good vs great

I've been thinking about poetry, art, and music, or collectively, Art. It seems to me that an inundation of mediocre Art has diluted and dimmed our ability to appreciate - even distinguish - the differences between good Art and great Art.
Most people can tell if something's bad Art - that's not difficult. And even mediocre Art is usually recognizable. But what about good Art? What defines good? If a painting's lines are true, its colors are right, and all the details are just as they should be, it's good. Or if it's unique in topic or color or style, as long as it's well-executed, it's good. But even if all the details are just right, it's not necessarily great.
The same goes for music. If all the parts fit as they should, if it's playable, then it's good. For poetry, if the lines and words mesh as they were meant to, it's good. And for all forms of Art, if a piece is well-constructed, if to appearances it's just right, then it's good. But it isn't great.
For Art to be great, it must transcend all the lines and notes and words. If one views a painting that is good, one enjoys the viewing, and might even desire many more. But if the painting is great, one can barely leave. One is drawn, almost inexplicably, to it. If a work is good, reasons can be given: the colors are brilliant, the lines sing, it looks or sounds real... But the line which separates good from great defies explanation.
When one attempts to give reasons for the greatness of Art, one finds oneself at a loss for words. Why is that painting so great? I cannot tell. Why does one love listening to that aria over and over? One does not know. Why does the reading of that poem stir one so? It cannot be explained. Great Art changes the observer. Good Art merely upholds previous notions. Great Art touches something deep inside one, makes one cry or laugh or feel able to conquer all. Or it leads one to quietly sit and think. But to be great, it must touch one. Somehow. Good Art affects the surface, the logic of a person. Great Art reaches inside, beyond the logic, beyond the sense, beyond one's reasons, to affect something else.
Good Art is merely beautiful, nothing more. That which leaves one with joy, with sorrow, with longing, with questions - that is great. Art which can be defined is not great. Art which can be ignored is not great. Art which leaves an indelible mark inside its viewer (or listener or reader) - that Art which transcends - that is great.
And that which is great is that which cannot be explained or expressed. Great Art is beyond, or above, my feeble attempts to explain. Good Art is pretty. Great Art just is. And one never can say more.