 Marx provides a two-stage argument for the labour theory of value. The first stage is to argue that if two objects can be compared in the sense of being put on either side of an equals sign, then there must be a “third thing of identical magnitude in both of them” to which they are both reducible. As commodities can be exchanged against each other, there must, Marx argues, be a third thing that they have in common. This then motivates the second stage, which is a search for the appropriate “third thing”, which is labour in Marx’s view, as the only plausible common element. Both steps of the argument are, of course, highly contestable.