ABSTRACT
Recurring crises slowing down economic activities and the resulting consequences paint the picture of modern economies. Therefore, it seems justified to draw on the achievements of economics with a view to explaining the causes of socio-economic crises. The article uses a descriptive method. The objective of the article is to understand the substance of the phenomenon;therefore, the author seeks certain analogies in the past and refers to the work of the Polish scholar Michal Kalecki. The basic conclusion of the article is: J.M. Keynes built a new macroeconomic theory based on the fundamental proposition that manufacturers' supply (the levels of national income and employment) depended on aggregate demand. In the mid-1930s, a similar thought was formulated by Poland's most eminent economist, Michal Kalecki. He constructed his business cycle model based on the aforementioned fundamental proposition. It meant that profits and national income were determined by effective investor and consumer demand. J.M. Keynes's and M. Kalecki's stressing the pivotal role of aggregate demand was a revolution in economic thinking. It should be noted that the COVID-19 pandemic is not merely a short-term change in economic activity. As predicted by various international institutions, it will have a lasting impact on the functioning of the global economy.