Christian Science teaches the allness of God, good, and the consequent nothingness of evil, or error. It does not theorize on this question; it demonstrates the difference between the real and the unreal through healing.
One who honestly applies the fact of God's allness to disease may prove that the disease is but an error of belief. A clear consciousness of the allness of good excludes the possibility of the consciousness of disease. If the disease were real, it would not disappear; but when understanding brings the consciousness of good's allness the disease does disappear, and health is established as ever present.
The healing power of Christian Science can be demonstrated only in the degree that the allness of good is actually perceived. We cannot demonstrate it by merely talking about or thinking about the allness of good; good must be discerned, understood, realized. In order to do this, we must remove all that would obstruct a clear view of good as God, Spirit, Mind, Principle.