Well actually Sun Tzu encouraged the use of spys because he wasn't in favor of engagements with enemy armies because there were great consequences for the victor and the loser.
Sun Tzu states that the highest excellence is to defeat an army without fighting and bloodshed.He even goes to state ingeniously that the greatest crime against humanity is the disuse of spys,because the effort of a single spy could save hundreds or thousands of lives depending on the situation.
"It is a good viewpoint to see the world as a dream. When you have something like a nightmare, you will wake up and tell yourself that it was only a dream. It is said that the world we live in is not a bit different from this." -Hagakure