As the main subject of the market economy, participators are supposed to exteriorize the own cost only if the external production cost less than the environment cost and the society cost generated by his own action choice. 处于市场竞争中的经济行动主体有足够的动力将自己的生产成本外部化,只要外部化生产成本的代价小于由他的行动选择而产生的环境成本和社会成本。