It all started with a footnote. On p. 59 of his Architectural Writers of the Nineteenth Century, Nikolaus Pevsner wrote, ‘I have gone through the Willis papers but I cannot pretend to have read every page. There are thousands of them and a student in search of a thesis would find them rewarding.’ It's now more than twenty years since my curiosity was piqued by Pevsner's suggestion; this book is the result of many hours, both rewarding and frustrating, spent in the Cambridge University Library and the various other repositories holding dispersed fragments of Willis's archive.

But it really started at Lincoln Cathedral, where I was taken on a student excursion by my tutor, Christopher Wilson, just as he had been taken there as a student by Nikolaus Pevsner. Like them, I became fascinated by the intriguing series of vaults whose three-dimensional geometry became the subject of my undergraduate dissertation. This took me to Willis, whose work on vaults had tried to reconstruct the mental processes used by medieval masons in designing and building complex rib vaults. He thereafter re-enacted these thoughts for himself when physically reconstructing the lost vaults of Prior Crauden's Chapel at Ely and that of the Great Gate at Trinity College, Cambridge. As I read Willis's paper, I felt an intellectual kinship with the author: an awareness of mutual aims, despite our chronological distance. Just as Willis hoped he could enter into the mind of the medieval mason, so he deliberately communicated his ideas in a way that forces his readers into active engagement with his argument.