Psychology is one of the key factors in warfare for Clausewitz. He refers to psychology as “moral quantities,” or the human mind (49). The intricacies of human behavior are also one of the subjective factors in war. They complicate the theory of war since they cannot be reduced to a mathematical equation, which leads Clausewitz to admit “[t]he difficulty of Theory as soon as moral quantities come into consideration” (49). However, he insists that “the moral cannot be omitted” (41) because psychological factors are “the spirits which permeate the whole element of war” (86). Of all the psychological characteristics, it is military virtue that is “one of the most important moral powers in war” (90).

There are certain objective factors that may contribute to (but do not guarantee) victory in battle and in the overall war. These factors include advanced technology and numeric superiority. However, Clausewitz argues, the inequality between the physical forces and capabilities of the two opponents “could be balanced by moral forces” (14). For example, individual courage may play a decisive role in battle and surpass the objective factors. In his view, war is not limited to physical combat, but is also “a trial of strength of the moral and physical forces” (41).