The Treaty establishing the European Union assigned to the ECB unambiguous responsibility for the maintenance of a stable scale of value which agents can internalise in their expectations. And it has shielded monetary policy from political interference. Price stability diminishes investment risk, reduces the cost of capital and lengthens the horizon over which lenders and borrowers wish to engage in credit arrangements.
The original speech, which contains various links to the documents mentioned, can be found on the US Federal Reserve System’s website. *  *  *  What's past is prologue; what to come, in yours and my discharge.