For Cicero it seems that the our recognition of our standing towards one another, as rational creatures who are part of the 'community of gods and men', is expressed by our refraining from harming one another.
But (as noted in the Mayweek seminar) here Cicero is rigorous and insists that we commit an injustice not merely when we directly injure another, but also when we refrain from protecting another from injury, when it was in our power to do so:
Sed iniustitiae genera duo sunt, unum eorum, qui inferunt, alterum eorum, qui ab iis, quibus infertur, si possunt, non propulsam iniuriam. Nam qui iniuste impetum in quempiam facit aut ira aut aliqua perturbatione incitatus, is quasi manus afferre videtur socio; qui autem non defendit nec obsistit, si potest, iniuriae, tam est in vitio, quam si parentes aut amicos aut patriam deserat. (I 23)It's remarkable that Cicero likens a failure to defend anyone ('who is my neighbor?'), to a failure to defend those closest to us; and he apparently regards our duty to defend as not made proportionately weaker or stronger, depending upon the closeness of our relation to the person under attack. Thus here, clearly, the 'impersonal point of view' is making a showing.
Of injustice there are two types: men may inflict injury; or else, when it is being inflicted upon others, they may fail to deflect it, even though they could. Anyone who makes an unjust attack on another, whether driven by anger or by some other agitation, seems to be laying hands, so to speak, upon a fellow. But also, the man who does not defend someone, or obstruct the injustice when he can, is at fault just as if he had abandoned his parents or his friends or his country.
Or is it?--since I wonder whether Cicero didn't intend the qualifications, si possunt and si potest, to allow some wiggle room. After all, interventions have various degrees of risk, and someone might claim that he wasn't able to intervene on behalf of a stranger, because it was too risky to do so. ("I couldn't very well fight off so many attackers" might be an allowable defense for inaction if a stranger is under attack, but not a family member.) If so, then the equal claim that all of us have to be guarded against attack by any other is a theoretical ideal rather than an immediate, practical principle of action.