https://www.opendemocracy.net/ourkingdom/steve-keen/keen-krugman-debate

Krugman thus supports what monetary reformers like Prof Steve Keen identify as the orthodox ‘Loanable Funds Theory’ – often (wrongly) defined as “Keynesian”. Professor Mankiw of Harvard, a prominent neoliberal economist, defines ‘Loanable Funds Theory’ this way: “Saving is the supply of loans – individuals lend their saving to investors or they deposit their saving in a bank that makes the loans for them.  Investment is the demand for loans – investors borrow from the public directly by selling bonds or indirectly by borrowing from banks.” (Mankiw, Macroeconomics, p.65).
According to this theory money for loans is provided by savers, and the ‘price’ (or rate of interest) for loans is determined by changes in demand and supply of ‘loanable funds’. Credit does not appear to enter into the theory. Mankiw and Krugman do not appear to accept that banks create credit--“out of thin air”--on the guarantee of collateral, a contract to repay over a fixed time period, and at a certain rate of interest. Instead, according to this view, they are simple intermediaries between savers and borrowers; between those who are ‘patient’ savers and ‘impatient’ borrowers.