In both these, the foundation whereon Epicurus built was Reciprocity: not pure sacrifice to others, but partnership with others, beneficial to all.

He kept the ideas of self and of others inseparably knit together in one complex association: he did not expel or degrade either, in order to give exclusive ascendancy to the other.

The dictate of Natural Justice was that no man should hurt another: each was bound to abstain from doing harm to others; each, on this condition, was entitled to count on security and relief from the fear that others would do harm to him.
Such double aspect, or reciprocity, was essential to social companionship: those that could not, or would not, accept this covenant, were unfit for society.

If a man does not behave justly towards others, he cannot expect that they will behave justly towards him; to live a life of injustice, and expect that others will not find it out, is idle.

The unjust man cannot enjoy a moment of security.
Epicurus laid it down explicitly, that just and righteous dealing was the indispensable condition to every one's comfort, and was the best means of attaining it.
The reciprocity of Justice was valid towards all the world; the reciprocity of friendship went much farther; it involved indefinite and active beneficence, but could reach only to a select few.