In the Indian context they are two sides of a single movement. Buddhi, in the Gita implies an Intelligent-Will. They should work together. What the mind knows to be the right thing to do, the will must follow. The two operations implied are, first, to know, to understand what is the right thing. The second is to follow the choice reflected in the understanding. However the Buddhi is often clouded and usurped by the lower nature, its desires and the ego-self. Hence the understanding gets clouded and the will, disconnected from the intelligence. Buddhi is the power of discernment while the will is the power of execution without which one cannot advance an inch upon the path even if one knows the right thing to be followed.
Affectionately,
Alok Da
