The view from nowhere does not exist; it is accessible to no one.
I think that justice—the character trait—is arrived at through a lifelong dialogue among three points of view.
The first is your own, honed through proper notions of prudence, courage, temperance, charity, inheritance and hope.
The second is the person or people you are trying to figure out what you owe, or what they owe you. Hume’s faculty of sympathy, or what we call sympathy today plus what we call empathy, is your primary tool here, as well as a lifetime of experience attempting to understand yourself and others.
The final one is Smith’s impartial spectator; impartial not in the sense of objective but in the sense of not being partial, not having a stake in the outcome.
All three are in a way mental constructs, as we must have mental models of ourselves, the people we are dealing with, and an imagined impartial judge observing the matter. And all three require nourishment through use and persistent critical evaluation over time.
You arrive at just decisions through the effective weighing of the claims made by these inner agents; you become truly wise when these inner constructs closely approximate real people who exist outside of your mental world.