Aesthetics
One theory in aesthetics is called subjectivism. Subjectivism says that each person has a sense of beauty. So, the only way of judging something's beauty is if people say it is beautiful. A common way of describing this is "Beauty is in the eye of the beholder". Subjectivism also says, for the same reason, that something is art simply because someone says it is art. This is backed up by the experience. Even though people feel that something is beautiful, they may not be able to explain why.