The theory
Standard monetary theory says that national income is in equilibrium only when the demand to hold money balances is equal to the quantity of money created by the banking system. This may sound like gobbledygook, but it is the "fundamental proposition of monetary theory" referred to by Keynes in the quote from The General Theory below. Our work is nevertheless distinctive in emphasizing the key role of all money balances (i.e., a broad money measure) and the relevance of money to asset price movements. Of course asset price movements have a powerful impact on expenditure and income.
"[I]ncomes and [the] prices [of securities] necessarily change until the aggregate of the amounts of money which individuals choose to hold at the new level of incomes and prices thus brought about has come to equality with the amount of money created by the banking system. This, indeed, is the fundamental proposition of monetary theory."
- Penultimate paragraph of chapter 7 of The General Theory of Employment, Interest and Money by John Maynard Keynes