That which acts naturally has a natural life. A natural life is that life which develops itself in accordance with the principles of its own nature, and which, in doing so, is true and harmonious to itself. The sinner, in his unregenerated state, lives and acts naturally in sinning; because that which he does is not only his own doing, but is done voluntarily and easily, and harmonizes with its own central principle of movement. The central principle in fallen man is self. The great law of selfishness, which requires him to place himself first, and God and humanity under him, regulates all his actions. From this principle, which operates as an internal and life-giving force, his actions flow out as constantly and as naturally as trees grow in a soil which is appropriate to them, and as waters flow from mountains to the ocean.
— edited from A Treatise on Divine Union (1851) Part 8, Chapter 9.