Epictetus, Golden Sayings 82

And now, when you see brothers, apparently good friends and living in accord, do not immediately pronounce anything upon their friendship, though they should affirm it with an oath, though they should declare, "For us to live apart in a thing impossible!" 

For the heart of a bad man is faithless, unprincipled, inconstant, now overpowered by one impression, now by another. Ask not the usual questions, were they born of the same parents, reared together, and under the same tutor; but ask this only, in what they place their real interest—whether in outward things or in the Will. 

If in outward things, call them not friends, any more than faithful, constant, brave or free. Call them not even human beings, if you have any sense. . . . 

But should you hear that these men hold the Good to lie only in the Will, only in rightly dealing with the things of sense, take no more trouble to inquire whether they are father and son, or brothers, or comrades of long standing; but, sure of this one thing, pronounce as boldly that they are friends as that they are faithful and just.

For where else can Friendship be found than where Modesty is, where there is an interchange of things fair and honest, and of such only?