Men say that one ought to love best one's best friend, and man's best friend is one who wishes well to the object of his wish for his sake, even if no one is to know of it; and these attributes are found most of all in a man's attitude towards himself, and so are all the other attributes by which a friend is defined; for, as we have said, it is from this relation that all the characteristics of friendship have extended to our neighbours. All the proverbs, too, agree with this, e.g. 'a single soul', and 'what friends have is common property', and 'friendship is equality', and 'charity begins at home'; for all these marks will be found most in a man's relation to himself; he is his own best friend and therefore ought to love himself best. It is therefore a reasonable question, which of the two views we should follow; for both are plausible.Plutarch, De Amicorum Multitudine 96F
[I]n our friendship's consonance and harmony there must be no element unlike, uneven, or unequal, but all must be alike to engender agreement in words, counsels, opinions, and feelings, and it must be as if one soul were apportioned among two or more bodies.Euripides, Electra 1045
My dearest, you who have a name that sounds most loved and sweet to your sister, partner in one soul with her!A helpful discussion of friendship in antiquity, from a biblical scholar, comes to us from Luke Timothy Johnson.
This illuminates the genre of Philippians as a "Friendship" letter (Fee, Paul’s Letter to the Philippians, 2-7 and deSilva, An Introduction to the New Testament, 653). It also suggests that the theme of unity in Philippians (O’Brien, Philippians, 38) is a central concern. I would also suggest that the element of κοινωνίᾳ (partenership/fellowship), is central in Philippians. I'm struggling to find an adequate translation for μιᾷ ψυχῇ as one soul doesn't capture the concept in contemporary usage. One life is not much better. If anyone has ideas, let me know... Back to the drawing board... conceptually that is.
In the Greek world, friendship was among the most discussed, analysed and highly esteemed relationships. Epicurus included it among the highest goods available to humans. The Pythagoreans founded a way of life on its basis. For Plato, it was the ideal paradigm for the city-state. Even the more pragmatic Aristotle considered friendship the prime metaphor and motive for society. The word “friendship” was not used lightly in these circles, nor was friendship considered simply a casual affection. On the contrary, it was regarded as a particularly intense and inclusive kind of intimacy, not only at the physical level, but above all, at the spiritual…
To be “one soul” with another meant, at the least, to share the same attitudes and values and perceptions, to see things the same way. Indeed, the friend was, in another phrase frequently repeated, “another self.”
L. T. Johnson, Brother of Jesus, Friend of God: Studies in the Letter of James (Michigan: Eerdmans, 2004), 213-4.