Charles F. Haanel 
The Master Key System 
With questionnaire and glossary

As the most powerful forces of Nature are the invisible forces, so we find that the most powerful forces of man are his invisible forces, his spiritual force, and the only way in which the spiritual force can manifest is through the process of thinking. Thinking is the only activity which the spirit possesses, and thought is the only product of thinking.

The real secret of power is consciousness of power. The Universal Mind is unconditional; therefore, the more conscious we become of our unity with this mind, the less conscious we shall become of conditions and limitations, and as we become emancipated or freed from conditions we come into a realization of the unconditional. We have become free!