Various heated arguments have developed on realism and many scholars have come out their own construed versions of the perceived realism. Kenneth Waltz argues that all of the various explanations for the causes of war can be easily filtered into three images: that of man, the state and the international system.
Applying Waltz’s logic to the realist camp, distinct categories of explanations can be readily identified. For example, traditional realism, with its emphasis on individuals, explains international phenomena and state behaviour from the point of view that human nature is intrinsically evil. Structural realism on the other hand throws out this assumption, and conducts an analysis based on the objective nature of the international structure, a system-level analysis. From the perspective of units of analysis, the different casual stories told by realist thinkers can be divided into three distinct categories: human nature realism, state-centric realism and system-centric realism2.
Human nature realism, also known as biological realism, 3Donelly emphasizes man's biological abilities and intrinsically evil nature. Most classical realists (including the many modern political philosophers who have contributed to the development of realist thought), including especially Machiavelli, Herbert Butterfield, Reinhold Niebuhr, and Hans Morgenthau fall into this category. Morgenthau's theory is the best example of human nature realism in international relations.
In “Politics among Nations”, Morgenthau. presents a systematic discussion of the basic principles of realism.