Can anyone point me to the primary source for the theory that underpins the Fed's policy of interfering with interest rates (instead of letting the market decid

Can anyone point me to the primary source for the theory that underpins the Fed's policy of interfering with interest rates (instead of letting the market decide)? At best I understand it's Keynes, but last I spoke to an economist, they said he was only concerned with fiscal policy, not monetary.