Why is the classification system of psychoactive drugs presented not a perfect system?

After school, several students skateboard at the nearby park. your best friend, erwin, is a good skateboarder and happens to fall when attempting a fancy trick. after falling, erwin gets back up and starts repeating the trick over and over again. how would a cognitive psychologist interpret erwin getting back up and repeating the trick many times? erwin has seen other people fall and knows they get back up, so he does the same thing and models their repetitive behavior. erwin is motivated to learn the trick to perfection and knows that practice makes perfect. erwin has repressed desires and, through introspection, he knows that he will only get better if he keeps trying until he skateboards perfectly