Complex sentence: Quimby argued that spiritual or "scientific man" is the perfect embodiment of Spirit, Wisdom, Principle, Truth, Mind and Science, and cannot be sick, but is overwhelmed by the "natural man" of matter and false beliefs.
Split into simpler sentences: Quimby argued that the real person--spiritual man or "scientific man"--is the perfect embodiment of Spirit, Wisdom, Principle, Truth, Mind, Science. Scientific man cannot be sick, but he is overwhelmed by the man of matter and false beliefs.