Explaining his attachment to Dorian, Basil tells Harry Wotton: "Dorian Gray is merely to me a motive in art. He is never more present in my work than when no image of him is there. He is simply a suggestion, as I have said, of a new manner. I see him in the curves of certain lines, in the loveliness and the subtleties of certain colors. That is all."
What does the word "motive" mean?