In the world of physics we watch a shadowgraph performance of the drama of familiar life. The shadow of my elbow rests on the shadow table as the shadow ink flows over the shadow paper. It is all symbolic, and as a symbol the physicist leaves it.... The frank realization that physical science is concerned with a world of shadows is one of the most significant of recent advances.

The Nature of the Physical World: Gifford Lectures (1927) (ed. Cambridge University Press, 2012) - ISBN: 9781107663855