Marginal thinking about the Fiscal stimulus - Paul Krugman

"The figure above is the way economists are taught to think about, well, everything. Greg Mankiw:

Rational People Think at the Margin.
A rational decision-maker takes action if and only if the marginal benefit of the action exceeds the marginal cost.

Yet when it comes to fiscal stimulus, there doesn’t seem to be a lot of marginal thinking going on."