Theoretical thinking is the key question for science development. Compass and gunpowder belong to the four great inventions of ancient China, but from Tang dynasty to Qing dynasty, no any theoretical questions on compass and gunpowder were ever studied. This article lists a few incisive theoretical examples in modern chemistry to illustrate the very importance of theoretical questions. Perceptual knowledge needs to rise to rational knowledge that must be completely consistent with experimental facts. Any correct theory should grasp the essence of things. From philosophical opinion, practice is the first; however, theoretics have important instructive effects on new and follow-up practices. Therefore, the tree of rational theory is ever green.

scientific methods  /  theoretical thinkings  /  scientific practices