In his 2009 book "Parkour", David Belle stressed that the most important aspect of parkour is not the physical movements, but rather the practitioner's mentality and understanding of its principles. "When young trainees come to see me and give me videos telling me to check out what they are doing, I just take the tape and throw it away. What I'm interested in is what the guy's got in his head, if he has self-confidence, if he masters the technique, if he has understood the principles of parkour. I just can't deal with guys who do Parkour because they saw videos on the Internet and thought it was kinda cool and want to do even better." Further, he states the importance of traceurs being aware of their abilities and limitations, and developing in their own way. "When a young person asks me: 'Can you show me how to do this?' I simply answer: "No, I am going to show you how "I" do it. Then, you'll have to learn with your own technique, your own way of moving, your style, your abilities and your limitations. You are going to learn to be yourself, not someone else along the way."