Discernment is the Mother of all Virtues
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 on 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 virtues; everyone needs either to guide the lives of others or to direct and reform his own life.
Baldwin of Canterbury
from The Liturgy of the Hours, vol. 3, p. 312