The outward facts of nature are insufficient to satisfy the mind. We cannot be content with knowing that the light and heat of the sun illuminate and warm the world. We are led irresistibly to enquire, 'What is light, and what is heat?' and this question leads us at once out of the region of sense into that of imagination.

Fragments of Science: A Series of Detached Essays, Addresses, and, Reviews (Volume 1), Chapter II (p. 72)