample wealth, is unhappy, though he be master of the whole world." Or, if the following seems to you a more suitable phrase,—for we must try to render the meaning and not the mere words: "A man may rule the world and still be unhappy, if he does not feel that he is supremely happy." In order, however, that you may know that these sentiments are universal, suggested, of course, by Nature, you will find in one of the comic poets this verse [Ribbeck, Comicorum Romanorum ... Fragmenta, 3rd ed., p. 147, no. LXIV]:
Unblest is he who thinks himself unblest.
"si cui," inquit, "sua non videntur amplissima, licet totius mundi dominus sit, tamen miser est." vel si hoc modo tibi melius enuntiari videtur,—id enim agendum est, ut non verbis serviamus, sed sensibus,—: "miser est, qui se non beatissimum iudicat, licet imperet mundo." ut scias autem hos sensus esse communes, natura scilicet dictante, apud poetam comicum invenies:
non est beatus, esse se qui non putat.
Buecheler thought he saw a Greek original behind the comic verse and suggested:
μακάριος οὐδεὶς ὅστις οὐχ αὑτῷ δοκεῖ.