Pigou argues that marginal private net product does not equal marginal private net product for some goods owing to externality.Therefore, governments should interfere and levy a Pigovian tax.

• 庇古认为，庄于外部性等原因，部分产品的边际私人纯产值与边际社会纯产值并不相等，因此政府必须界入进行管制，对外部性产品收取庇古税；