There are three aspects of knowledge: self-knowledge, the knowledge of another person, and the knowledge of the collectivity. Also, there are three ways of looking at the world: its past, its present, and its future. By yesterday is meant the past; by today is meant the present; and by tomorrow is meant the future. The knowledge of the past gives wisdom; the knowledge of the present gives power; the knowledge of the future gives peace. The one who is anxious to acquire knowledge must consider all these three aspects to be equally important.