To state that national saving is equal to investment, for a closed economy, is to state an accounting identity.

National Saving

The total amount of saving in a country, which includes both private savings and public (government) savings.

Closed Economy

An economic system that does not engage in international trade with outside countries, relying on its own resources.

Accounting Identity

A fundamental equation that must always hold true, often without regard to time or specific instances, in accounting and economics.