Reading the Commentary on the Sentences is like reading another mode of St. Thomas: he still speaks as an authoritative master, but he is speaking in a more youthful and ecstatic key. He more emphasizes the role of the divine light as the source of theological truth, and he gives a striking proof for the existence of God from beauty.
The fourth [road to seeing the existence of God] is through eminence in knowledge. In everything in which is found a more and less beautiful, there is found some principle of the beauty, and something is called beautiful by its nearness to it. Now we see that a body is beautiful in a sensible species [or as sensible]; a spirit more beautiful in its intelligible species [or as intelligible]. Therefore it is necessary that there be something by which both are beautiful, to which created spirits more draw near to.
Quarta sumitur per eminentiam in cognitione, et est talis. In quibuscumque est invenire magis et minus speciosum, est invenire aliquod speciositatis principium, per cujus propinquitatem aliud alio dicitur speciosius. Sed invenimus corpora esse speciosa sensibili specie, spiritus autem speciosiores specie intelligibili. Ergo oportet esse aliquid a quo utraque speciosa sint, cui spiritus creati magis appropinquant.
St. Thomas here uses “speciosus” for beautiful, which seems to more emphasize the visible character of beauty. Had he used “pulcher” this proof would probably be better known, not because it would change the sense (“pulcher” and “speciosus” are synonyms), but simply because more people would have found it in word searches for “beautiful” in St. Thomas.