A single / healthy eye
Here’s what it means
An undivided, morally sound, or generous way of looking at life.
Why the Bible says it this way
In Matthew 6 the phrase is tied to treasure and masters, so it likely concerns a clear, loyal outlook rather than merely eyesight.
The writer uses vivid or unusual wording to make the point clear and memorable.
A helpful detail: Translations vary among 'single,' 'healthy,' 'sound,' and 'generous.'.