What is the secret in Book 1, Chapter 3 of "Tale of Two Cities", and what is it referring to?
As Lorry rides in the mail-coach on the way to fulfill his secret obligation, he reflects on the isolation in which every member of the human race exists. He thinks, "every human creature is constituted to be that profound secret and mystery to every other".
Lorry is riding with two other passengers, and he has a secret mission of which the other two are not aware. His philosophical thoughts refer specifically to his own situation in the beginning, and as he thinks about how his own clandestine errand remains completely hidden from the others, his reflections lead him to wonder more generally at the secrets every other man holds in his heart. He ruminates that it is a fact of the human condition that man is fated to exist alone, unknown by all around him. Lorry concludes that it is man's "natural and not to be alienated inheritance" that each person, rich or poor, remain "mysteries to one another", the innermost secrets of their being never to be completely revealed or shared (Chapter 3).