Question

Why would an economist choose either the neoclassical perspective or the Keynesian perspective, but not both?

Short answer

Economists take opinions of the neoclassical perspective or the Keynesian perspective depending on whether they focus more on the supply side and the long run (neoclassical) or the demand side and the short run (Keynesian).

Step-by-step solution

Keynesian approach and neoclassical approach : 

The supply-oriented neoclassical approach considers supply as an economic tool for influencing demand. The Keynesian method, on the other hand, is demand-driven, which has an influence on the economy.

 Step 2 : Explanation :

Say's Law is held by neoclassical economists. A useful shorthand for it is that supply generates demand, whereas the Keynesian approach claims that demand generates supply.

Keynes' perspective applies more accurately in the short run while the neoclassical perspective is more accurate in the long run.