For the arch-Romanian saint John Cassian, discernment has to do with every aspect of both the interior and exterior life of the monk and the Christian in general. He sets a great deal of emphasis on the necessity of discernment in the monk’s process of achieving such goals as purity of the heart and the kingdom of God, and this emphasis is so strong that his contribution to the understanding of the concept of discretio has been very appreciated in the East. The author has many times underlined the exquisiteness of discernment and how it is so necessary in the development of virtues. Discernment is needed for the development of the perception and knowledge of the spiritual realm and puts at the monk’s disposal the power to detect demonic deceit. Discernment operates inside the mind and reason and offers the monk the ability to make rightful choices and both examine and understand his interior state. It thus guides him and counsels him throughout all his decisional processes in such way that both his interior life and his exterior life are balanced. The monk has to cultivate discernment and learn how to use it efficiently: it must be sought after, kept and constantly used.