There are many dimensions of the principle of equality among humans. The most difficult is expressed by the  Biblical command “Love your neighbor as yourself” (Leviticus, 19:18; Matthew 22:39). The Martin Buber quotation above points to some of the bias toward self that would have to be overcome to actually obey this command. 

A much more modest demand is to treat the interests and concerns of two humans that come before you for judgment equally. This idea provides part of the context for the Levitical command “Love your neighbor as yourself.” Three verses earlier, Leviticus reads: 

You shall do no injustice in court. You shall not be partial to the poor or defer to the great, but in righteousness shall you judge your neighbor. (Leviticus 19:15)

Of course, a key question, as I put it in “Us and Them” is “whose well-being counts: who is in the charmed circle of people whose lives we are concerned about and who is not.” Jesus was asked almost exactly this question by a student of the Law of Moses who knew his Leviticus well: 

And, behold, a certain lawyer stood up, and tempted him, saying, Master, what shall I do to inherit eternal life? He said unto him, What is written in the law? how readest thou? And he answering said, Thou shalt love the Lord thy God with all thy heart, and with all thy soul, and with all thy strength, and with all thy mind; and thy neighbour as thyself. And he said unto him, Thou hast answered right: this do, and thou shalt live. But he, willing to justify himself, said unto Jesus, And who is my neighbour? (Luke 10:25-29)

Jesus answered by telling the story of the Good Samaritan–a despised outsider who was kinder to a man beaten by thieves than those of his own ethnicity. The rhetorical force of the Good Samaritan story, as I see it, is that someone who is kind to all human beings seems nobler than someone so ready to draw a line between the people who count and those who don’t that a few lines later the charmed circle of those who count has contracted perilously close to being a circle enclosing a single ego. 

Not just equal concern for the welfare of those in outgroups, but any significant concern for those in outgroups is very much still at issue in the modern world. The second figure above doesn’t plumb the full depth of unconcern for outgroups that we are still wrestling with. Take as given for the sake of argument that the welfare of citizens ought to count in political decision-making more than the welfare of non-citizens. It then makes a huge difference whether the welfare of non-citizens counts zero or counts at a fraction–say one-hundredth as much as the welfare of citizens. Why? Because as a practical matter there are many policies that raise the welfare of citizens a tiny bit, or seem to, but without doubt grievously hurt the welfare of non-citizens.

The dimension of equality most directly relevant for political philosophy is the one pointed to by Thomas Jefferson in the third figure above. There is not some person or group of people who have the inherent right to be rulers. Although this is the type of equality that John Locke needs most to make his argument in his 2d Treatise on Government:“On Civil Government,” he begins with the stronger “Love your neighbor as yourself” version of equality, pointing in section 5 to the theology of Richard Hooker:  

This equality of men by nature, the judicious Hooker looks upon as so evident in itself, and beyond all question, that he makes it the foundation of that obligation to mutual love amongst men, on which he builds the duties they owe one another, and from whence he derives the great maxims of justice and charity.

John Locke then quotes Richard Hooker as follows:       

The like natural inducement hath brought men to know that it is no less their duty, to love others than themselves; for seeing those things which are equal, must needs all have one measure; if I cannot but wish to receive good, even as much at every man’s hands, as any man can wish unto his own soul, how should I look to have any part of my desire herein satisfied, unless myself be careful to satisfy the like desire, which is undoubtedly in other men, being of one and the same nature? To have any thing offered them repugnant to this desire, must needs in all respects grieve them as much as me; so that if I do harm, I must look to suffer, there being no reason that others should shew greater measure of love to me, than they have by me shewed unto them: my desire therefore to be loved of my equals in nature, as much as possible may be, imposeth upon me a natural duty of bearing to them-ward fully the like affection; from which relation of equality between ourselves and them that are as ourselves, what several rules and canons natural reason hath drawn, for direction of life, no man is ignorant.  Eccl. Pol. Lib. i.

How is it that we human beings have the concept of human equality at all? I don’t know. But I have the sense that the way we see other human beings when we look at a crowd of strangers we know nothing about, who are all of a relatively homogeneous social group, has a lot to do with it. Because we have “theory of mind”–including a model in our own heads of how things appear to other people–we know that each of us, too, could seem like just a face in a crowd. That picture of just another face in a crowd is a starting point for conceiving of human equality.