Principle of Political Realism

As I wrote down before, realism is one of schools of international relations.
By reading Hans Morgenthau's thesis, I took a hint of the core idea of realism.
The key word for realism is 'reason.' Hargenthau explains in his book that realism believes 'truth' which is truly objective and rational through the filter of evidence and reason. It does not include prejudice and wishful thinking which were separated from the facts. This is called 'opinion' according to Hargenthau. Realism examines political acts in the past imagining why politicians acted in this way or in that way. And from the knowledge derived from that process, realism can foresee what will happen in the future.