Intense physical labor causes parts of the mind to go dormant; there simply is not enough energy available to maintain them. So one experiences a sort of fog when one's mind drifts in a certain direction. But the senses and thoughts attending to the labor itself become ever more illuminated and vivid.
Like the intensity of a light source, which reveals its surroundings with the spread of its emitted light, the intensity of one's activity in a given sphere reveals and advances the development of that activity.
