The professional performer is the tree in the philosopher’s human forest. If there’s no opportunity to sing or act or dance in front of an audience, are they still a performer at all? In the spring of 2020, when most of his colleagues shrugged and started making banana bread, the tenor Ian Bostridge took an altogether more existential approach to isolation, writing a series of lectures for the University of Chicago exploring the relationship between self and singer, silence and song.