As light and heat happen to be part and parcel of the blazing sun, karma is inherent in Jiva. Between the Jiva and his karma, the explanation of one necessarily contains the explanation of the other. Thought or mentation is a subtle form of karma. Verily each activity is actuated by will. There is no karma independent of the will.
Man wills to walk, to eat, to sleep, to wake up and so on, and as a result action ensues.
The gross manifestation of the will is action. The entire creation is the manifestation of the cosmic will. All forms of will resolve themselves ultimately into the will to live. It is the innate desire to preserve life that transforms itself into innumerable activities. Existence, not extinction is the essential nature of the soul.