This lecture explains that the essence of Keynesian economics is complexity - the whole is not the sum of the parts, and the macroeconomy does not behave like the sum of the individual microeconomics. Thus Aggregate Demand cannot be computed as the sum of the individual demands, and the Aggregate Supply cannot be computed as the sum of the individual firm supplies. This is because the firm supply is not a well-defined concept without specifying what the other firms are doing. Textbooks get around this by the representative firm device, which makes each firm a replica of the other, and thereby forcing the equality of the supply curve with the sum of the firm supplies. But the representative firm hides the complexity problem, the main discovery of Keynes.
Here are the two video lectures:
The slides for lecture 9A above are here (9A)