Directions: Answer the given question based on the following passage:
Some of the profoundest ideas relating to the philosophy of work are to be found in the Bhagvad-Gita. It is unique in its stress on work-ethics. Among the great religious books of the past, this is the only book whose central theme is work, man at work, and man achieving from work a double benefit, namely, social welfare outside, and spiritual growth within. All other religious books deal with man at worship, at prayer, at meditation, at ritual. But Gita deals primarily with man at work, and presents worship, prayer, meditation and ritual as means for man`s spiritual growth, development and realization on one hand and for increasing his work-efficiency, leading to social welfare, on the other. This is yoga.
What is the underlying idea of the passage?