Art historian and curator Mark Rosenthal. He is curator of the new exhibition "Abstraction in the Twentieth Century: Total Risk, Freedom, Discipline" at the Guggenheim Museum in New York (Feb 9 - May 12). There's also a companion book to the exhibition, same title, published by Guggenheim. Rosenthal is Curator of Twentieth Century at the National Gallery of Art, Washington D.C. and formerly Consultative Curator at the Solomon R. Guggenheim Museum.