The central bank is the sole issuer of banknotes and bank reserves. That means it is the monopoly supplier of the monetary base. By virtue of this monopoly, it can set the conditions at which banks borrow from the central bank. Therefore it can also influence the conditions at which banks trade with each other in the money market.
In the short run, a change in money market interest rates induced by the central bank sets in motion a number of mechanisms and actions by economic agents. Ultimately the change will influence developments in economic variables such as output or prices. This process – also known as the monetary policy transmission mechanism – is highly complex. While its broad features are understood, there is no consensus on its detailed functioning.
It is widely agreed that in the long run – after all adjustments in the economy have worked through – a change in the quantity of money in the economy will be reflected in a change in the general level of prices. But it will not induce permanent changes in real variables such as real output or unemployment.
This general principle, referred to as "the long-run neutrality of money", underlies all standard macroeconomic thinking. Real income or the level of employment are, in the long term, essentially determined by real factors, such as technology, population growth or the preferences of economic agents.
In the long run a central bank can only contribute to raising the growth potential of the economy by maintaining an environment of stable prices. It cannot enhance economic growth by expanding the money supply or keeping short-term interest rates at a level inconsistent with price stability. It can only influence the general level of prices.
Ultimately, inflation is a monetary phenomenon. Prolonged periods of high inflation are typically associated with high monetary growth. While other factors (such as variations in aggregate demand, technological changes or commodity price shocks) can influence price developments over shorter horizons, over time their effects can be offset by a change in monetary policy.