My favorite yoga sutra and quote:
Yogaś citta vrtti nirodhah (Pantañjali’s Yoga Sutra 1.2) Yoga is the stilling of the changing states of the mind. (Translation-Edwin F. Brant-The Yoga Sutras of Pantañjali, 2009)
However beautifully we carry out an āsana, however flexible our body may be, if we do not achieve the integration of body, breath, and mind we can hardly claim that what we are doing is yoga. What is yoga after all? It is something that we experience inside, deep within our being. Yoga is not an external experience. In yoga we try in every action to be as attentive as possible to everything we do. Yoga is different from dance or theatre. In yoga we are not creating something for others to look at. As we perform the various āsanas we observe what we are doing and how we are doing it. We do it only for ourselves. We are both observer and what is observed at the same time. If we do not pay attention to ourselves in our practice, then we cannot call it yoga. The Heart of Yoga-TKV Desikachar (pg, 23, 1995).