Evagrius was a desert monk who lived from 345-399. He was well known for his academic abilities and was frequently sought after for his wisdom but fled the life of popular acclaim upon being tempted to have an affair (think Joseph and Potiphar’s wife).
An anonymous history, Historia Monachorum testifies of him, “We also visited Evagrius, a wise and learned man who was skilled in the discernment of thoughts, an ability he had acquired by experience. He often went down to Alexandria and refuted the pagan philosophers in disputations…He taught us much else about ascesis, strengthening our souls.” His work was described as “training his intellect to examine his thoughts systematically (Palladius’ Coptic Life). This “thinking about thinking” was seen as a direct continuation of Jesus’ command to repent, because the Greek word behind it means, “rethink your thoughts.”
Evagrius’ Core ideas (In his book, “153 sayings on Prayer):
He would often use the word “demons” for bad thoughts.
The pursuit of the good (especially spiritual prayer) is hindered by the passions: “What is it that the demons wish to excite in us? Gluttony, unchastity, avarice, anger, rancor, and the rest of the passions, so that the intellect grows coarse and cannot pray as it ought. For when the passions are aroused in the non-rational part of our nature, they do not allow the intellect to function properly.”
“When the demons see you truly eager to pray, they suggest an imaginary need for various things, and then stir up your remembrance of these things, inciting the intellect to go after them; and when it fails to find them, it becomes very depressed and miserable.”
This is analogous to your experience sitting down to do homework or deciding to clean your room, or resolving to exercise, etc.
What is spiritual prayer? “Whether you pray with brethren or alone, try to pray not simply as a routine, but with conscious awareness of your prayer. Conscious awareness of prayer is concentration accompanied by reverence, compunction and distress of soul as it confesses its sins with inward sorrow.”
How does one dispell the passions and distracting thoughts which stir them up? “He who has mastery over his incensive power has mastery also over the demons. (Discrimination of Passions and Thoughts)”
Finally, here’s his thought on how the Christian interested in daily growing in Christlikeness ought to live, “A monk should always act as if he was going to die tomorrow; yet he should treat his body as if it was going to live for many years. The first cuts off the inclination to listlessness, and makes the monk more diligent; the second keeps his body sound and his self control well balanced” (Texts on Watchfulness).