What did Keynes believe caused recessions and depressions?

As per Keynes, depressions and recessions are caused by a fall in aggregate demand. When the demand for products falls, the production falls, when the production falls there will be loss of revenue which will lead to laying off of workers. You can read about the Monetary System – Types of Monetary System (Commodity, Commodity-Based, Fiat Money) in the given link.

Further readings:

1. National Institution for Transforming India (NITI Aayog) – A Brief Overview
2. Monetary Policy – Objectives, Roles and Instruments (UPSC Indian Economy)