The most significant contribution of the physiocrats was their emphasis on productive work as the source of national wealth. This is in contrast to earlier schools, in particular mercantilism, which often focused on the ruler's wealth, accumulation of gold or the balance of trade. A chief weakness from the viewpoint of modern economics is that they only considered agricultural labor to be valuable. Physiocrats viewed the production of goods and services as consumption of the agricultural surplus, while modern economists consider these to be productive activities which add to national income.