Jean-Baptiste Say was a French philosopher and economist in the late 18th century. He is buried in a Paris Cemetery near Jim Morrison, lead singer of the Doors. Jim Morrison gets more visitors.
Say's main contribution to economic thinking was that a free market economy always tends to full employment. If markets are left to weave their magic then the incomes created by production will ensure sufficient demand to buy the total output. Say did not deny that there could be temporary excesses of supply in particular markets but overall the economy would always tend to full employment.
Say's law underpins free market ideology. Its modern translation is that large scale unemployment is the result of rigidities in the labour market. These take the form of minimum wages, worker rights, unemployment benefits and union power. Unemployment is due to the unwillingness of workers to accept the wages that a truly competitive labour market would offer.
In the 1930s Depression, John Maynard Keynes attacked Say's law. The lengthy mass unemployment of this period suggested that free markets do not always ensure full employment. Keynes' insight was that an economy could spend a significant period with mass unemployment. There was a need for activist government policy to break this stalemate.
Say's law rests on the premise that net incomes are either spent or saved. Income that is saved is then used for investment in new factories, machinery, houses and other productive investments which create employment, output and incomes. Keynes suggested that during a severe recession, the incomes that are saved may not be reinjected into the economy to create more production and employment. The reason is that businesses lose confidence. They are unwilling to borrow and spend on new production because the demand is not there. Savings just pile up or are used for speculative none-productive investments such as existing housing or shares.