The economic case for an open trading system based on multilaterally agreed rules is simple enough and rests largely on commercial common sense.
But it is also supported by evidence: the experience of world trade and economic growth since the Second World War.
Tariffs on industrial products have fallen steeply and now average less than 5% in industrial countries.
During the first 25 years after the war, world economic growth averaged about 5% per year, a high rate that was partly the result of lower trade barriers.
World trade grew even faster, averaging about 8% during the period.