《逸周书·卷三·小开武解》
四察是：一，眼睛察的是颜色；二，耳朵察的是声音；三，嘴里察的是言语；四，心里察的是意念。
四察，一目察维极，二耳察维声，三口察维言，四心察维念。
The four observations are: first, the eyes observe color; second, the ears observe sound; third, the mouth observes speech; fourth, the heart observes thoughts.