A deconstruction of the Marxian theories of value and exploitation is attempted by arguing, first, that the labour theory of value is logically and methodologically inconsistent as a basis for a theory of capitalist exploitation and, second, that it is founded on an ontology of the social being which is not plausibly justified. A counterfactual model economy is then built, called Utopia, in which the workers receive the whole net output while commodities exchange at labour values. This model is used as a benchmark to evaluate a capitalist economy where commodities exchange at production prices and workers are exploited. It is shown that the factor of exploitation can be expressed as a ratio between the labour commanded by, and the labour contained in, the net output, or between the value added produced in a capitalist economy and that produced in Utopia. The resulting theory of exploitation entails a weak ontology of the social being, expressing a worker's point of view.