Luther on prayer being undistracted

I read this helpful quote from John Piper's Brothers, we are not professionals (pp.62-63) today. By the way, this book has just been published in Spanish, which is great news!

Martin Luther was once asked by his barber, "Dr. Luther, how do you pray?" Astonishingly, one of the busiest men of the Reformation wrote a forty-page response for his barber, Peter Beskendorf. His words are a great inspiration for us to beware of sacred substitutes.

"A good clever barber must have his thoughts, mind and eyes concentrated upon the razor and the beard and not forget where he is in his stroke and shave. If he keeps talking or looking around or thinking of something else, he is likely to cut a man's mouth or nose - or even his throat. So anything that is to be done well ought to occupy the whole man with all his faculties and members. As the saying goes: he who thinks of many things thinks of nothing and accomplishes no good. How much more must prayer possess the heart exclusively and completely if it is to be a good prayer!"