A true, brave and honest fighter must principally know that the first enemy is his best friend, what’s more he is the same person, that is himself.
Carl William Brown
Thus we may know that there are five essentials for victory:
(1) He will win who knows when to fight and when not to fight.
(2) He will win who knows how to handle both superior and inferior forces.
(3) He will win whose army is animated by the same spirit throughout all its ranks.
(4) He will win who, prepared himself, waits to take the enemy unprepared.
(5) He will win who has military capacity and is not interfered with by the sovereign.
Hence the saying: If you know the enemy and know yourself, you need not fear the result of a hundred battles. If you know yourself but not the enemy, for every victory gained you will also suffer a defeat. If you know neither the enemy nor yourself, you will succumb in every battle.
Sun Tzu The Art of War