School is the main channel for disseminating knowledge, but it is not the only channel. Learning book knowledge is important, but learning is also important in practice. No scientist's knowledge is completely learned from school, and no scientist's knowledge is completely Learned from books. What is the main point of the above materials? ['Studying book knowledge and participating in practice are two sources of knowledge.', 'The experience people learn through their own practice is real and reliable, and book knowledge is unreliable.', 'It is impossible for a person to gain knowledge by doing everything himself, and it is not necessary to do so.', 'We must constantly learn from books and continuously enrich our knowledge through practice.']