Philosophy is lonely, because it must have that kind of critical spirit, and only critical philosophy can become a force for progress. Only criticism can make progress, which is the professional quality that all philosophers should have. The first step of criticism is to doubt, not only to treat the real world with a positive attitude, but also to doubt. Of course, the criticism of philosophy is not a complaint like a dissatisfied wife, nor is it a dispute of interests. The most important thing is to get the law and practice it with the help of the light of reason, and get the truth from the law. Descartes' famous saying "I think, therefore I am" is based on universal suspicion. On the other hand, Marx mercilessly criticized capitalism and brought us Marxist philosophy and the pursuit of future socialism. Criticism is an inexhaustible motive force for philosophy to remain young forever, making philosophy an eternal living fire or a running water that stirs the soul. Philosophy is not a history of philosophy, nor is it a few words of some philosophers. Philosophy pays attention to the present, history and future. Many discoveries of philosophers, even if they could not be proved at that time, will become truth in the future. Therefore, great philosophers do not belong to themselves now, but to all mankind in the future. They have pointed out the direction for mankind.