In May 1953 he was commissioned by Lincoln Kirstein, director of the New York City center of Music and Drama, for a three-act opera to a libretto by Chester Kallman based on a story by Boccaccio, to be titled "The Tuscan Players". Intended to be finished in August 1954, it was first postponed to April 1955, but only finally completed in 1956, by which time the title had been changed twice, first to "Pánfilo and Lauretta", then to . The City Center waived its rights to the first performance, which was given under the title "Panfilo and Lauretta" in the Brander Matthews Theatre at Columbia University in New York on May 9, 1957, under the baton of Howard Shanet. Stage direction was by Bill Butler, scenic design by Herbert Senn and Helen Pond, and costumes by Sylvia Wintle. The principal singers were Sylvia Stahlman, Frank Porretta, Craig Timberlake, Mary McMurray, Michael Kermoyan, and Thomas Stewart . The opera would be revised twice more and the title changed again to ("The Visitors"), for productions in 1968 and 1973, in Mexico City and Aptos, California, respectively (; ). From 1958–1959 he was the Charles Eliot Norton professor at Harvard University, and the public lectures he gave there were published as a book, "Musical Thought" .