This result proposes that private parties (consumers and producers) can solve the problem of externalities on their own. a tariff scarce theorem social optimum theorem coase theorem

This result proposes that private parties (consumers and producers) can solve the problem of externalities on their own. = Coase theorem.
Coase theorem proposes that "when there is a disagreement about property rights, those parties concerned can find a way to come to a mutually beneficial outcome by means of bargaining or negotiating terms," according to its definition.