The key idea is that ex ante we have a better idea of the risks of small changes than we do of the risks of large changes, and ex post we can contain the damage of small changes better than the damage of large changes. Given reasonable loss aversion in the face of genuine uncertainty, this implies that we should be more careful about larger than about smaller changes to our institutions. Note that the argument is not that major changes will have bad effects, but that we should be more uncertain about the effects of major changes than about the effects of minor changes, and that this asymmetric uncertainty should make us more cautious about undertaking these larger changes.
There is a whole question here about how to define “small” and “large,” but let’s let that slide for the moment, since there are clearly some cases where the argument seems to make sense. Something like it forms part of Burke’s case against revolution: there are many potential bad consequences from trying to completely change an entire form of life, but given our epistemic limitations we can hardly know which major changes will be on balance good and which will be on balance bad. And today we might see this argument deployed to justify less action than some economists might like on restarting the global economy, or less action on climate change than some environmentalists might like, though whether the argument works in these cases depends in part on our estimates of the costs of inaction, estimates that may also be subject to our epistemic limitations. (A full formal investigation of the problems here would presumably use Bayesian analysis. But here I reveal my on epistemic limitations – I do not know enough to use it).
This is a sort of “selection” argument for conservatism, though it does not look like one at first sight. In order for this argument to justify gradualism, however, rather than rigid immobility, we have to assume that small deleterious changes are either eliminated quickly or else that they cannot accumulate and interact in collectively very harmful ways. For example, a firm that produces bad products should go bankrupt without affecting the ability of other firms to operate too much, and small regulatory changes that do not work must be quickly identified and eliminated through some explicit mechanism (e.g., litigation, as in some defences of the common law). Whenever there is no selection mechanism to get rid of such minor but potentially harmful changes (and there may not be one, especially if the changes are only truly harmful in connection with many other changes), they can accumulate until they reach a kind of threshold (when they become really bad). The point is that if our epistemic limitations are binding with respect to the consequences of large “revolutionary” changes, they are also binding with respect to the consequences of collections of small changes that interact in complex ways with one another. Thus, in the absence of knowledge about the interactions of many small changes, the argument would seem to justify rigid immobility, not gradualism.
Indeed, an awareness of this problem seems to have led most classical political thinkers to a position that is in a sense the reverse of the Burkean “gradualist” position, i.e., willing to contemplate major changes to institutions under some limited circumstances (when the legislator has good knowledge) but extremely wary of small changes that might accumulate and interact in unexpected ways (since the presence of knowledgeable individuals capable of understanding their effects in the long run cannot be assumed). Plato, for example, while quite willing to recommend radical institutional innovation in cities (e.g., equal education for men and women, major censorship, new religious institutions, etc.) insisted that a city must be very careful about changing its “educational” laws, in part because the deleterious effects of any minor changes would not be visible until many years later, and they could easily accumulate in destructive ways. Similarly, Aristotle was not shy about recommending large institutional changes to political communities, but nevertheless urged cities to be vigilant about institutional drift, the minor changes that seem harmless individually but may collectively lead to the destruction of a politeia.
The point is not that they believed that major change was epistemically “safe” while gradual change was not; they thought both major and minor change was epistemically problematic. (Plato’s insistence on the need for knowledge in politics is grounded precisely on an extreme distrust of normal human epistemic capacities; we would only need philosopher kings if we were effectively incompetent in political matters). But they did not think gradual change was any less exempt from the problems caused by human epistemic limitations than revolutionary change, since they did not think that there were appropriate “selection mechanisms” able to weed out deleterious gradual institutional change under normal circumstances (other than “state death,” which is of course not especially ideal from the point of view of the state in question; as Josiah Ober has noted, whereas bankruptcy may be the consequence of “failure” in a market economy, “state death” in the classical world typically implied the death or enslavement of much of the population). From this point of view, major but planned institutional change had a slight “epistemic advantage” over unplanned, gradual change, at least so long as gradual changes could accumulate and interact in deleterious ways that could not be easily identified either at the time or in advance.