Answer:
Equality.
Step-by-step explanation:
The principle of equality, as the exercise develops, states that everyone should be able to access to the same facilities and to each other. The quote shows said principle: it is unjust to treat people differently in ways that deny to some of them significant social benefits unless we can show that there is a difference between them that is relevant to the differential treatment. Different access to facilities, social benefits, social treatment, etc. should not exist as everyone is equal.