Every man being conscious to himself that he thinks
 John Locke, An Essay Concerning Human Understanding (1689). copy citation

Context

“and this is enough for a man who professes no more than to lay down candidly and freely his own conjectures, concerning a subject lying somewhat in the dark, without any other design than an unbiassed inquiry after truth. BOOK II—OF IDEAS CHAPTER I.—OF IDEAS IN GENERAL, AND THEIR ORIGINAL. 1. Idea is the Object of Thinking. Every man being conscious to himself that he thinks; and that which his mind is applied about whilst thinking being the IDEAS that are there, it is past doubt that men have in their minds several ideas,—such as are those expressed by the words whiteness, hardness, sweetness, thinking, motion, man, elephant, army, drunkenness, and others:” source