Final answer:
Keynes' law applies to the short run as it accounts for immediate changes in demand and supply constraints, while Say's law is applicable in the long run, where the economy self-adjusts and supply creates its own demand.
Step-by-step explanation:
Keynes' law is primarily applicable in the short run, which includes the period of a few months to a few years. During this time, companies often face fluctuating demand which can lead to underproduction during economic booms or decreased demand during recessions. In the short run, factors like constrained production capacity, labor, physical capital, and technology determine how much an economy can produce, and these constraints do not immediately adjust to changes in demand.
By contrast, Say's law is based more on macroeconomic supply and is believed to apply more accurately in the long run. In the long run, an economy tends to adjust and supply creates its own demand. Neoclassical economists believe that prices and wages will eventually adjust to shifts in supply or demand, leading the economy to reach equilibrium at potential GDP, which aligns more closely with Say's law.
Say's law and Keynes' law offer perspectives that highlight the importance of both supply and demand in different time frames. Therefore, both laws are essential in analyzing macroeconomic issues, with each providing more explanatory power in the different contexts of the short and long run.