To be a realist is to have a dark pessimistic and generally harsh view of humanity. Men like Thomas Hobbs authored early classical realist work on the subject of human kind. However other roots can also be found in the work of Machiavelli. His work "The Prince" establishes a core theme of realist theory, that the morality that one attaches to society and the interactions of humans on the domestic level can not be applied to international relations and the behavior between states. Early realist theory also describes the world in a state of chaos where men and states are in a constant struggle for power the end result being war and international conflict. Modern realist theory taking the same premise, moves further to try and explain international conflicts but with one major digression with classical realists. Power, classical realist saw it as the end of which all of man's regrettable qualities are focused towards, however in modern realist theory power is only seen as a means to a greater goal of which all states living in anarchy are trying to achieve, security. .
It is ironic that a theory used to try and establish a system or set of circumstances to explain certain events in international relations and predict the outcomes of others, would be entirely based on the a belief that all states in the international level exist in a environment of anarchy. However this is exactly what a realist or realism sets out to do. .
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It is off that initial premise that realist then establishes that all states must provide for their own security and that, above all else, drives their behavior and explains almost every international conflict. This is a general overview behind the main belief the theory is based on, but to really get a better understanding of realism a closer look into the main specifics and general assumptions that guide it are needed. After doing so it is clearer to see exactly what the theory tries to achieve and what different criticisms are attached to realist thought.