Claiming that blood circulated fundamentally questioned all understanding of what blood was, and what it did. Scientists in Oxford began to assess two possible routes of investigation — infusion and transfusion. In about 1639, the year before Denis was born, Oxford scholar Francis Potter began to consider the ideas of transfusing blood from one animal to another. He claimed that this occurred while he was thinking about an event that occurs in Publius Ovidius Naso's story of Jason and the Argonauts. In Ovid's tale, Medea restored the youth of Jason's aged father by giving him a brew incorporating moonlight and hoar frost, bits of owls, tortoises, wolves, stags and crows. Medea poured her brew into his mouth while letting his blood flow from a cut in his neck. According to legend, the old man's youth was instantly restored; his white hair resumed its former black colour, his veins filled with blood and his paleness was replaced with a healthy complexion.
Potter wondered if there could be any way of replacing old and worn blood. Trying to solve recurrent illness was a personal issue, as he regularly suffered from epileptic fits. His 27 years at Trinity College, Oxford, encompassed the siege of the city, and it was through this period of history that Potter scribbled remarks on the sides of his note books commenting on the number of times that Harvey came to look at the hens' eggs that were incubating in the college. This was because Harvey was looking at the way that chick embryos develop in the egg, and was making almost daily visits to the chambers of his colleague Ralph Bathurst. By 1651, Potter and Harvey had become good friends, and the two enjoyed stretching the thinking of one another. This proved particularly useful for, in contrast to Harvey, Potter never had much interest in reading.