The four great instruments of Yoga-Siddhi

Yoga-Siddhi, the perfection that comes from the practice of Yoga, can be best attained by the combined working of four great instruments. There is, first, the knowledge of the truths, principles, powers and processes that govern the realisation—śāstra. Next comes a patient and persistent action on the lines laid down by this knowledge, the force of our personal effort—utsāha. There intervenes, third, uplifting our knowledge and effort into the domain of spiritual experience, the direct suggestion, example and influence of the Teacher—guru. Last comes the instrumentality of Time—kāla; for in all things there is a cycle of their action and a period of the divine movement.

Ref: The Synthesis of Yoga