The relation between the soul and the body is that of the spark and the charcoal. When the spark of the fire touches the charcoal then it is caught by the charcoal. The saying "it has caught fire" means that the fire, which was apart from it, has been caught by it; and so the soul is caught by the body. One can also look at it in another way: that the body is caught by the soul, or better, that the body is used by the soul. And as the charcoal turns into ashes, so the body in the end is destroyed. But the fire is not lost, it has disappeared to its own element, which is heat.