 Concerns about exploitation often take the form of unfair economic exchange. Attempts to specify the principles that render an exchange fair or unfair can be traced back at least as far as Aristotle, who argued that a just exchange will embody a kind of reciprocity such that the values of the goods exchanged are proportional (Nicomachean Ethics, Book V, Part V). But while the notion of proportionality is intuitively appealing, it is somewhat unclear precisely what Aristotle had in mind by it, or what the most defensible explication of the idea would be. To borrow Aristotle’s own example, if a shoemaker and a builder trade, how many pairs of shoes is proportional to a single house?