The fourth step of Ashtanga Yoga (Raja Yoga) is Pranayama, in which one harnesses the most powerful energy within us so that one can then access the following steps of Yoga (dharana, dhyana, samadhi). Pranayama stabilizes the mind and energy within us, making meditation possible. Includes explanations of prana, ojas, brahamacharya, nadis, and how to perform a basic pranayama exercise.
"Or [peace of mind comes] by exhalation and retention of breath... After securing that steadiness of posture [asana] follows regulation of breath or the control of Prana, the cessation of inhalation and exhalation." —Patanjali, Yoga Sutras 1.34 and 2.49
“You can hardly make any spiritual progress without the practice of Pranayama.” - Swami Sivananda
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"No one can achieve illumination without having been previously purified. Only those that have achieved purification and sanctity can enter into the hall of illumination."