Chapter 5, Verses 10 and 11 - Sun Tzu

In battle, there are not more than two methods of attack -- the direct and the indirect; yet these two in combination give rise to an endless series of maneuvers. The direct and the indirect lead on to each other in turn. It is like moving in a circle -- you never come to an end. Who can exhaust the possibilities of their combination?
~ from The Art of War, Giles translation ~

Go here to read the introductory post to the verses of The Art of War.