The 'Sphere of Life and Death' is an onomantic divinatory device present in around sixty-two manuscripts of late medieval English provenance. It is an example of illicit divination, and as such is condemned in various theological and legal tracts. However, at the same time, one of the most frequent manuscript contexts of this device is medical, as it claims to predict the life or death of a sick person. This article aims to show that the 'Sphere' was at the same time both licit medicine and illicit divination, and that we must take a less rigid approach to the categorization of such devices.At the end of the fourteenth century, John Mirfield (d. 1407), chaplain of the hospital of St. Bartholomew in Smithfield, London, produced two Latin encyclopedias, the Breviarium Bartholomei, dedicated to bodily health, and the Florarium Bartholomei, dedicated to spiritual health. In a chapter of the Breviarium entitled 'De signis malis', Mirfield lists a plethora of methods available to a physician to predict the life or death of a sick person, and includes an experiment to prognosticate life or death using the numbers that correlate to the letters of personal names: 'Take the name of the patient, the name of the messenger sent to summon the physician, and the name of the day upon which the messenger came to you; join all their letters together, and if an even number result the patient will not escape, if the number is odd then he will recover'.