Let us review the case for the budget the Liberals have just unveiled.Canada's economy, on this view, has been struggling under the yoke of years of Conservative austerity, public services deliberately starved of funds by years of Conservative tax cuts.On the other hand, Canada's public debts are modest, by international standards, and interest rates are low. Therefore the government has both the opportunity and the obligation to borrow now, to invest in the kinds of public infrastructure that will stimulate economic growth.Even if the resulting deficits are larger than advertised in the last election, this is what Canadians voted for. Any other course would not only have required the government to impose deep and damaging cuts in spending, but to break important campaign promises.