Beauty has always been a problematic topic for philosophers. It is a concept that provokes pleasure and satisfaction (see the famous sigh that some people make in front of works of art), but it can also be subject to criticism or even destruction — as with the French Revolution’s rejection of Rococo style paintings, which were hedonist expressions of wealth, or the destruction of beauty in a modern context of war, censorship and pollution. It seems to be a very unstable concept, shifting with fashion and cultural contexts, and it can be difficult to define.
Philosophers have tried to give beauty some stability by linking it to a specific sort of pleasure. The ancient Greeks were particularly interested in the pleasurable experiences that beautiful things must evoke. The classic conception of beauty was that a thing is beautiful when its parts are in the right proportion to each other, forming an integrated harmonious whole. This can be seen in the symmetry of human bodies or in the proportions of a musical composition or piece of architecture. It is this harmony that evokes the pleasures of beauty, according to the ancient Greeks, and so it is the basis of most philosophical treatments of the subject.
This idea of beauty was Christianized by Thomas Aquinas, who connected the beauty of objects to the Second Person of the Trinity. Aquinas gives three requirements for beauty: integrity, conformity to the laws of nature, and due proportion or consonance. He also adds that beauty must be clear and radiant.
Another approach to beauty is that it is a subjective experience of pleasure. This view, which is sometimes associated with George Santayana, holds that beauty is a sort of pleasure that we feel when we perceive a beautiful object. But this claim is based on an error: it ascribes subjective states to objects that in fact are not capable of having them.
For example, a work of art is not beautiful to someone with color blindness, and the same object can be perceived in different ways by the same person at different times of day. Even if one accepts that these kinds of variations are an inherent part of beauty, this does not save beauty from the charge of subjectivism and emotivism that Kant levels against it.
This is not to say that we should reject the question of beauty or treat it as a trivial issue, although most twentieth-century philosophers did just that. There was a revival of interest in something like the classical philosophical sense of beauty in the 1990s, including some feminist reconstruals and reappropriations of the concept (see Hickey 1994, Brand 2000, Irigaray 1993). But this new attention to the question does not mean that we have finally solved the problem or rescued it from the skepticism of Hume and Kant.