But the scanty wisdom of man, on entering into an affair which looks well at first, cannot discern the poison that is hidden in it, as I have said above of hectic fevers. Therefore, if he who rules a principality cannot recognize evils until they are upon him, he is not truly wise; and this insight is given to few. And if the first disaster to the Roman Empire should be examined, it will be found to have commenced only with the enlisting of the Goths; because from that time the vigour of the Roman Empire began to decline, and all that valour [virtu] which had raised it passed away to others.--Machiavelli, The Prince, Chapter 13.
Thanks to John Pocock and his followers, the direct and indirect impact of Machiavelli on Scottish Enlightenment thought does not surprise anymore. So, sometimes these digressions are no more than a footnote. In the quoted passage, Machiavelli offers an unintended consequence explanation. The effect (the disaster to the Roman Empire) had a first cause (the hiring of mercenary Goths), which undermined what we may call 'sustaining forces" -- vigour and valour -- of the Roman Empire. However, the effect was unforeseen by those that were responsible for the first cause (in this case Emperor Valens) and those that continued it.
Before I get to the Scottish Enlightenment, it is worth noting seven features of Machiavelli's claim. First, Machiavelli relies on his reader's knowledge to understand that the cause (hiring mercenaries) and the effect (fall of western Roman empire) are about a century apart. Second, (a) the causal explanation he offers here is mediated by intermediary causes (some of which I have tried to capture in terms of my nod to sustaining forces) and (b) the main explanation involves a cause whose impact is felt finally over a lengthy amount of time. There is no temporal contiguity between cause and effect (even if one can grant that the existence of intermediary mechanisms prevents us from positing distant temporal action). Third, Machiavelli relies (with -- here and elsewhere in The Prince -- a nod to medicine) on a (tacit) distinction between the surface phenomena and the hidden causal order. Fourth, that is, most political action is at the surface, but it can both alter the underlying political body, and be affected by it. The exception to that rule (see chapter VI) is the founder/legislator/re-newer of a polity, who can order and (re-)arrange the body politic. Fifth, the cause and the effect are, in a certain sense, disproportionate. Hiring mercenaries may well be an important policy decision, but it seems rather small compared to the fall of a whole (long lasting) political order.
Sixth, it is also important to note that the hidden (long-term) causes here operate by undermining (poison) existing mechanisms that sustain the ordinary functioning of the roman empire. That is to say, the unintended consequence is itself the effect of the breakdown of stable background conditions. The original cause is explanatory because it helps explain why the once stable and regular background conditions [themselves many cause-effect relations rooted in human nature] stop working. (I return to the seventh below.)
In my book on Adam Smith, I introduce a distinction between two kinds of unintended consequence explanations: (A) short-term invisible hand explanations and what I call (B) “Smithian social explanation.” The latter is characterized by four features:
i. It is causal (“necessary consequence”). Thus, it generates consequences such that the outcome could not be otherwise — presumably as necessary as the fact that all humans are mortal. But, like human mortality, the exact timing of a particular outcome is ordinarily unknown in advance to mortals.
ii. Smith’s account is a historical explanation. By “historical” I mean to capture two features: (a) that the stable consequence would not have been predictable to observers at an early time and, so, are also not capable of being intended; (b) that to be a cause does not require temporal contiguity between the cause and the effect. The same cause(s) can do their work over enormous expanses of time.
iii. Smith’s account does require that after certain consequences become visible to observer-participants they become self-reinforcing and generate a form of lock-in. Presumably, this self-reinforcement is due to the fact that those who benefit from the cumulative consequences will help prevent backsliding from new social arrangements.
iv. It supports counterfactuals. That is, it is a form of historical explanation that can help you specify what would have happened given certain features about human nature and social causation.
These four features of Smithian social explanation* have remarkable overlap with Machiavelli's explanation of the fall of the Roman Empire. There is, however, an important difference. And to see that, we need to return to Machiavelli's analysis.
Machiavelli treats the mistake of the Roman emperors as an instance of the all-too-common poor judgment of the Roman emperors. (It's so common he does not name the particular emperor.) But, seventh, he contrasts this with true wisdom. One property of such practical wisdom is, thus, the ability to foresee long-term (intrinsic) effects of one's actions. Machiavelli is neither claiming on one must foresee all the (accidental) effects, nor claiming that one must foresee (say) the timing of, say, predictable effect (i.e., fall of Roman Empire). Strikingly enough, at the start of Wealth of Nations, Smith characterizes philosophy in terms of one's ability to connect such distant cause and effect relationships: "philosophers...are often capable of combining together the powers of the most distant and dissimilar objects." What's key for Machiavelli, and in Machiavelli, it comes close to a definition of wisdom, is the ability to foresee evils and, if one is prudent, to take precaution against them (see especially chapter 3).** Obviously, Machiavelli is not claiming the possibility of perfect foresight. Intervening causes can undermine the workings of such long-term causal relations.
By contrast, Smith denies that when it comes to these cases (of Smithian social explanations) such long-term historical effects can be known in advance. They can only be known after the fact. This is not because Smith thought, as is sometimes claimed, that all foreknowledge impossible. As I have argued, it is, for example, characteristic of invisible hand explanations -- these involve unintended consequences that occur in the short-term -- that the outcome can known in advance to the right observer in real time.
*I have made no mention of the disproportion between cause and historical effect in this list. Both Hume and Smith emphasize it. Here's Smith one of his favorite, unintended consequence explanations: "Having sold their birth-right, not like Esau for a mess of pottage in time of hunger and necessity, but in the wantonness of plenty, for trinkets and baubles, fitter to be the play-things of children than the serious pursuits of men, they became as insignificant as any substantial burgher or tradesman in a city. A regular government was established in the country as well as in the city, nobody having sufficient power to disturb its operations in the one, any more than in the other. say, when they note."
**It is worth quoting the full passage (see here for the Italian): "The Romans did in these instances what all prudent[/wise savi] princes ought to do, who have to regard not only present troubles, but also future ones, for which they must prepare with every energy, because, when foreseen, it is easy to remedy them; but if you wait until they approach, the medicine is no longer in time because the malady has become incurable; for it happens in this, as the physicians say it happens in hectic fever, that in the beginning of the malady it is easy to cure but difficult to detect, but in the course of time, not having been either detected or treated in the beginning, it becomes easy to detect but difficult to cure. Thus it happens in affairs of state, for when the evils that arise have been foreseen (which it is only given to a wise[/prudent prudente] man to see), they can be quickly redressed, but when, through not having been foreseen, they have been permitted to grow in a way that every one can see them. there is no longer a remedy."