So many different people have said it; it's impossible to count them all. If you want to know what good art is, look at a lot of pictures....hundreds, thousands. It WILL sink in eventually. And 99 times out of a hundred, you'll agree with the others who have done the same. The only thing to keep in mind is that art evokes something. If it makes you want to puke it's probably art, just not pretty. Francis Bacon (the modern one)did some wonderful stuff - ugly and scary though it was.