From The Art Spirit (1923), by Robert Henri.

When the artist is alive in any person, whatever his kind of work may be, he becomes an inventive, searching, daring, self-expressive creature. He becomes interesting to other people. He disturbs, upsets, enlightens, and opens ways for better understanding. Where those who are not artists are trying to close the book, he opens it and shows there are still more pages possible. The world would stagnate without him, and the world would be beautiful with him; for he is interesting to himself and he is interesting to others. He does not have to be a painter or sculptor to be an artist. He can work in any medium. He simply has to find the gain in the work itself, not outside it.