I don't buy Thoreau's premise actually, much too narrow a definition for me.

Understand is a reasonable word though. Using the McCurry example I think he understands his subject matter, he combines elements that "belong" to the same subject matter, say India, but that might not normally cross paths, like a particularly photogenic person being in a particularly photogenic setting. McCurry's art, his act of improving upon nature, IMO is in combining the all elements required into a single frame.