The word "De" in Oracle Bone Inscriptions has the shape of "Chi" on the left, which is a symbol of road and action in ancient Chinese. On its right is an eye, and there is a vertical line above the eye, which means looking straight. So the meaning of this word is: look straight at the direction of "the road you are taking" and conform to nature. This is "virtue".
-The understanding in bronze inscriptions is more comprehensive. The word "heart" is added under the word "eye", which means to follow the true heart and nature is "virtue".
In Biography, it is still knowledge: its upper right side becomes "straight" and "straight heart" is "virtue".