In fact, wherever information – or comparison – is of the essence of our explanation, there, for me, is mental process. Information can be defined as a difference that makes a difference. A sensory end organ is a comparator, a device which responds to difference. Of course, the sensory end organ is material, but it is this responsiveness to difference that we shall use to distinguish its functioning as "mental." Similarly, the ink on this page is material, but the ink is not my thought. Even at the most elementary level, the ink is not signal or message. The difference between paper and ink is the signal.

Gregory Bateson & Mary Catherine Bateson, Angels Fear