This study documents the discovery of Peter Lombard's long-thought-to-be-lost lectures on the Old Testament, which were hidden in plain view in the Old Testament lectures of Stephen Langton, who lectured on the Lombard's lectures. The presence in the Lombard's lectures on Genesis of the logical theory of supposition, the single greatest advance in logical theory during the High Middle Ages, means that those lectures not only postdate the Sentences but also represent the beginning of a radical advance in speculative theology that would continue to develop through the end of the High Middle Ages. This means in turn that lectures on the Bible from the 1150s to 1200, and in particular those of the School of Paris, headed by Peter Lombard, play a central role in one of the greatest speculative developments — logical, philosophical, and theological — of the Middle Ages.