Proving Matilda

My senior thesis at Harvard (1967) was a study of the brief career of Geoffrey, duke of Brittany from 1181 to 1186.  He ruled Brittany iure uxoris, through his marriage to Duchess Constance, heiress of Duke Conan IV.  Two children of Geoffrey by Constance are well known.  Arthur, Geoffrey's posthumous son, was captured by his uncle, King John, at Mirebeau in August 1202.  Arthur never emerged from captivity, and he was probably murdered at the beginning of April 1203, aged 16.  His sister...