reciprocity.

It is plain that the promise, or extension of responsibility through time,
is what chiefly distinguishes us, I will not say from savages, but from
brutes and reptiles. This was noted by the shrewdness of the Old Testament,
when it summed up the dark irresponsible enormity of Leviathan in the words
"Will he make a pact with thee?" The promise, like the wheel, is unknown in
Nature: and is the first mark of man. Referring only to human civilisation
it may be said with seriousness, that in the beginning was the Word. The
vow is to the man what the song is to the bird, or the bark to the dog; his
voice, whereby he is known. Just as a man who cannot keep an appointment is
not fit even to fight a duel, so the man who cannot keep an appointment
with himself is not sane enough even for suicide. It is not easy to mention
anything on which the enormous apparatus of human life can be said to
depend. But if it depends on anything, it is on this frail cord, flung from
the forgotten hills of yesterday to the invisible mountains of to-morrow.
On that solitary string hangs everything from Armageddon to an almanac,
from a successful revolution to a return ticket. On that solitary string
the Barbarian is hacking heavily, with a sabre which is fortunately blunt.

Any one can see this well enough, merely by reading the last negotiations
between London and Berlin. The Prussians had made a new discovery in
international politics: that it may often be convenient to make a promise;
and yet curiously inconvenient to keep it. They were charmed, in their
simple way, with this scientific discovery, and desired to communicate it
to the world. They therefore promised England a promise, on condition that
she broke a promise, and on the implied condition that the new promise
might be broken as easily as the old one. To the profound astonishment of
Prussia, this reasonable offer was refused! I believe that the astonishment
of Prussia was quite sincere. That is what I mean when I say that the