I was set this essay title at school in the philosophy class, the term we studied æsthetics. I have continued to ask the question while studying literature and then history of art at university, whilst working at the Arts Council dispensing public money into the arts, and now as we work with lighting.
There is no one answer, but there are many.
For example, it would appear that when you put a light in a sculpture, it is no longer a work of art: it becomes a light and the price you have to pay for it drops by 90%.
On the other hand, if you show a light in an art gallery, it becomes a work of art (and the price goes back up!). For example, the lights at the Hayward's Gallery's Light Show. Or lights from people like Johanna Grawunder and Studio Drift at the Carpenters Workshop Gallery.
At the head of this post you can see a Black Diamond from below. Here are two more angles:
More than once when we have shown pictures of Adam's work to clients, they have asked us to send them a straightforward picture taken with a phone, because they assume that the real thing cannot look as amazing as the images -- that the images must have been doctored. But they haven't -- they do!
Have a look at this Black Diamond 1 -- it really does look like this:
And just look what happens when Hoets broadens his colour palette from the the black and white of the Black Diamonds! This is a Mandala:
And the best thing is that you can have stunning wow factor pieces like this for your own project -- and they are priced as Lights, rather than as Works of Art!
Here is a custom piece in the same idiom as Mandala: