John Emerson Roberts, freethought preacher (see Books) believed in clear thinking.  He believed that encouraging people to think for themselves was his mission, his contribution to society.  He lifted up free thinking against all doctrine, all dogma.  He could not lecture without metaphor.  Sometimes he even mixed his metaphors, as in the following example:

“The brain is the sun. Civilization is its light. Thought is the mother of progress. The mother must be free in order that the child may be well-born.”  (Roberts, Lecture on Ingersoll, 1902.)

Light is a popular metaphor among freethinkers.  Robert Ingersoll used it extensively.  Motherhood was a beloved concept of the latter half of the nineteenth century.  There is more to say on both of these.