The Commens Dictionary
Quote from ‘The Fourth Curiosity’
Any object whose attributes, i.e. all that may truly be predicated, or asserted, of it, will, and always would, remain exactly what they are, unchanged, though you or I or any man or men should think or should have thought as variously as you please, I term external, in contradistinction to mental. For example, a dream is mental, because it depends upon what passed in the thoughts of the dreamer whether it be true that the dream was of a dog or was of the Round Table of King Arthur or of anything else. On the other hand, the colors of objects of human experience and in particular the contrast between the color of the petals of a Jaqueminot rose and that of the leaves of the bush, although it is relative to the sense of sight, is not mental, in my sense of that word.