Plato defined the world as consisting of two distinct realms : Idea and phenomena. The Idea is the world of perfect truth and phenomena are imperfect replications of Idea. Aristotle, however, refused the separate existence of Idea, making the material world observed by senses the basis of his natural science. In this paper, I will compare the two philosophers only in regard to the world of phenomena or material, without concern for the world of Idea. They have different opinions of the material world. Plato`s world of phenomena is the beat replication of Idea, these two not contradicting each other. And all phenomena, although different in kind and hierarchy, advance in harmony toward the Idea. But Aristotle regarded the primary characteristic of nature as dualistic. The four basic elements consist of one each of the pairs of opposites, coldfiot, dry-wet, and also form contrasts between purity and uncleanlineas, nobility and depravity, etc. Furthermore, the difference between the two philosophers is applicable to their concepts of social systems. Plato idealized the Republic where the division of function and harmony through cooperation among various social classes is realized. The important criterion whereby he judges the level of wisdom, courage, discipline and justice is the harmony within the community. The degree of social vice in contradiction to the merits of a community is not a matter of any social class, but is related to the principles upheld by the community. According to Aristotle, however, a society can be best understood by the subjugation relation between the ruler and the ruled, and a society is premised upon the difference in external material conditions and internal spiritual values. Examples of such power relation are the dualities of men and women, free men and slaves, the ruler and the ruled. The basic elements in the evaluation of Aristotle are wealth, family, power, etc. We can see the difference between the two philosophers in their opinions about the possession of political power and wealth, as well as various kinds of virtue such as justice, goodness, discipline, happiness and friendship. In summary, the Idea Plato strives to approach is the world which all of us should advance toward. The phenomena of nature and society are the places where harmony aloud be realized. And the mindful community advances to absolute values through education. Aristotle, on the other hand, defined both nature and society as consisting of opposite elements. He is a conservative who believes in the inevitability of inequality of power and wealth among the social classes, rather than an idealist who makes efforts to overcome inequalities.