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An eternalism is a belief system based on some eternal ordering principle that supposedly gives everything a definite meaning. It can be useful to look at eternalisms as wrong-way reductions: they turn somewhat-difficult problems into more difficult (or impossible) ones.
Obviously, wrong-way reduction is worse than useless. However, it can be emotionally compelling, as a way to deny nebulosity. The aim is to turn a messy problem into a tidy one. Rationalists are particularly averse to mess, and may be willing to overlook the fact that the new tidy problem is provably insoluble.
In religious eternalisms, the ordering principle (often personified as God) is supposed to act autonomously. In that case, one can imagine the principle fulfilling eternalism’s promises of certainty, understanding, and control without humans having to do the work. This is not a wrong-way reduction; it “reduces” a hard problem to maintaining faith, which is easier (although ineffective).
In a rationalist eternalism, certainty, understanding, and control must be available to us directly. And so we must be able to get certainty, understanding, control of the principle itself, by rational means. For example, utilitarianism is supposed to deliver certainty, understanding, and control of ethics, through mathematical calculation.
Unfortunately, the utilitarian calculations are more difficult than effective moral reasoning, so this is a wrong-way reduction. More generally, in other rationalist eternalisms, the problem of accessing the eternal ordering principle is more difficult than solving the practical problems the eternalism is supposed to address. In fact, in each case, it is outright impossible—and has to be.
Rationalist eternalism promises to eliminate nebulosity, so it is attractive when nebulosity is unattractive. However, nebulosity is a brute fact that cannot be eliminated; and so no eternal ordering principle can exist. Rationalist eternalism fails precisely because of its attractive promise.
The rationalist eternalist proposition is: “If we can just eliminate nebulosity using math, then solving the actual problem will be easy, at least in principle.” But mathematics can’t eliminate nebulosity, so this is always a wrong-way reduction.