“The rules of justice are accurate in the highest degree, and admit of no exceptions or modifications, but such as may be ascertained as accurately as the rules themselves.”
Smith contrasts justice with looser virtues by arguing that its external obligations require unusually precise rules if people are to rely on one another.
Source details
The Theory of Moral Sentiments, Part III discussion of the general rules of morality; Smith contrasts exact rules of justice with more flexible rules of prudence and generosity.