In the article Beau of the French Encyclopédie, mention is made of a treatise on the beautiful, by St Augustine, which is now lost. Some idea, however, we are told, may be formed of its contents from different passages scattered through his other writings. [Stewart in note: "Augustin [sic], in his Confessions, records the purport of his treatise, De Apto et Pulchro"] The idea here ascribed to St Augustine amounts to this, that the distinctive character of beauty is, that exact relation of parts of a whole to each other, which constitutes its unity.
Even in the works of nature, one of the chief sources of their Beauty to a philosophical eye, is the Unity of Design which they everywhere exhibit. -On the mind of St Augustine, who had been originally educated in the school of the Manicheans, this view of the subject might reasonably be expected to produce a peculiarly strong impression.
Dugald Stewart, vol 5 Collected Works, pp.453-4, 'Note QQ, (p358), Essay III, chap. 3 -The Beautiful and St Augustine' (1855) here
I excerpt this with no guarantee that it is an adequate representation of St Augustine's views (this seems to be the best way to explore that topic), but rather because I find it interesting in two ways:
a) as a matter of the history of ideas, it suggests the close link between the idea of God and the idea of the (albeit limited) rational comprehensibility of the universe that is key to a lot of Stewart's thought: the task of the philosopher is simply to discern the rules which God, as designer, has laid down without expecting fully to be able to discern the reasoning behind those rules. God as designer and knowledge as the discernment of the pattern of that design are fundamental to his thought. (So yet again, the importance of religion to the Scottish 'Enlightenment' is clear.)
b) as a matter of philosophy, it suggests the way that the arguments for the existence of God in natural theology need to be read in both directions: that (eg) the argument from design not only shows the existence of God, but the existence of God shows the correct way of seeing the world -of seeing it as designed/beautiful/unified.