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Agathos (ἀγαθός) in Matt 20:15 means "good" which also appears in Mark 10:18 where Jesus rebukes a person for calling him good because God is the only one who is good. In the parable of vineyard workers, the landlord insists that he is good because he cares about all who need work and the daily wage. He is good because he cares about distributive justice (or economic justice for all). He is good because he cares about full employment and makes sure that all will get daily food.
Otherwise, this parable is not about attributive justice in which the logic is "the more one works, the more one is paid." This view is held by those who argue that the landlord abused the earlycomers by not paying enough or more than the rest.
But this is not the case. First, there is no guarantee that earlycomers worked harder than latecomers. Moreover, they agreed to work with the usual daily wage, which is not too much or too little. Their grumbling against the landlord does not stand their ground. Finally, the vineyard owner says: "Am I not allowed to do what I choose with what belongs to me? Or are you envious because I am good (agathos)?" (Matt 20:15). Unfortunately, the NRSV, NIV, and CEB, and most English Bibles translate agathos as "generous" which connotes a sense of charity, defending the landlord's wealth or power. Generosity is not the same as justice.