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.