Every man of reflection is vaguely conscious of an imperfectly-defined circle which is drawn about his intellect.
 Oliver Wendell Holmes Sr., The Autocrat of the Breakfast-Table (1858). copy citation

Context

“The forms or conditions of Time and Space, as Kant will tell you, are nothing in themselves,—only our way of looking at things. You are right, I think, however, in recognizing the category of Space as being quite as applicable to minds as to the outer world. Every man of reflection is vaguely conscious of an imperfectly-defined circle which is drawn about his intellect. He has a perfectly clear sense that the fragments of his intellectual circle include the curves of many other minds of which he is cognizant. He often recognizes these as manifestly concentric with his own, but of less radius.” source