This is not true, and there is a simple antidote to such caricatures of Keynesianism. It is available in (of all places!) an “Austrian Economics Newsletter” of 1983, where George L. S. Shackle summed up the essence of Keynes’ theory. I reproduce it here:
[sc. Keynes’s] ... theory of involuntary unemployment is perfectly simple and can be expressed in a paragraph, or in a sentence. If you express it in a sentence, you simply say that enterprise is the launching of resources upon a project whose outcome you do not, and cannot, know. The business of enterprise involves investment, the investing of large amounts of resources--huge sums of money--in things whose outcome you cannot be certain of, which could perfectly well turn into a disaster or a brilliant success.This is actually a splendid summing up of what Keynes’s theory is about: rises and falls in the aggregate level of investment.
The people who do this kind of investing are essentially gamblers and they can lose their nerve. And if they decide to withdraw from trade, they sweep their chips up from the table. If they decide it’s too risky, if their nerve gives out and they can’t bring themselves to go on investing, they cease to give employment and that is the explanation. When business is at all unsettled--when there’s any sign at all of depression--or when there’s been a lot of investment and people have run out of ideas, or when their goods are not selling quite as fast as they have been, they no longer know what the marginal value product of an extra man is—it’s non-existent. How can you say that a certain number of men have a certain marginal productivity when you can’t know what the per unit value of the goods they would produce if you employed them would sell for?”
“An Interview with G.L.S. Shackle,” The Austrian Economics Newsletter, Spring 1983.
There are many things that affect the level of that investment, but a crucial one is the expectations of people make the investment decision. Their expectations are, in the end, subjective.
When investment collapses the question then becomes: how can that private sector investment be restored? Is there a substitute for it in the meantime to increase employment, income and output?
The answer is yes. And consumption is certainly one factor, but not the only one. In fact, the major Keynesian method to restore the health of economies historically has been stimulus programs that take the form of public infrastructure/public investment.