Ernulf, bishop of Rochester, died aged eighty-four, on 15 March 1124. In the course of his life, he studied under Lanfranc and was a close friend of Anselm at Bec. One-time prior of Christ Church, Canterbury, his advice was apparently sought by the king; he became a much respected abbot of Peterborough; and, as bishop, he instigated the important collection of secular and ecclesiastical law, the Textus Roffensis. Of his own writing, only three letters survive: one to Anselm, pleading with him to return from exile; one to the monk Lambert of St-Bertin, answering four questions on the eucharist and a fifth concerning a passage from the prophet Joel; and the third to Walkelin of Winchester, dealing with the case of canon law which the two men had previously discussed. It is this last letter, appearing in the manuscripts with the title De incestis coniugiis, which makes of Ernulf something more than a shadow among the Anglo-Norman theologians and men of letters who came to England in the aftermath of conquest. It is in this letter-treatise that Ernulf emerges as an accomplished lawyer and juridical thinker, whose approach has departed radically from that of Lanfranc, his former teacher, and is closely comparable to the principles for legal judgement set down by Ivo of Chartres in the preface to his Decretum and Panormia. Ernulf's use of such methods, grounded in, and made possible by, the new systematic collections of canon law, helps to confirm what has already begun to be evident, that this systematic, deliberately and self-consciously rational, jurisprudence finds its way into England well before the dissemination of Gratian's Decretum in the mid-twelfth century.