"The eye is the lamp of the body.  So, if your eye is healthy, your whole body will be full of light; but if your eye is unhealthy, your whole body will be full of darkness. If then the light in you is darkness, how great is the darkness!"

The desert fathers understood this passage to refer to discretion (see John Cassian, The Conferences, Conference II, 2).  Discretion is the ability to judge the thoughts and intentions of the heart before acting.  It is the guardian of virtue.

Also according to the ancients, there are three sources of our thoughts: God, the devil and ourselves.  We must never blindly allow ourselves to entertain just any of the constant stream of thoughts which are available for our consumption but like a careful gatekeeper accurately judge which are healthy for us.  During my stay at a monastery once, after having meditated on this God taught me the following simple prayer which I prayed constantly to great effect: "O God, take my thoughts, give me your thoughts and keep far from me the devils thoughts."  In this way the Lord taught me humility through submission of all my thoughts to him and to the monk set over me as spiritual guide.