As may be gathered from the words of Dionysius (Div. Nom. iv), beauty or comeliness results from the concurrence of clarity and due proportion. For he states that God is said to be beautiful, as being "the cause of the harmony and clarity of the universe." Hence the beauty of the body consists in a man having his bodily limbs well proportioned, together with a certain clarity of color. On like manner spiritual beauty consists in a man's conduct or actions being well proportioned in respect of the spiritual clarity of reason.
Thomas Aquinas, Summa Theologiae 2-2.145.2