My theory is that art is what happens when the design is to be artistic. For example, there might be books made to entertain people. You can find beauty in how well a book is written to be entertaining. Then you can start writing books specifically for that beauty. Then you find beauty in how well a book is written to be beautiful. Pretty soon, you end up with didactic books that are useless for entertainment, unless you're specifically into the beauty of didactic literature.