But it's really just poison with a sweet perfume. To say with Hume that "beauty in things exists merely in the mind which contemplates them" is to say that there is nothing we call beauty which exists as an independent, objective quality or attribute of a thing. Which is another way of saying that God (being transcendent) is not beautiful, although some of us may have beautiful feelings about Him.
Beauty, like truth, is a transcendental thing. And all Beauty, all Truth, all Goodness, and perfect Unity is found in God. No reasonable person defends himself against the charge of bearing false witness with the argument that "truth is in the mind of the thinker." No parent, telling his child to "be good" would consider that "Goodness is whatever the child thinks it is." So also with Beauty. It's no good trying to discuss whether something is beautiful with statements such as, "I like it," or, "it moves me," or, "I think it's pretty."
To the extent that things conform to and reflect the Beauty of God, they are beautiful. Thus, the criteria for judging Beauty are found in God Himself. God is a God of order. Therefore, disorderly things are not beautiful. God is not an abstraction; He is real. It could even be said that He is Reality. So we have the principle that abstract "art" is not beautiful. (Although, to be fair, the purpose of the artist in producing abstract art is not to make something beautiful, although he is presumably trying to communicate something about another transcendental quality.)
For another time: Beauty & Architecture, Beauty & Liturgical Music