—the veils that Allah has placed upon his heart.

To illustrate with a material example: when a human being first comes into existence, his flesh is tender and soft. As he grows older, however, the flesh firms up and the bones harden. Even so, people do not all develop in the same way. Consider someone who immerses himself in a particular field of study: what grows stronger in him? It is his intellect; the parts of the brain engaged by that discipline become more robust because he focuses all his effort there. Physically, though, his body gains no added strength and his muscles do not develop.

Now take the opposite case. Another person devotes himself to the physical side, performing athletic exercises every day, as we say nowadays. His muscles tighten, his body grows strong, and he attains a physique we sometimes witness in real life or see in photographs. Those champions, for instance, whose bodies seem all muscle—were they created that way, or did they acquire that powerful, heavily muscled build? They reached it through their own effort and their own choice.

The same holds true for the person who persists in his misguidance and stubbornness,