Now, the end of our desires is God; hence, the act whereby we are primarily joined to Him is basically and substantially our happiness. But we are primarily united with God by an act of understanding; and therefore, the very seeing of God, which is an act of the intellect, is substantially and basically our happiness.
Quodlibetal Questions (c.1256) vol. 8, bk. 9, pt. 19 (translated by Bourke)