No person can ever hope to gain an adequate idea of the religious movement which has been called by the name of Quakerism until he has discovered what is meant by the "Inner Light." It is the root principle of an important historic faith, and it deserves a careful examination.
The term "Inner Light" is older than Quakerism, and the idea which is thus named was not new when George Fox began to preach it. But this idea received a meaning and an emphasis from the Quakers which make it their own peculiar principle and their distinct contribution to religious thought.

Social Law in the Spiritual World (1904)