The human brain performs the function of thinking as involuntarily as the chest the function of breathing. However, we can, by our will, stop breathing for a while, and accelerate or retard the breathing movements. In the same way, the will can control the thoughts. We may choose any object as the subject matter of our thought, and yet we may quickly convince ourselves that the power of our will and the freedom of the mind are not any greater than the freedom of the chest in breathing.