He assumed that workers could be paid wages as low as was necessary for their survival, which was later transformed by David Ricardo and Thomas Robert Malthus into the "iron law of wages". His main emphasis was on the benefit of free internal and international trade, which he thought could increase wealth through specialisation in production. He also opposed restrictive trade preferences, state grants of monopolies and employers' organisations and trade unions. Government should be limited to defence, public works and the administration of justice, financed by taxes based on income.