"…possess a good eye…." (Avot 5:19)

It is written, "Let your eyes be in the field that they are reaping" (Ruth 2:9) which is to be explained with the verse, "He who has a good, generous eye will be blessed". (Proverbs 22:9) The eye denotes wisdom, and in truth "looking" is the lower level of wisdom; as one looks and regards something, he brings blessing into that thing. For when he looks at an object, he knows that object is as nothing before the blessed L-rd - meaning that it is truly naught and nothing except for His divinity which is manifest within it….Without Him it is utterly naught, in keeping with the verse, "…but what are we?" (Ex. 16:7) Through this kind of look and regard, he draws down supernal energy to that object from the divinity of the blessed L-rd. This is the recondite sense of the verse, "He who has a good, generous eye will be blessed": he brings blessing to that object.

The opposite is equally true. When a person with an evil, ungenerous eye stares at the object and is impressed by it, thinking, "How lovely [or handsome] that object is," he makes it a matter of worth by itself. Thus he is a "whisperer who separates away the Divine Friend". (Proverbs 16:28) This is because, by his regard and stare, that object is cut off from its root-source of energy - divinity. As it is known, such staring forms a "vessel" [a place or situation for bad influence in the world] and as a result, the "evil eye" prevails (May Heaven spare us!). [Likkutei Amarim, citing the Baal Shem Tov]

[From "Baal Shem Tov on Pirkei Avot"]