Speaking about Friction in War, Clausewitz (2014, p.36) states:

“Everything is very simple in War,but the simplest thing is difficult. These difficulties accumulate and produce a friction which no man can imagine exactly who has not seen War,…”

“Activity in War is movement in a resistant medium. Just as a man immersed in water is unable to perform with ease and regularity the most natural and simplest movement, that of walking, so in War, with ordinary powers, one cannot keep even the line of mediocrity. This is the reason that the correct military theorist is like a swimming master, who teaches on dry land movements which are required in the water. This is also why theorists, who have never plunged themselves, or cannot deduce any generalities from their experience, are unpractical and even absurd, because they only teach what every one knows – how to walk.”

“Besides, it can never be learnt  theoretically; and if it could, there would still be wanting that experience of judgement which is called tact, and which is always more necessary in a field full of innumerable small and diversified objects than in great and decisive cases, when one’s own judgement may be aided by consultation with others.”

Reference

von Clausewitz, C. & Graham, J.J. (trans) (2014) On War. CreateSpace Independent Publishing Platform.