Final answer:
John Maynard Keynes is associated with the shift from classical economics to modern economic theory, emphasizing economics as a method of thought and advocating for governmental intervention in the economy.
Step-by-step explanation:
The transition from classical economics to modern economic theory in the twentieth century is primarily associated with British economist John Maynard Keynes. Keynes, who lived from 1883 to 1946, revolutionized economics by introducing new ideas that prompted a rethinking of economic policies and practices. His perspective that economics is a method rather than a doctrine emphasized that the discipline is a tool for thought, aiding in drawing correct conclusions, rather than imposing a set of rigid beliefs.
Keynes challenged the prevailing notions of his time by arguing for governmental intervention in the economy to moderate boom and bust cycles, particularly during periods of economic downturn. This approach formed the basis of Keynesian economics, which has had a profound impact on both economic theory and policy since the Great Depression. His theories contrasted sharply with classical economics, which advocated for minimal government interference and believed that markets would naturally adjust back to full employment.