In the so-called moral (or social) sciences it is sometimes very difficult to measure the facts. In his essay, “The Populousness Of Ancient Nations,” (hereafter Populousness) Hume points out that when facts are uncertain, it is appropriate to “intermingle the enquiry concerning the causes with that concerning facts” (381, see also 404). [We wannabe post-Malthusians forget that population-size/growth and political economy were once fully integrated sciences.] Hume admits that this is a second-best-scenario: such intermingling both “ought never to be admitted, where the facts can be ascertained with any tolerable assurance.” Hume speaks of the problem of unreliability of data in his discussion “Of Miracles,” to make clear how to rule out potentially unreliable testimony about uncertain ‘facts’. But his treatment does little to help one decide between competing data that are in no sense so implausible that, by themselves, they violate a known law of nature. In his political economy, Hume needs, thus, to offer a more subtle, methodological account. One must design theory in such a way that it enables principled constraints on accepting data. Hume is quite explicit that he is dealing with limited reliability of data: “the facts, delivered by ancient authors, are either so uncertain or so imperfect as to afford us nothing positive in the matter … [And] the very facts, which we must oppose to them, in computing the populousness of modern states, are far from being either certain or complete” (Populousness, 421).
After ruling out the applicability of the most “general physical causes,” that is, whether the world is still in its youth or decaying (378), Hume looks at “particular physical causes” (e.g., diseases ) that influence population size (379; see also his discussion of effects of climate 448-452). But he realizes that he cannot say much about them without a principled way to assess the available data. To do that, Hume needs to make assumptions about the “moral causes” that influence population growth (383).
Hume, thus, infers what would be the case based on empirical observations of what is the case in, say, colonies and the quick rebound in population after plagues (381) as well on facts about human nature as supplied by, for example, the Treatise, which is based on “experiment and observation.” Then, Hume postulates a ‘natural’ rate of propagation: “the human species, at this rate of propagation, would more than double every generation” (381). Hume stipulates that “everything else being equal” (vegetation, climate, etc.), this rate can be achieved only under “wise, just, and mild government” with the “wisest institutions;” this “seems natural to expect” (382). “Difficulties” in “the police [i.e., administrative policies] … manners, and the constitution” act as “restraints” on this rate (381; While Hume draws extensively on his theories in political economy, some of his claims that provide him with principled constraints, while perhaps credible in his own day, appear naïve to us, e.g., his claim that 700,000 citizens is the largest ‘natural’ size for a city (447-448)). It is worth noting that he believes that income equality is one of the major sources of population growth (403, although it can also lead to more civil strife, which will lower the rate, 413). Thus, the natural rate is for Hume an optimal rate. (I call it “optimal” to call attention to two-fold nature of Hume’s ‘natural’ rate: Hume implies that this is rate is the largest/fastest possible (under any conditions) and that it is instantiated only under particular and, as it happens, politically desirable conditions with the right policy/administration/constitution, etc. That these conditions are desirable is not argued in the essay itself, but it is asserted by the use of such descriptions as “wise” and “just.” That is to say, “Populousness,” Hume presupposes his moral (in his 18th century sense) and political views. In the remainder of his essay, Hume can use this natural rate to reason from facts to causes as well as from causes to facts.
But first Hume deals with an ‘unnatural’ situation: the impact of slavery on population (383-397), which in “the first appearance of things” (387) may (despite its “cruel and oppressive nature” (383, see also, 386) make one believe that it is conducive to population growth (387); the slave-owner has an “interest” in breeding slaves as if they were cattle (386-387). Hume argues “by a parity of reason,” (387), that is, by (tacitly) employing his own fourth and fifth “Rules,” that if in rich countries cattle barely propagate, if at all, since it is expensive to breed them there, then it is extremely unlikely that in the “richer” countries of the Ancient world (i.e., Rome) slavery would have caused abundant population. Hume draws then on an extremely wide variety of Ancient and Modern sources to make the analogy plausible and support his claim about the bad effects of slavery (387-398). He then very briefly discusses the possible impact of convents and the exposure of children (398-400) and then the impact of “political customs and institutions” in the remainder of his essay.
Acceptance of the relationship between the nature of government and population size provides Hume with an important constraint on accepting the facts provided by various literary and political texts. (In this context Hume does not use historical facts to improve or refute the rule that he employs to interpret history.) Hume self-consciously offers a principled evidential strategy to deal with a situation in which only limited data are available (recall 381 and 421). So, while comparing the size of the standing mercenary army of Dionysius the Elder (of Syracuse) with that of the contemporary United Provinces, he rules out Philistus’ claims (425-426) based on nature and size of the polities, and their economic policies.
The logic of Hume’s evidential strategy is that if there is reliable information about the existence of a mild government with sound economic policy, then he will accept evidence for claims that population is or was increasing in some locale, while he can infer the converse, too, although then he is likely to be extremely cautious. In practice, Hume rarely employs an argument from the natural rate to its full extent since he is more interested in absolute population numbers and in non-dynamic comparisons between ancient and modern population numbers at fixed times then in figuring out population trends. Most of his arguments rely more on a direct application of the assumptions (i.e., his political economy) that went into formulating the natural rate as a way to evaluate historical evidence than on the full potential of the argument. Nevertheless, his positing the natural rate allows Hume to infer about the past and present; he can reason from facts to causes and from causes to facts. (Section 11.25 of the first Enquiry allows for such reasoning in “works of human art and contrivance,” but rejects it in our “reasonings from the work of nature” especially when dealing with the Deity (11.26, see also Hume’s accompanying footnote).) Regardless of Hume’s practice, the articulation of this strategy anticipated elements of and inspires Adam Smith’s counterfactual theorizing in the Wealth of Nations, but about that elsewhere more.