"From him exact usury whom it would not be a crime to kill."

The truth about usury lies somewhere beyond St. Ambrose's condemnation and Jeremy Bentham's cavalier apologetics. In a very brief but valuable essay, Francis Bacon counselled,

It is good to set before us the incommodities and commodities of usury, that the good may be either weighed out or culled out; and warily to provide, that while we make forth to that which is better, we meet not with that which is worse. 

Strictly speaking, compound interest is usury. Discounting is compound interest, ergo discounting is usury.  Bentham, who upheld usury in a series of letters addressed to Adam Smith, also was a pioneering proponent of cost-benefit analysis for public investments. Considering that usury has both incommodities and commodities, a proper cost-benefit analysis would need to evaluate the costs as well as the benefits that arise from the discounting of future value.

The typical way of handling traditional objections to usury is to cite scripture and the interpretations of it offered by religious authorities. This was the method followed by Benjamin Nelson in The Idea of Usury, whose analysis was taken up by Lewis Hyde in The Gift and by David Graeber in Debt: the first 5000 years. But the biblical injunctions are laconic and subsequent interpretations may partake more of rationalization than impetus. Bentham was right when he observed,

It is one thing, to find reasons why it is fit a law should have been made: it is another to find the reasons why it was made: in other words, it is one thing to justify a law: it is another thing to account for its existence. 

Bentham's defence of usury, though, was as verbose and meandering as the infamous passage from Deuteronomy about brethren and strangers was terse. His account of the grounds for the prejudice against usury was frivolous and dismissive. "To trace an error to its fountain head," Bentham cited Lord Coke, "is to refute it." What Bentham meant by "trace" was "assert." According to him, the prohibition of usury was motivated by the perverse asceticism of early Christians, foolish abstractions of Aristotle and ill-tempered envy toward the wealthy by the profligate debtors.

More concisely and substantively, Francis Bacon presented, in one paragraph, a catalogue of seven disadvantages arising from usury. A second paragraph elaborated on three advantages. Bacon's fourth criticism of usury is of particular interest:

…it bringeth the treasure of a realm or state into a few hands; for the usurer being at certainties, and others at uncertainties, at the end of the game most of the money will be in the box; and ever a state flourisheth when wealth is more equally spread…

In favour of usury, Bacon's second point is his most compelling:

…were it not for this easy borrowing upon interest, men's necessities would draw upon them a most sudden undoing, in that they would be forced to sell their means (be it lands or goods), far under foot; and so, whereas usury doth but gnaw upon them, bad markets would swallow them quite up.

In modern parlance, Bacon's most compelling arguments, both for and against usury, refer to what Marshall called the "external economies" -- or positive and negative externalities -- of the loan transactions. For better or worse then, compound interest is a vehicle for the shifting of costs and benefits. It is well to remember, in this connection, Joan Martinez-Alier's observation that "one can see externalities not as market failures but as cost-shifting successes."

One doesn't need to assume that cost shifting is necessarily a bad thing. Insurance, including social insurance, is a form of cost shifting. But when the project being evaluated in a cost-benefit analysis has the overt purpose of internalizing the cost of externalities -- such as in the analysis of abatement of greenhouse gas -- it is disingenuous to overlook the role of compound interest in enabling the social cost shifting in the first place and of perpetuating it over the period being analyzed. In other words, part of the value allegedly being "added" by capital in the analysis is not in fact being produced but is merely being appropriated by capital through social cost shifting.

(See also "Why Is the Discount Rate So Important?" page 9 in "More than Meets the Eye: The Social Cost of Carbon in U.S. Climate Policy, in Plain English.")