The standard answer goes like this: interest rates are already very low, so the Bank of Japan has done all it can. Meanwhile, the government has a severe fiscal problem, so it cannot increase spending or cut taxes. There is, in short, nothing to be done except pursue structural reforms and hope for an eventual turnaround. This answer sounds hard-headed and responsible. In fact, however, it is based on a completely false premise - the idea that the Bank of Japan has reached the limits of what it can do.
The simple fact is that there is no limit on how much a central bank can increase the supply of money. Could the Bank of Japan, for example, double the amount of monetary base - that is, bank reserves plus cash in circulation - over the next year? Sure: just buy that amount of Japanese government debt. True, even such a large increase in the money supply might not drive down interest rates very much, since they are already so low. But an increase in Japan's money supply could ease the economic problem in ways other than lower interest rates. It is possible that putting more cash in circulation will stimulate spending directly - that the extra money will simply "burn holes in peoples' pockets". Or banks, awash in reserves, might become more willing to lend; or individuals, with all that cash on hand, will bypass the banks and find other ways of investing."
But this was written in the mid 1990s. And about Japan. So I guess the rules are different or something.