St Ignatius of Loyola began to learn about the discernment of spirits while convalescing from serious battle injuries. He noticed different interior movements as he imagined his future. In his autobiography, Ignatius writes ( in the 3rd person).
He did not consider nor did he stop to examine this difference, until one day his eyes were partially opened and he began to wonder at the difference and began to reflect on it. From experience he knew some thoughts left him sad, while other made him happy. And little by little he came to perceive the different spirits that were moving him, one coming from the devil and the other from God. Good and evil spirits.
Ignatius believed that these interior movements were caused by “Good Spirits” and “evil spirits”. We want to follow the action of a Good spirit and reject the action of an evil one. Discernment of Spirits is a way to understand God’s Will and desire for us in our lives.