I think of the key assumptions of standard economic theory as three pillars: utility maximization, equilibrium and agent beliefs. These three ideas are taught in all economics textbooks and form the basis for all mainstream theory.

However, something important was still missing. Up until that point, economic models had assumed a world of perfect certainty. Most models were either set in the present, where the utility of each choice is known, or they assumed a deterministic world where the future was perfectly known by every agent. But in the real world, the future is unknown.