Perelman & Olbrechts-Tyteca introduce the rule of justice as a fundamental argumentative principle, “all beings of the same category must be treated in the same way”. The rule is illustrated by some categories that have historically regulated the distribution of benefits, “to each according to his merit; to each according to his birth; to each according to his needs” (Perelman [1963], p. 26).

The rule founds claims such as “equal pay for equal work”. It involves distinct operations.

(i) A categorization — First, individuals are categorized as members of a general category, “to be born”; “to have needs”; “to have merit” (admitting that one can deserve a punishment and that to demerit is to have a negative merit); “to be an employee, having worked such and such hours and produced such and such assessable products”.
General rights and duties can be defined with recourse this first level, “all born human beings have the same right to life”. The following practice refers to a strict a pari argument, referring to thieves as a non-hierarchized category, “a thief is a thief”.

General Baclay was also quite a character, but a funny woman, very just in her own way. She shot in the same way women and men, all thieves, whether they had stolen a needle or an ox. A thief is a thief and all were shot. It was fair.
Ahmadou Kourouma, Allah is not obliged. 2000.[1]

(ii) An equality relation — Secondly, there is an equality relation defined as “equality of birth; of needs; of merits; of work”. This relation determines a hierarchy between workers, “P has worked as much as Q or R…; more than A or B…; less than X or Y…”.
Such equipped categories can be represented on oriented scales. The position of an individual upon this scale can be debated, “has X more/less merit than Y?”.
The metric is easy to define in cases of work, when determined by the weight of the fruit picked from the trees for example. Things become more complicated when it comes to scientific production, or when it comes to needs and merits. In any cases, the criteria for prioritizing one individual over another one must be set.

(iii) An allocation scale — Another quite different scoring method must be established in order to define the parallel scales of punishments and rewards (what wages for that level of work?), and the two scales must be coupled.

These two independent rankings ((ii) and (iii)) make the rule of justice more complex than an a pari argument. Gross a pari holds that “work must pay”:

if P works, P has a right to be paid for this work (except if P is a voluntary worker serving a non-profit organization),

while the rule of justice connects two graduated scales.

In addition, it is supposed that the rule of justice is to be applied to all members of the group in a linear order, but actual rules include thresholds. Regarding a tax level, the rule “to each according to his or her income” applies only beyond a certain threshold, and contains tax brackets and smoothing principles.

Other categories may be considered, showing that the rule of justice can also serve in support of injustices:

To each according to his or her gender
To each according to his or her color of skin

The rule of justice excludes arbitrariness, but not injustice. According to the principle “who favors disfavors”, the rule of justice, necessarily creates innumerable injustices. If the benefits are distributed according to merit, they are not distributed according to birth or according to need.
The rule of justice is said to be “just” because it excludes the arbitrariness of the principle “to everyone according to my convenience”; and because the category and the hierarchy have been defined by disregarding the cases to be judged, “the decision is just because the rule existed before your case.” This “justice” is formally just because it allows the application of a legal syllogism.

[1] Ahmadou Kourouma, Allah n’est pas obligé. Paris: Le Seuil, p. 111