On His Children Playing Football, by Theodore Roosevelt
"I am delighted to have you play football. I believe in rough, manly sports. But I do not believe in them if they degenerate into the sole end of any one's existence. I don't want you to sacrifice standing well in your studies to any over-athleticism; and I need not tell you that character counts for a great deal more than either intellect or body in winning success in life. Athletic proficiency is a mighty good servant, and like so many other good servants, a mighty bad master. "
-Theodore Roosevelt, Theodore Roosevelt’s Letters to His Children