Gewu, investigating things, comes from the Great Learning. Zhu Xi made it the first step of self-cultivation: by examining the principle of things one by one, one eventually reaches a sudden, comprehensive understanding. Wang Yangming, after trying to investigate bamboo for seven days, reinterpreted it as rectifying the mind.
Gewu
The investigation of things: the Neo-Confucian method of learning by examining the principle in each thing.
Also: ko-wu, investigation of things