B.K.S. Iyengar, P. 54 Light on Life:
"Sometimes our body is willing, but our mind is weak and says, "We don't have time," or "Forget it, it's not worth all the effort." Sometimes it is our mind that is willing, but our body is weak and says, "I'm really to tired for all this trouble." A practitioner must focus between the mind and the body, listening to the counsel of each, but letting the intelligence and the soul make the true decision, for this is where real will power and real dedication are found. Do to your capacity while always striving to extend your capacity. Ten minutes today. After a few days, twelve minutes. Master that, then again extend. It is better to do a good pose minimum than a bad pose maximum."