what is an artist?

What is an artist? What makes a person an artist?
H.L. Mencken said:

It is almost as safe to assume that an artist of any dignity is against his country, i.e., against his environment in which God hath placed him, as it is to assume that his country is against the artist. The special quality which makes an artist out of him might almost be defined, indeed, as an extraordinary capacity for irritation, a pathological sensitivity to environmental pricks and stings. He differs from the rest of us mainly because he reacts sharply and in an uncommon manner to phenomena which leave the rest of us unmoved, or, at most merely annoy us vaguely. He is, in brief, a more delicate fellow than we are, and hence less fitted to prosper and enjoy himself under the conditions of life which he and we must face alike. Therefore, he takes to artistic endeavor, which is at once a criticism of life and an attempt to escape from life.

From The Vintage Mencken by Alistair Cooke, New York: Vintage Books, 1958