Sun Tzu means very specific and important things when he says that "we must know ourselves and our enemies." The problem is that most of us lack a deep understanding of the concepts of which Sun Tzu speaks. This is why our victories are usually far from painless.

“Know the enemy and know yourself.
Your victory will be painless."
Sun Tzu’s The Art of War 10:5.15-16

The two lines above appear rather late in Sun Tzu's work, the tenth chapter out of thirteen. At this point in the book, every term in these lines has a very specific definition. To understand these lines, you must first be familiar with the concepts "to know," "enemy," "victory," and "pain."