The concept of rationality represents the foundation of much of Western epistemology that can be traced to ancient Greek philosophers, and the claim that humans are innately rational. Rationality is grounded in reason, a mental power used to form judgments, make inferences, reach conclusions, and construct arguments. If an individual possesses a belief and needs to defend it, that individual can resort to reason to support, justify, or explain their position. To be declared lacking in reason by others means that an individual is stupid, irrational or insane. Western theologians use reason to support their positions on the doctrine of God, faith, and revelation during the Middle Ages by arguing in part that theology is rational and that theological issues do not contradict reason.