Martin Luther Quote
When speaking of the spiritual nature or the soul, we are referring to that which is inner or new. When speaking of the bodily nature, or that which is flesh and blood, we are referring to that which is called sensual, outward, or old. Paul writes in 2 Corinthians 4:16: Even though our outer nature is wasting away, our inner nature is being renewed day by day.
p. 51 - The Freedom of a Christian (1520)