We have both a head and a body. They are joined by the neck. The mind that wills and thinks is found in our head, and the power that acts and carries out is found in our body. If therefore we had only benevolence, or thoughts based on goodwill, but we did not do anything good or produce anything useful as a result, we would be like head by itself or a mind by itself, which could not continue to exist on its own without a body. Surely everyone can see from this that goodwill and faith are not goodwill and faith when they are only in our head and our mind but not in our body.
Under those circumstances goodwill and faith are like birds flying in the sky that have no home of their own on the ground. They are like birds that are about to lay eggs but have no nests; the eggs slip out of the birds into the air or onto a twig of some tree and then fall and smash on the ground.
All things in our mind have a corresponding element in our body. The corresponding thing could be called an embodiment. Therefore when goodwill and faith are only in our mind, they are not embodied in us. Under those circumstances we could be compared to the airy human figures known as ghosts, as Fama was depicted by the ancients, with a laurel wreath on her head and a horn of plenty in her hand. Because we would then be ghosts and yet would still be able to think, we could not help being constantly hounded by mental images (a problem also caused by false inferences based on various kinds of sophistry). We would be much like swamp reeds blown around by the wind that have shells at their base underwater and frogs croaking at the surface. Surely we can see that things like this happen when people merely know some ideas from the Word about goodwill and faith but do not practice them.
In fact the Lord says, “Everyone who hears my words and does them I will compare to a prudent man who built his house on a rock. But everyone who hears my words and does not do them will be compared to a foolish man who built his house on the sand” or “on the ground without a foundation” (Matthew 7:24, 26; Luke 6:47-49). Goodwill and faith and made-up ideas about them, when we do not put them into practice, can also be compared to butterflies in the air that a sparrow sees, flies toward, and eats. Likewise, the Lord says, “A sower went out to sow. Some seeds fell on hard ground, and the birds came and ate them” (Matthew 13:3-4).
from True Christianity, Section 375