Answered in example sentence
One of these times, as I was pondering with the paper before me, a pen in my ear, my elbow on the desk, and my cheek in my hand, thinking of what I should say, there came in unexpectedly a certain lively, clever friend of mine, who, seeing me so deep in thought, asked the reason; to which I, making no mystery of it, answered that I was thinking of the Preface I had to make for the story of "Don Quixote," which so troubled me that I had a mind not to make any at all, nor even publish the achievements of so noble a knight.