On the evening of April 18, 1775, Samuel Johnson accompanied his friends James Boswell and Joshua Reynolds on a visit to Richard Owen Cambridge’s grand villa on the banks of the Thames outside London. They were shown into the library, where Cambridge was waiting to meet them, and after a brief greeting Johnson darted to the shelves and began silently reading the spines of the volumes arrayed there. “Dr. Johnson,” said Cambridge, “it seems odd that one should have such a desire to look at the backs of books.” Johnson, Boswell would later recall, “instantly started from his reverie, wheeled about, and replied, ‘Sir, the reason is very plain. Knowledge is of two kinds. We know a subject ourselves, or we know where we can find information upon it.’”