This is from Paul Krugman’s introduction to The General Theory:

Stripped down, the conclusions of The General Theory might be expressed as four bullet points:

• Economies can and often do suffer from an overall lack of demand, which leads to involuntary unemployment

• The economy’s automatic tendency to correct shortfalls in demand, if it exists at all, operates slowly and painfully

• Government policies to increase demand, by contrast, can reduce unemployment quickly

• Sometimes increasing the money supply won’t be enough to persuade the private sector to spend more, and government spending must step into the breach

To a modern practitioner of economic policy, none of this – except, possibly, the last point – sounds startling or even especially controversial. But these ideas weren’t just radical when Keynes proposed them; they were very nearly unthinkable. And the great achievement of The General Theory was precisely to make them thinkable.

There it is. It could not be put more perfectly than that. And the very first point is precisely what all pre-Keynesian not only denied, but described as utterly fallacious. This is Say’s Law which is as integral to modern economic theory as it was anathema to those who came before. More from Krugman:

[Keynes] quickly shows that the conventional view that wage cuts were the route to full employment made no sense given the realities of the time. . . .

Read Keynes’s speculations on the virtues of mercantilism [ie protectionism] and the vanishing need for thrift. . . .

The real classical model, as Keynes described it, was something much harder to fix. It was, essentially, a model of a barter economy, in which money and nominal prices don’t matter, with a monetary theory of the price level appended in a non-essential way, like a veneer on a tabletop. It was a model in which Say’s Law applied: supply automatically creates its own demand, because income must be spent. And it was a model in which the interest rate was purely a matter of the supply and demand for funds, with no possible role for money or monetary policy. It was, as I said, a model in which ideas we now take for granted were literally unthinkable. . . .

Keynes confronts naïve beliefs about how a fall in wages can increase employment, beliefs that were prevalent among economists when he wrote, but play no role in the model we now call ‘classical.’ . . .

The crucial innovation in The General Theory isn’t, as a modern macroeconomist tends to think, the idea that nominal wages are sticky. It’s the demolition of Say’s Law and the classical theory of the interest rate in Book IV, ‘The inducement to invest.’ One measure of how hard it was for Keynes to divest himself of Say’s Law is that to this day some people deny what Keynes realized – that the ‘law’ is, at best, a useless tautology when individuals have the option of accumulating money rather than purchasing real goods and services. Another measure of Keynes’s achievement may be hard to appreciate unless you’ve tried to write a macroeconomics textbook: how do you explain to students how the central bank can reduce the interest rate by increasing the money supply, even though the interest rate is the price at which the supply of loans is equal to the demand? It’s not easy to explain even when you know the answer; think how much harder it was for Keynes to arrive at the right answer in the first place. . . .

But the classical model wasn’t the only thing Keynes had to escape from. He also had to break free of the business cycle theory of the day.

There wasn’t, of course, a fully-worked out theory of recessions and recoveries. But it’s instructive to compare The General Theory with Gottfried Haberler’s Prosperity and Depression3, written at roughly the same time, which was a League of Nations-sponsored attempt to systematize and synthesize what the economists of the time had to say about the subject. What’s striking about Haberler’s book, from a modern perspective, is that he was trying to answer the wrong question. Like most macroeconomic theorists before Keynes, Haberler believed that the crucial thing was to explain the economy’s dynamics, to explain why booms are followed by busts, rather than to explain how mass unemployment is possible in the first place. And Harberler’s book, like much business cycle writing at the time, seems more preoccupied with the excesses of the boom that with the mechanics of the bust. Although Keynes speculated about the causes of the business cycle in Chapter 22 of The General Theory, those speculations were peripheral to his argument. Instead, Keynes saw it as his job to explain why the economy sometimes operates far below full employment. That is, The General Theory for the most part offers a static model, not a dynamic model – a picture of an economy stuck in depression, not a story about how it got there. So Keynes actually chose to answer a more limited question than most people writing about business cycles at the time. . . .

Rather than getting bogged down in an attempt to explain the dynamics of the business cycle – a subject that remains contentious to this day – Keynes focused on a question that could be answered. And that was also the question that most needed an answer: given that overall demand is depressed – never mind why – how can we create more employment?