It is argued, in the context of a heterogeneous oligopoly with perfect information, that rational behaviour should lead to an equilibrium that is invariant to the choice of instrument, price or quantity, by firms. The Cournot oligopoly and the Bertrand oligopoly fail this test; as both solutions are considered an example of a Nash noncooperative equilibrium, this result casts doubt on the appropriateness of the latter concept. The conjectural variations oligopoly passes the test. A natural consistency condition on the conjectural variations implies that the solution is in fact the contract (hyper)surface. The non-uniqueness of the equilibrium can be resolved by introducing a concept of economic force exerted by agents analogous to the force of gravity exerted by massive objects. This leads to the conclusion that the joint profit maximizing point is the equilibrium, where the joint profit includes the profits of the consumers. The equilibrium depends on cardinal properties of the consumers’ utility functions. The emphasis is on the exposition of ideas rather than on technical detail.Keywords: Conjectural variation, Contract curve, Force, Game, Nash noncooperative equilibrium, OligopolyJEL: C70, D21, D43, L13