I haven’t read the book. I am still working on a long post about T. Krishnamacharya, who was instrumental in collecting and teaching asana as part of yoga practice in 20th century India. He taught those who brought yoga to the west — Jois, Iyengar, and Devi. Apparently Singleton wrote quite a bit about that, and his book is definitely on my reading list.
If you’re not a yogi, you may not know that what we call yoga in the West is actually one of six schools of Hindu philosophy in India. What we call yoga here is actually asana, one of the eight limbs of yoga, which is a holistic practice with ethical, social, physical, mental, and spiritual dimensions.
My opinion at this time about whether yoga is physical or spiritual: Most of us in the west first encounter yoga as physical exercises that relieve stress and build strength and flexibility. That’s okay. That may be the only way into our culture.
The physical body is but one layer of our beings. A regular asana practice brings changes to the physical body as well as the other layers. Once your body has gotten accustomed to doing yoga, doing yoga feels good. You miss a few days or a couple of weeks, and you notice the loss of well-being. It is meant to be a practice, and it affects more than just the physical body.
Whether you ever study yoga philosophy or not, a regular asana practice eventually opens you to notice your chakras and understand that you are much more than matter.
And after awhile, you may become fascinated with your subtle bodies, and you will want to meditate.