The "Natural Interest Rate" Is Always Positive and Cannot Be Negative

Some economists have been arguing that the “equilibrium real interest rate” (that is the “natural interest rate” or the “originary interest rate”) has become negative, as a “secular stagnation” has allegedly caused a “savings glut.”1
The idea is that savings exceed investment, and that a negative real interest rate is required for bringing savings in line with investment. From the viewpoint of the Austrian school, the notion of a “negative equilibrium real interest rate” doesn’t make sense at all.2