Auto-suggestion is a term which applies to all suggestions and all self-administered stimuli which reach one’s mind through the five senses. Stated in another way, auto-suggestion is self-suggestion. It is the agency of communication between that part of the mind where conscious thought takes place, and that which serves as the seat of action for the subconscious mind.
Through the dominating thoughts which one permits to remain in the conscious mind (whether these thoughts be negative or positive, is immaterial), the principle of auto-suggestion voluntarily reaches the subconscious mind and influences it with these thoughts.
No thought, whether it be negative or positive, can enter the subconscious mind without the aid of the principle of auto-suggestion, with the exception of thoughts picked up from the ether. Stated differently, all sense impression which are perceived through the five senses are stopped by the conscious thinking mind, and may be either passed on to the subconscious mind, or rejected, at will. The conscious faculty serves, therefore, as an outer-guard to the approach of the subconscious.
Nature has so built man that he has absolute control over the material which reaches his subconscious mind, through his five senses, although this is not meant to be construed as a statement that man always exercises this control. In the great majority of instances, he does not exercise it, which explains why so many people go through life in poverty.
~ Napoleon Hill, Think & Grow Rich