The present work tries to see whether Leonardo Polo´s proposal expands synderesis in a way that, assuming the medieval traditional views, it can be the ontological basis for grounding a personalistic view of ethics.Synderesis seems to be a controversial topic. The human reality behind the term was touched upon by Greek and Latin philosophers but the term was not used in classic philosophy. It was Jerome the first to use this term in his commentary to Ezequiel 1:4-14. This was a theological treatise. Synderesis was a popular term among philosophers in the Middle Ages especially after Peter Lombard included St. Jerome’s commentary in his Sentences. Peter Lombard’s Sentences was a common textbook in most universities and therefore synderesis became a compulsory and therefore well know term in medieval philosophy. Aquinas used it extensively as well as his commentators. Aquinas considered synderesis an intellectual habit responsible for the first principles of morality. For Bonaventura synderesis was the will’s inclination to do good. Ockham does not mention synderesis in his works. Luther in his early works mentions it but later breaks with tradition and no longer accepts synderesis as he defended that nature was totally corrupt after the original sin, and synderesis was seen as an unpolluted remain of the original nature which kept men aware of sins. From there on the topic, with rare exceptions, was only developed by those who followed medieval traditions. It looks as if Leonardo Polo not only regains the traditional view of synderesis but makes it a pivotal topic in anthropology. For him synderesis becomes the innate habit through which the personal act of being activates all human faculties. Synderesis then, is at the root of all human decisions, because it becomes the unifying structure of the intellect and will. Because of this it also becomes the root of ethics because it allows viewing human decisions in their relationship with the human integral good