The macroeconomic ruel: SPENDING = INCOME = OUTPUT, WHICH DRIVES EMPLOYMENT is presented here in a schematic form. Output can be taken to be equivalent to  GDP. In order to maintain output it is necessary for all the income earned to be spent. If this is not the case, then companies find they have excess unsold stock on their hands and will cut back on production. This, in time, will lead to an increase in unemployment as companies need fewer employees. The shortfall in spending can be made up by any of the three sectors that contribute to total output. However, in cases where  a country has a trade deficit and where the private sector is not spending or investing enough, the only option is for the government to Net Spend i.e. to spend more than it collected in taxes causing a fiscal deficit.

Goodwin cycle IM-2010 with debt and taxes added, modified from Steve Keen's illustration of Hyman Minsky's Financial Instability Hypothesis "stability begets instability". This can be extended by adding the Ponzi effect of borrowing for speculative investment.

Regulation of resource allocation to production in response to inventory adequacy and delivery delay. A non-price-mediated resource allocation system. From Sterman JD Business Dynamics p172 Fig 5-27

MODERN MONETARY THEORY SHOWS HOW FULL EMPLOYMENT CAN BE ACHIEVED!

POTENTIAL GDP is a level of overall spending - by the government and the non-government sector - at which there is full employment. If the economy is not operating at its potential, then the  private sector has failed to invested or spend enough to generate the necessary growth nor has income  from net exports contributed enough. This only leaves the government to close the spending gap. Conceptually, a government disposing of its own freely floating currency could act using two powerful tools -  spending in excess of tax revenue, and taxation - to ensure that the gap between the actual economic activity and potential GDP is quickly closed. Achieving the  full employment that prevailed for 30 years between 1945 and 1975 in western economies is definitely possible!