Tsukahara Bokuden, one of Japan's greatest swordsmen, wanted to test the self-defense abilities of his three sons, all of whom he had trained in the way of the samurai. To do this, Bokuden placed a pillow, which in those days were cylinders made of wood, over the curtain on the door to his room so that when the curtain was raised, the pillow would fall on the head of the person entering.
Bokuden first called his oldest son, who saw the pillow, took it down, entered the room, and replaced the pillow over the curtain.
As the second son entered, the pillow fell, but the son caught it in his hands and placed it back over the curtain.
As the youngest son rushed in, the pillow fell squarely on his head, but the son cut it in half with his sword before it hit the floor.
To the first, oldest son, Bokuden gave his sword, saying, "You are a great swordsman."
To the second son, he said, "You will one day become a great swordsman, but you must yet train very hard."
To the third, youngest son, he said, "You are a disgrace to this family, and are not qualified to even hold a sword." So saying, he took his youngest son's sword away from him and cast him out of the house.
This story has been told to countless Karate students by their instructors, in one form or another, for as long as karate has existed. The reason is that it perfectly illustrates the essence of self defense -- because of their potential to deliver extreme impact force, karate techniques should be used only as a last resort against surprise attacks. What Bokuden's sons faced was a surprise attack, and the eldest son displayed the perfect defense -- he avoided the situation entirely. The more you increase your awareness of potentially dangerous situations, the more you decrease your vulnerability to being attacked.