This work seeks to recover the importance and value of the knowledge produced by medieval philosophy. This book deals with the classical concept of soul brought by Aristotle, which explains the soul's way of being in beings, in other words, its ontology. St Thomas Aquinas adopts some Aristotelian concepts and with the refinement of his theories, passing through Boethius, brings us the definition of person. This concept is still used today to guarantee people's rights and protect humanity, and is the basis for the Social Doctrine of the Church (DSI), the Universal Declaration of Human Rights (DUDH) and even the 1988 Constitution of the Federative Republic of Brazil.