Addressing myself to you, as teachers, I would say, mere book learning in physical science is a sham and a delusion — what you teach, unless you wish to be impostors, that you must first know; and real knowledge in science means personal acquaintance with the facts, be they few or many.
Collected Essays (Volume 8), Discourses, Biological and Geological, A Lobster; or, The Study of Zoology (p. 227)