Thomas Willis's description of the intercostal nerves has not received much attention by historians of medicine. Yet the intercostal nerves are of paramount importance for his neurology. Willis explained that via these nerves, which connect the brain to the heart and lower viscera, the brain controls the passions and instincts of the lower body. In other words, Willis believed that the intercostal nerves mediate a kind of rationality and that therefore they make a human a rational being. Willis's theory, I argue, must be seen in the context of the early modern mind-body problem. In the second part of the article I discuss how Oxford theologian Samuel Parker took up Willis's argument while stating that the intercostal nerves are the most important instruments (reins) of the soul. They control the bodily passions so that humans can transform into more virtuous beings. The explanation of the intercostal nerves offered by Willis and Parker fits the Anglican optimism about the abilities of human reason as well as about the moral potential of humankind.