"The eye is the lamp of the body; so then if your eye is clear, your whole body will be full of light. "But if your eye is bad, your whole body will be full of darkness. If then the light that is in you is darkness, how great is the darkness! (Matthew 6:22-23 NASB)
The Sermon on the Mount as recorded in the Gospel recorded by Matthew has a very logical flow. It is not a random sampling of great one-liners from Jesus but an actual Sermon that flows logically from one teaching to the next. This is critical for our understanding of this teaching. The various points in the Sermon on the Mount must be viewed in light of the teachings around those points. A prime example of that is the section we just finished teaching about hypocrisy. The Lord's Prayer was tucked into that section and if we avoid the context surrounding the Lord's Prayer we will miss the whole point Jesus is making when He is teaching how to pray. The reason I bring this up is because Matthew 6:22-23 is the one text in the entire Sermon on the Mount that seems out of place to me, which leads me to believe I do not have a very good handle on what these two verses mean.
Preceding Matthew 6:22-23 we read about not storing up treasure on earth, but in heaven, and following Matthew 6:22-23 we read that you cannot serve both God and money. It is relatively easy to make the 'treasure in heaven', and 'cannot serve God and money' connection and their proximity to each other in the Sermon on the Mount just makes sense. The difficult thing for my understanding is that they are divided by the verses at the top of this post. When reading this verse as a standalone verse and not considering the context I would view as a verse about lust, or about taking care of what you set your eyes upon, thinking I could apply it to television, literature, advertising, and so on. The verse makes a lot of sense in that light. The things we look upon shape us and illumine us. If it is our eyes that we use to bring light (righteousness, purity, etc...) into ourselves, how great will our wickedness be if we have a 'bad eye' that looks upon wickedness? In that light this verse seems to make a simple command to look upon what is good in the sight of God, and certainly we are called in scripture to do just that. At the same time, given the context around this verse gives it an entirely different purpose.
This idea of 'if your eye is clear' is a call to discernment, and it is a call to discernment with regard to what is treasure on earth vs. what is treasure in heaven. The command in the preceding verses is to not lay up treasure on earth but in heaven, this verse is a call to discern what heavenly treasure is and what it is not. Moreover this passage explains the great darkness in lacking this discernment or having an 'eye that is bad'. The next verse in the logical flow will be an aid in this discernment concerning treasure in heaven vs. treasure on earth and will point out incisively that one cannot serve both God and wealth. Now I will agree wholeheartedly that these verses apply in the realm of lust, television, reading, and any other activity of the eyes, however the intent of these verses must be determined by their context. The intent is to teach that the eye must be good to discern the treasure which should be sought after, and the consequences of a 'bad eye' with regard to heavenly vs. earthly treasure.