The natural life... has a close connection with the natural desires. Just so far as such desires are inordinate in their action, they are the result of unsanctified nature, and not of the Spirit of God. The root, however, the original and fruitful source of that state of things in the natural heart, which is conveniently denominated the Natural Life, is the inordinate action of the principle of SELF-LOVE; denominated, in a single term, selfishness. The pernicious influence from this source, with the exception of what has become sanctified by the Spirit of God, reaches and corrupts every thing. Hence the importance of the process of excision. It is not only important, but indispensably necessary, that this evil influence should be met and destroyed wherever it exists. A process often exceedingly painful; but inevitable to him, who would be relieved from his false position, and put in harmony with God. There must be a CUTTING OFF, and a renewed and repeated CUTTING OFF, till the tree of Self, despoiled of its branches and foliage, and thus deprived of the nourishment of the rain, the sun, and the atmosphere, dies down to its very root; giving place, in its destruction, to the sweet bloom of the tree of life.
A life of practical holiness depends essentially upon two things: FIRST, upon an entire consecration of ourselves, body and spirit, to the Lord; and SECOND, upon a belief that this consecration is accepted. We must, in the first place, offer up our whole being as a sacrifice to the Lord, laying all upon his altar. But we should remember, it is laid there, in order that the natural life may be consumed, and that there may be a resurrection of the true spiritual life from its ashes. He, therefore, who has consecrated himself to God, must expect that the truth of the consecration will be tested by the severity of an interior crucifixion, which is the death of nature, but in the end present and everlasting life. It is not till the flame has come upon us, and we have passed through the fire of the inward crucifixion, which consumes the rottenness and the hay and stubble of the old life of nature, that we can speak, in a higher sense, of the new life; and say, CHRIST LIVETH IN ME.
— edited from The Interior or Hidden Life (1844) Part 2, Chapter 10.