But if a man walk in the night... — He passes in this verse from the material to the spiritual truth. This first clause still holds of the natural night, and the danger to men who walk in it, but it holds, too, of the darkness in which men walk who do not see, as He is seeing, the light of heaven falling upon the moral path. In the second clause the moral truth is expressed with a prominence which excludes the other.
Because there is no light in him. — The light is now not that “of this world,” but that which is within man.