If the man has the right stuff in him, his will grows stronger and stronger with each exercise of it—and if he has not the right stuff in him he had better keep clear of dangerous game hunting, or indeed of any other form of sport or work in which there is bodily peril.
 Theodore Roosevelt, Theodore Roosevelt; an Autobiography (1913). copy citation

Context

“He must, by custom and repeated exercise of self-mastery, get his nerves thoroughly under control. This is largely a matter of habit, in the sense of repeated effort and repeated exercise of will power. If the man has the right stuff in him, his will grows stronger and stronger with each exercise of it—and if he has not the right stuff in him he had better keep clear of dangerous game hunting, or indeed of any other form of sport or work in which there is bodily peril. After he has achieved the ability to exercise wariness and judgment and the control over his nerves which will make him shoot as well at the game as at a target, he can begin his essays at dangerous game hunting, and he will then find that it does not demand such abnormal prowess as the outsider is apt to imagine.” source