The war-god Ares assumed the likeness of a bull and came down to challenge the bull of Paris. And Ares, in the shape of a bull, did contest with the bull of Paris and emerged the victor. Now Paris, seeing that Ares had defeated his champion bull, placed the crown between the war-god’s horns to acknowledge his victory. Word of this contest spread, and because of his impartiality, as a judge he was said to be just.
Thus Paris was chosen by Zeus to adjudicate upon the three goddesses.