"Physical fitness is not only one of the most important keys to a healthy body, it is the basis of dynamic and creative intellectual activity. The relationship between the soundness of the body and activities of the mind is subtle and complex. Much is not yet understood. But we do know what the Greeks knew: that intelligence and skill can only function at the peak of their capacity when the body is healthy and strong." - President John F. Kennedy

John F. Kenndy is saying fitness is not the only key to a healthy body. He is saying be also smart and energetic. For example, being very fit can be good for you in some situations. But sometimes it won't help you understand things around you. Like knowing how the world is working around you. Knowing simple things like science or History. Know how to read is very important in life.