Security against defeat implies defensive tactics; ability to defeat the enemy means taking the offensive.

不可胜者，守也；可胜者，攻也。

How are we gonna work on our defensive tactics if we can't even climb a fence?

如果我们连一个栅栏都爬不过去还谈什么防御策略?

The different measures suited to the nine varieties of ground; the expediency of aggressive or defensive tactics; and the fundamental laws of human nature: these are things that must most certainly be studied.

九地之变，屈伸之力，人情之理，不可不察也。