When making the analysis of matter we impliedly admitted two propositions: first, that sensation is the tertium quid which is interposed between the excitant of our sensory nerves and ourselves; secondly, that the aggregate of our sensations is all we can know of the outer world, so that it is correct to define this last as the collection of our present, past, and possible sensations.

p. 60; Definition of sensation - The Mind and the Brain, 1907