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One who is intent on practice will obtain Siddhi, not one who is idle. Yoga-Siddhi is not obtained by a mere reading of the scriptures (Shastra's). Siddhi is not achieved by wearing the dress [of a Yogin], or by talking about it; practice alone is the cause of success. This is the truth, without doubt.

Hatha Yoga Pradipika I. 65-66
(translation by Srinivasa Iyangar/Tookaram Tatya)