Symbolism, Imagery, Allegory
You know how Hume sees sentiment as part of our nature? Well, in his view, that's why some images are immediately pleasing to us.
Say, for instance, that we enter a warm apartment that's in a neat location and arranged in a way that's well-suited to its purpose. Hume believes that this instantly transmits pleasant emotions to us. And if the landlord's friendly and good-natured? Even better. This isn't just about reason or intellect. It's a natural response. The same applies in reverse: if the doors and windows of a building aren't designed in a way that's suited to human use, we see them as ill-fitting. Check out some of these designs. The Extreme Makeover team would have their hands full.
To back up his point, Hume refers to the way in which architects describe a building. They might go into all the proportions of the base, the pillars, blah, blah, blah, but this doesn't explain the beauty of the building. As Hume sums up:
The beauty is not in any of the parts or members of a pillar, but results from the whole, when that complicated figure is presented to an intelligent mind, susceptible to those finer sensations. Till such a spectator appear, there is nothing but a figure of such particular dimensions and proportions. (AI.15)
Hume's not arguing that details aren't important or necessary. His point is that these can't account for the appeal of a building. This depends on the presence of a human being who feels a pleasing (or not so pleasing) sentiment when they look at it. Hume says it best when he declares:
Propositions in geometry may be proved, systems in physics may be controverted; but the harmony of verse, the tenderness of passion, the brilliancy of wit, must give immediate pleasure. (I.5)
There you have it folks, straight from the horse's mouth.