From 21st Century Science and Health:
The physical affirmation of disease should always be met with the mental negation. Whatever benefit is produced on the body must be expressed mentally and thought should be held fast to the improved ideal. If you believe in inflamed and weak nerves, you are liable to an attack from that source. You will call it neuralgia, but I call it a belief. If you think that cancer is hereditary in your family, you are liable to the development of that thought, unless Science shows you otherwise. If you decide that the weather or air quality is unhealthy, it will be so to you. Your decisions will control you, whichever direction they take.
Guarding the access
Improve the case. Safeguard, or firewall your thought. Admit only those conclusions that you wish realized in bodily results and you will control yourself harmoniously. When the condition is present which you say induces disease, whether it is air, exercise, heredity, contagion, or accident, than actively keep those unhealthy conclusions and fears out of thought. The issues of pain or pleasure must come through mind. Be on guard, do not abandon the mental post and do not forget that through divine help we can stop intruders. Exclude from human mind all offending errors; then the body cannot suffer from them.
The strength of Spirit
The body seems to be self-acting, only because mortal mind is ignorant of itself. The human mind is unaware of its own actions, and of their results—unaware that the inclining, outlying, and triggering cause of all bad effects is a law of so-called human mind, not of matter. Mind is the master of the physical senses and can vanquish sickness, sin, and death. Exercise this God-given power. Take possession of your body, and direct its feeling and action. Advance in the strength of Spirit to resist all that is unlike good. God has made you capable of this, and nothing can stop the ability and power divinely bestowed on your spirituality.