Hume's observation on natural law does not show that there is or must be an unbridgeable gap between what is and what ought to be.
Aristotle reports an ancient example of rational choice theory in international affairs. Indeed, if rational choice theory had existed at the time, Aristotle’s report would have begged the complaint of caricature given the parties to the conflict not only viewed their goals instrumentally, but even reduced their goals to cash equivalents.
In chapter 7 of Book II of his Politics, discussing Phaleas’s proposed constitution, Aristotle develops several objections, and in passing mentions the following:
Enough [property] needs to be available for use within the city-state, but also to meet external dangers. That is why there should not be so much property on hand that more powerful neighbors will covet it, and the owners will be unable to repel the attackers, nor so little that they cannot sustain a war even against equal or similar people. . . . Perhaps the best limit is such that those who are stronger will not profit if they go to war because of the excess, but as they would if the property were not so great.
For example, when Autophradates was about the lay siege to Atarneus, its ruler, Eubulus, told him to consider how long it would take to capture the place, and then to figure what such time would cost, for he said he was willing to abandon Atarneus at once for less. These words caused Autophradates to have second thoughts and to abandon the siege.
The transaction between the two rulers is an interesting one, not least because it is so coldly instrumental.
One explanation for the exchange is Eubulus knew he could not outlast Autophradates’s siege; Autophradates would ultimately take the city. This would cause untold suffering to the people of Atarneus, and the outcome would still be the loss of the city. So Eubulus offers to surrender the town to Autophradates at a lower cost than the siege would cost Autophradates if he had to carry it out. The city would still go to Autophradates, although without the cost in lives and destruction to the people of Atarneus. Eubulus would escape with his life, presumably pocketing the price Autophradates paid to call off the siege.
Or we can develop a more public-choicey sort of story: Atarneus is worth more to Autophradates than it is to Eubulus. Autophradates aimed to strip the city of its wealth. Eubulus could not do the same as its ruler; its citizens presumably would not permit him to continue to rule if he attempted it. Nonetheless, as ruler, Eubulus is able to extract some stream of value as rent. So the deal here is that Eubulus would sell out his city for some current amount greater than the discounted value stream of income he could extract as rent from ruling the city. The present value to Eubulus of future rents is less than the cost of the siege to Autophradates plus the value of the city’s existing stock of wealth.
Beyond the exchange itself, two items draw interest. First, the story provides a counterpoint to the common view today that instrumental rationality is a modern phenomenon, one conceptually derived from the same spirit animating the creation of the modern economic or rational man. Ancients, the thought goes, valued honor and glory more than cash. Indeed, they were not even commensurable goods. Yet here we have an ancient ruler engaging in a coldly calculating transaction, perhaps for the benefit of his city, perhaps just for his personal benefit.
To be sure, analogous events are not unknown today. Consider the occasional dictator induced to leave his office (and country) with the promise of safe exile with the bulk of his kleptocratically derived wealth left intact.
But, indeed, there are counterexamples; counterexamples nonetheless that differ from the ancient example. We have seen sacrifices prompted not by aristocratic virtues like honor and glory (at least not personal honor and glory), but instead by ideology. Indeed, and this is the odd bit, it is in the nations in which instrumental rationality has taken deepest root that the sort of overt instrumental calculation of Aristotle’s example would be viewed as most offensive. Perhaps ancients and moderns aren’t so different after all. Or, perhaps, some of us who view ourselves as most modern in fact could out-ancient the ancients in willingness to sacrifice all for a cause. But with a difference as well. Modern causes tend not to involve sacrifice for honor and glory, but rather involve sacrifice for an idea, for a theory.