Thompson's desire to ground international relations thought in history led him to distrust mono-causal theories, whether the Marxist championing of class and economic relations, the liberal faith in democratic government, or the "scientific" theorizing of much of international relations thought since the behavioral revolution. For Thompson, theory picks out what is most essential from reality, but theory that strays too far from the complexity of reality revealed in history is more likely to be the projection of the theorist’s own prejudices than a useful way of understanding the international states-system.