This was quite clearly played out during the US subprime crisis. In this context he argues in favour of preventative measures, rather than to be reactive. We have learnt firstly that systemic risk cannot be prevented by focussing only on regulation and supervision, and secondly that achieving price stability does not equate to achieving financial stability.
Hence, the birth of macro-prudential policy which pursues financial stability as its objective, whereas the objective of monetary policy is price stability. The latter uses interest rates as its instrument, but it is well known that interest rates can be a blunt tool in dealing with excess leverage, extreme risk taking, or unwarranted deviations of asset prices from fundamentals.