Dr James Grotstein (1925-2015) was the foremost Bion scholar, and one of the most noted and honoured psychoanalysts in the world. His prolific writings and generous encouragement to other analysts has had an enormous impact. He was among the first to examine Bion’s most controversial concept - O - in particular the mystical aspects of O. The title of this book, Of Things Invisible To Mortal Sight: A Celebration of the Work of James S. Grotstein, inspired by a line from Milton’s Paradise Lost (Book III), reflects Grotstein’s decades-long examination of the most profound aspects of the human mind. Dr James Grotstein’s erudition and depth of understanding made him one of the most revered psychoanalysts throughout the psychoanalytic world. He was well known and appreciated for his prolific writings, so it was only fitting to honour him through writing, and the fifteen articles in Of Things Invisible To Mortal Sight are written by esteemed analysts from Italy, Brazil, Argentina, Israel, and throughout the United States.