Plato, Laws 657c

φαίνεται οὕτως ἂν ταῦτα ἔχειν ἐκ τῶν ὑπὸ σοῦ τὰ νῦν λεχθέντων. ἆρʼ οὖν θαρροῦντες λέγομεν τὴν τῇ μουσικῇ καὶ τῇ παιδιᾷ μετὰ χορείας χρείαν ὀρθὴν εἶναι τοιῷδέ τινι τρόπῳ; χαίρομεν ὅταν οἰώμεθα εὖ πράττειν, καὶ ὁπόταν χαίρωμεν, οἰόμεθα εὖ πράττειν αὖ; μῶν οὐχ οὕτως; οὕτω μὲν οὖν. καὶ μὴν ἔν γε τῷ τοιούτῳ, χαίροντες, ἡσυχίαν οὐ δυνάμεθα ἄγειν. ἔστι ταῦτα.

Such would evidently be the case, judging from what you now say. May we confidently describe the correct method in music and play, in connection with choristry, in some such terms as this: we rejoice whenever we think we are prospering, and, conversely, whenever we rejoice we think we are prospering? Is not that so? Yes, that is so. Moreover, when in this state of joy we are unable to keep still. True.