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