Bhagavad Gita 4.17

4.17 For verily (the true nature) of action (enjoined by the scriptures) should be known, also (that) of forbidden (or unlawful) action, and of inaction; hard to understand is the nature (path) of action.

The intricacies of action are very hard to understand. Therefore one should know properly what action is, what forbidden action is and what inaction is.