In the book of Matthew, Yeshua speaks about a "single" and an "evil" eye. In our Western culture this terminology can be difficult to understand, but in the time when Yeshua spoke these words His audience knew exactly what He was talking about. In fact, "eye" terminology is still used in the Middle Eastern countries today. In the Middle East, if someone were to ask you for a donation for the needy, they would ask you to give with a good eye.
Matthew 6:22-24 The lamp of the body is the eye: if therefore thine eye be single, thy whole body shall be full of light. But if thine eye be evil, thy whole body shall be full of darkness. If therefore the light that is in thee be darkness, how great is the darkness! No man can serve two masters: for either he will hate the one, and love the other; or else he will hold to one, and despise the other. Ye cannot serve God and mammon. (ASV)
In Hebraic understanding a reference to an "eye" referred to a person's relationship with others. Oftentimes, as in these scriptures it had to do with money. A stingy or self-centered person was one who would not give liberally to help the poor, needy, homeless, widow, fatherless, etc... and that person was said to have a bad or an evil eye. This type of person had a money problem that did not line up with Torah. In fact, one of the main reasons that Yahweh gives for driving His people out of the promised land was because they did not do these things. (Zech. 7:8-11 is just one scriptural example). This could have been the problem with the Rich Young Ruler of Matthew 19:16-22.
In Mark 7:21-23 an "evil eye" is again mentioned as that which comes from within a man and defiles him. Having a "single" eye denoted a good thing. It meant that the whole body was full of light. How did having a single eye or a "good eye" differ from having a bad or evil eye? A person with a good eye was not self-centered or stingy when it came to meeting the needs of others and truly caring about their fellow man. The word "single" are "singleness" is often used in the Bible and means a person who is "sincere" in their actions or motives. So, a person with a "single eye" is sincere and conscientious about their treatment of others.
The lamp of the body is the eye. The kind of “eye” (good/generous or evil/stingy) determines how your life’s path is illuminated. If your eye is stingy or bad/evil then you are not following Torah and your whole world (path) is full of darkness. Likewise, if your eye is good/generous it lights the path and illuminates your path and your walk. No one can serve two masters-Yahweh and money. The love of money is the problem, not the money itself. The word “mammom” is Chaldee and means wealth.
I like this verse to sum it up, Matthew 7:12 - "All things therefore whatsoever ye would that men should do unto you, even so do ye also unto them: for this is the law and the prophets." (ASV)