Consider a simple coordination game (the payoff matrix is to the right). The row player can play "a" if he can reasonably believe that the column player could play "A", since "a" is a best response to "A". He can reasonably believe that the column player can play "A" if it is reasonable for the column player to believe that the row player could play "a". She can believe that he will play "a" if it is reasonable for her to believe that he could play "a", etc.