The philosophy of parkour has been compared to that of martial arts. In an interview with "The New Yorker", David Belle acknowledges the influence: "There's a quote by Bruce Lee that's my motto: 'There are no limits. There are plateaus, but you must not stay there, you must go beyond them. A man must constantly exceed his level.' If you're not better than you were the day before, then what are you doing—what's the point?" In an interview with the press, Belle explained that parkour is a training method for warriors. "So many people try to train easy 'Come do parkour! It's really cool!' But if tomorrow I made you do real training, you would end up crying. That's what you need to know: you are going to cry, you are going to bleed and you are going to sweat like never before." In his book, Belle also quotes his father Raymond: "If two roads open up before you, always take the most difficult one. Because you know you can travel the easy one."