A man, if he compares two things together, can hardly be supposed not to know what it is wherein he compares them
 John Locke, An Essay Concerning Human Understanding (1689). copy citation

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Author John Locke
Source An Essay Concerning Human Understanding
Topic
Date 1689
Language English
Reference
Note
Weblink http://www.gutenberg.org/files/10615/10615-h/10615-h.htm

Context

“because the knowledge of one action, or one simple idea, is oftentimes sufficient to give me the notion of a relation; but to the knowing of any substantial being, an accurate collection of sundry ideas is necessary. A man, if he compares two things together, can hardly be supposed not to know what it is wherein he compares them: so that when he compares any things together, he cannot but have a very clear idea of that relation. THE IDEAS, THEN, OF RELATIONS, ARE CAPABLE AT LEAST OF BEING MORE PERFECT AND DISTINCT IN OUR MINDS THAN THOSE OF SUBSTANCES.” source