In verse 22, the word single means "whole," "undivided," implying seeing things in their reality since the eye is not diseased in any way. In verse 23, Jesus says, "if your eye be evil." Here the word evil means "divided" or "unhealthy." He is using this illustration to indicate that, if the eye is in good health and is able to see clearly, a person has a good chance of being able to see at least the physical reality clearly.
But if a person's eye is diseased—if there is macular degeneration or cataracts forming, or if the muscles of an old person's eye are not working properly so that he loses maybe his close-up or distant vision—we know that he will not have clear sight unless those conditions are corrected in some way.
The real subject of Jesus' saying is the importance of being able to focus in order to fulfill one's purpose in life.
John W. Ritenbaugh
Jesus urges single-mindedness! The teaching here involves simplicity of intention in living one's life. In light of verse Matthew 6:33, verse 24 shows we must focus our attention on our highest priority. When that is done, it indicates devotion to purpose and undivided loyalty to the object of that purpose.
In geometry, it is impossible to draw more than one straight line between two points. Though other lines may start at the same point, only one will reach the second point. All others end up somewhere else. Likewise, a person who tries to focus on several goals at once has no clear orientation, and he will wind up elsewhere.
Some commentaries note that the ancients believed that light entered a person through the eyes, the "windows" of the body. If the eyes were in good condition, the whole body benefited from the unimpeded light. If the eye were not sound or "single," the whole body's effectiveness was diminished. Thus a person who single-mindedly pursues God's Kingdom and His righteousness will have moral healthiness and simple, unaffected goodness.
John W. Ritenbaugh
Simplify Your Life!