The world is the field of action. Through action one acquires knowledge. The guru instructs the disciple to perform certain works and refrain from others. Again, he advises the pupil to perform action without desiring the result. The impurity of the mind is destroyed through the performance of duty. It is like getting rid of a disease by means of medicine, under the instruction of a competent physician.

Reference: The Gospel of Sri Ramakrishna (8.3 The Master's Birthday Celebration At Dakshineswar)