Impelling Spirit is a book about Jesuit spirituality as seen in its origins. As such it responds to the challenge of Vatican II that the appropriate renewal of religious life demands a return to the sources of Christian life and the spirit and aims of the founders of an institute. The instrument the author employs is a 1539 document Ignatius and his companions drafted for Pope Paul III as an apostolic letter addressed to themselves; this document - long neglected and largely unknown - clearly reveals how they understood themselves and their way of life. It demonstrates that the spirit and aims of the Society, though radical in 1539, were also deeply rooted in the Christian tradition.