The Lord knows the thoughts and intentions of our hearts. Without a doubt, every one of them is known to him, while we know only those which he lets us read by the grace of discernment. The spirit of man does not know all that is in man, nor all of the thoughts which he has, willingly or unwillingly. Man does not always perceive his thoughts as they really are. Having clouded vision, he does not discern them clearly with his mind’s eye.
Often under the guise of devotion a suggestion occurs to our mind—coming from our own thoughts or from another person or from the tempter—and in God’s eyes we do not deserve any reward for our virtue. For there are certain imitations of true virtues as also of vices which play tricks with the heart and bedazzle the mind’s vision. As a result, the appearance of goodness often seems to be in something which is evil, and equally the appearance of evil seems to be in something good. This is part of our wretchedness and ignorance, causing us anguish and anxiety.
It has been written: There are paths which seem to man to be right, but which in the end lead him to hell. To avoid this peril, Saint John gives us these words of advice: Test the spirits to see if they are from God. Now no one can test the spirits to see if they are from God unless God has given him discernment of spirits to enable him to investigate spiritual thoughts, inclinations and intentions with honest and true judgement. Discernment is the mother of all the virtues; everyone needs it either to guide the lives of others or to direct and reform his own life.
In the sphere of action, a right thought is one ruled by the will of God, and intentions are holy when directed single-mindedly toward him. In a word, we could see clearly through any action of ours, or into our entire lives, if we had a simple eye. A simple eye is an eye, and it is simple. This means that we see by right thinking what is to be done, and by our good intention we carry it out with simple honesty, because deceitful action is wrong. Right thinking does not permit mistakes; a good intention rules out pretence. This then is true discernment, a combination of right thinking and good intention.
Therefore, we must do all our actions in the light of discernment as if in God and in his presence.
From a sermon by Baldwin of Canterbury, bishop (Tract . 6: PL 204, 466-467)