The general principle latent in these diverse applications of
the idea of justice is that individuals are entitled in respect of
each other to a certain relative position of equality or inequality.
This is something to be respected in the vicissitudes
of social life when burdens or benefits fall to be distributed;
it is also something to be restored when it is disturbed. Hence
justice is traditionally thought of as maintaining or restoring
a balance or proportion, and its leading precept is often formulated
as 'Treat like cases alike'; though we need to add to the
latter 'and treat different cases differently'. So when, in the
name of justice, we protest against a law forbidding coloured
people the use of the public parks, the point of such criticism
is that such a law is bad, because in distributing the benefits
of public amenities among the population it discriminates
between persons who are, in all relevant respects, alike. Conversely,
if a law is praised as just because it withdraws from
some special section some privilege or immunity, e.g. in taxation,
the guiding thought is that there is no such relevant
difference between the privileged class and the rest of the
community as to entitle them to the special treatment. These
simple examples are, however, enough to show that, though
'Treat like cases alike and different cases differently' is a central
element in the idea of justice, it is by itself incomplete and,
until supplemented, cannot afford any determinate guide to
conduct. This is so because any set of human beings will
resemble each other in some respects and differ from each
other in others and, until it is established what resemblance
and differences are relevant, 'Treat like cases alike' must
remain an empty form. To fill it we must know when, f~r the
purposes in hand, cases are to be regarded as alike and what
differences are relevant. Without this further supplement we
cannot proceed to criticize laws or other social arrangements
as unjust. It is not unjust for the law when it forbids homicide to treat the red-haired murderers in the same way as others;
indeed it would be as unjust if it treated them differently, as
it would be if it refused to treat differently the sane and the
Insane.