One could of course see this as an observation on the patriarch’s eyesight… at least as far as one eye is concerned; nothing is said about the other. But this would be to misunderstand rabbinic symbolism.
To appreciate what the rabbis really meant it is necessary to go to the actual passage, a comparison between “the disciples of Abraham our father” and “the disciples of Balaam the wicked”.
It declares that “a good eye, a humble mind and a lowly spirit (are the signs) of the disciples of Abraham our father”, whereas “an evil eye, a haughty mind and a proud spirit (are the signs) of the disciples of Balaam the wicked”.
“A good eye” is a generous spirit; “an evil eye” is a grudging attitude. This is why the evil eye came especially in its Yiddish version, einahora, to mean wishing someone ill fortune, and all sorts of remedies were suggested in order to counter the evil eye.
The Talmud says that 99 people die through an evil eye for one who dies from natural causes (BM 107a); it also remarks that a person may not stand over his neighbour’s field when the crop is ripe, and Rashi explains that he might harm it through an evil eye (ibid.).
However, the concept of the good eye is not merely a general disposition towards benevolence but specifically defined.
In the Jewish agricultural system which required a portion of one’s crops to be given to the kohen, an average donor gave a 50th, a mean person (the one with an evil eye) a 60th and a generous person who had a good eye a 40th.
Abraham is the prototype of the person with a good eye.
When passers-by needed hospitality, he exerted himself for their benefit. Though he said he would fetch them “a morsel of bread”, he did more than expected and gave them a whole feast (Gen. 18).
He not only welcomed weary travellers but positioned himself outside his tent so that he could see them coming (ibid.).
When he and his nephew Lot had to part, he offered Lot the better quality pastures for his flocks and herds.
When his contemporaries needed spiritual guidance he went to them and brought them under the wings of the Divine presence: the first outreach program in history.
What do we learn from Abraham?
To cultivate a broad, generous heart which rejoices in the happiness of others and instinctively seeks their welfare.