John S. Mill, System of Logic, b. 1, ch. vii. § 4.
BOOK VIII.
THE PHILOSOPHY OF THE CLASSIFICATORY SCIENCES.
CHAPTER I.
The Idea of Likeness as Governing the Use of Common Names.
1. Object of the Chapter.—Not only the Classificatory Sciences, but the application of names to things in the rudest and most unscientific manner, depends upon our apprehending them as like each other. We must therefore endeavour to trace the influence and operation of the Idea of Likeness in the common use of language, before we speak of the conditions under which it acquires its utmost exactness and efficacy.
It will be my object to show in this, as in previous cases, that the impressions of sense are apprehended by acts of the mind; and that these mental acts necessarily imply certain relations which may be made the subjects of speculative reasoning. We shall have, if we can, to seize and bring into clear view the principles which the relation of like and unlike involves, and the mode in which these principles have been developed.