As long as the Prana does not enter and flow in the middle channel – Sushumna nadi – and the vindu does not become firm by the control of the movements of the Prana; as long as the mind does not assume the form of Brahma without any effort in contemplation, all the talk of knowledge and wisdom is merely the nonsensical babbling of a mad man.
— Svatmarama’s Hathapradipika, Chapter 4.113