Most often attention is confused with a kind of muscular effort. If one says to one's pupils: "Now you must pay attention," one sees them contracting their brows, holding their breath, stiffening their muscles. If after two minutes they are asked what they have been paying attention to, they cannot reply. They have been concentrating on nothing. They have not been paying attention. They have been contracting their muscles.
We often expend this kind of muscular effort on our studies. As it ends by making us tired, we have the impression that we have been working. That is an illusion.
Simone Weil, Waiting for God, Craufurd, tr. pp. 109-110.