How was it possible to have a ‘theory’ of war? ‘Theory and practice in war’ details Clausewitz's rejection of previous theories on the grounds that they did not take sufficient account of the unpredictability of war. To this unpredictability Clausewitz opposes moral forces: namely, the quality of the men and commanders that was expressed through intuition, determination and professionalism. A theory of war could be developed through studying the history of war, but this theory could only be used to educate judgement. War was an art, not a scientific system that could tell generals exactly what to do.