Abstract
It must be rare that discoveries which transform mathematics also undermine legal rules. Yet this is precisely what happened when probability was first developed in the second half of the 17th century and the first decades of the following one. The focus of this article is a doctoral thesis in law written in 1709 by Nicolaus Bernoulli, an important mathematician of the age. He highlighted the dramatic implications of the new mathematics of probability for a rule which was fundamental to contemporary contract law in continental Europe. This article reconstructs a remarkable story about the place of mathematics in the history of contractual justice and the place of contractual justice in the history of mathematics.