Exchange is the purest and most concentrated form of all human interactions in which serious interests are at stake. … In human relations, however, interaction generally appears in forms which lend themselves to being viewed as exchange. The ordinary vicissitudes of daily life produce a continuous alternation of profit and loss, an ebbing and flowing of the contents of life. Exchange has the effect of rationalizing these vicissitudes, through the conscious act of setting the one for the other.

Georg Simmel, on individuality and social…, loc. 217