Book VIII The Philosophy of the Classificatory Sciences
Where a certain apparent difference between things (although perhaps in itself of little moment) answers to we know not what number of other differences, pervading not only their known properties but properties yet undiscovered, it is not optional but imperative to recognise this difference as the foundation of a specific distinction.
John S. Mill, System of Logic, b. 1, ch. vii. § 4.
BOOK VIII.
THE PHILOSOPHY OF THE CLASSIFICATORY SCIENCES.