Mainstream economists regard various theories that they are employing as tentative.
This means that theories might be valid for a certain period of time. However, as time goes by, it is held, the changing world requires new theories. For instance an economist forms a view that people's consumption of goods and services is determined by their incomes. Once this view is validated by means of statistical methods it is employed as a tool to assess the future direction of people's consumption. The tentative nature, so it is argued, becomes apparent once the theory fails in its predictive capability.