We can trace the development of a nervous system, and correlate with it the parallel phenomena of sensation and thought. We see with undoubting certainty that they go hand in hand. But we try to soar in a vacuum the moment we seek to comprehend the connexion between them...Man the object is separated by an impassable gulf from man the subject.

'Address Delivered Before The British Association Assembled at Belfast', 19 August 1874. Fragments of Science for Unscientific People: A Series of Detached Essays, Lectures, and Reviews (1892), Vol. 2, 194–5