To make a physics analogy, work (analogous to positive trade) occurs when energy is transfered in an ordered fashion, and entropy is the waste of the energy without doing work. For work to be possible, an energy differential is necessary. For the physical world, there is a source and sink called our sun and empty space, and as rational souls we are able to tap into that energy differential to do work for ourselves and others. The farmer received the energy of the sun to produce edible energy for our bodies. The Christian receives the spiritual food of the Son to power his (and His) charity in the world.