When do governments have the right to be Keynesian?

The fiscal deficit can legitimately be increased: When there is a countercyclical objective, if there are temporarily excess savings over investment and high unemployment; if unemployment is high, but there is a shortage of savings over investment (excess domestic demand over supply), the problem is a shortfall in supply, not in demand; If there are structurally excess savings over investment, provided that the return on public investments, including the positive externalities that they generate, is higher than both the cost of the public debt and the return on private investments. If the return on private investments is higher than the return on public investments (including externalities), it is better to encourage the private sector to invest more.