Pesaq “changes” in a number of very different ways. I put the word in quotes, because I’m including things I do not consider an actual change in pesaq. R’ Herschel Schachter writes in a number of places of the difference between appropriate halachic innovation, “chiddush“, vs unauthorized change in established din, “shinui“. Much rests on whether one is actually changing established din, and whether that change is authorized.1- The realia change in some subtle but relevent way. What seems like a new pesaq for an old situation is actually a pesaq for a new situation. This isn’t what RHS would call chidush or shinui, it’s not really a change of halakhah. Chiddush — it’s a new pesaq for a new case.
If teaching girls Torah were declared “assur” rather than “tiflus“, the Chafetz Chaim’s grounds for backing Beis Yaakov would qualify. He held that universal secular education for girls was a change in realia which in turn changed the definition of “teaching them enough for them to observe Torah”. Teaching halakhah is no longer enough; they now must also see that Torah has greater beauty than the other systems of thought to which they are exposed. As it is, the Chafetz Chaim justified a change in minhag Yisrael, which is KEdin (like law) and thus follows the same rules — but what was changed wasn’t actually din itself.
1b- Technology advanced to make a new option possible. With a new set of options, we have a new reality. This may well call for a new pesaq.
Eg 1: If a woman ovulates before day 12, how do they ever have children? Many poseqim have historically permitted the couple not to fulfill the full 5+7 days of waiting, because the obligation to “be fruitful and multiply” outweighs a late practice of treating every period as though it might be zivah. However, with the advent of IVF-H (in vitro fertilization from the husband), a poseiq might decide not to wave the law, and require them to use IVF to fulfill the obligation to reproduce. If he so decides, it’s a new pesaq for a new case — not overturning precedent.
Eg 2: R’ SZ Aurbach held that modern ink making methods have advanced greatly due to superior grinding and therefore mixing of the ingrediants. In fact, he rules that indoor mezuzos ought not be checked. The chance of a letter cracking due to age are far smaller than the risk caused by unrolling and rerolling the parchment. Previous pesaqim are ignorable, since they speak to an inferior ink, not the situation now facing the rabbi.
1c- One can not overturn din because of the motivation of the din. If the motivation no longer applies, eg not using medicines on Shabbos lest someone may grind one (not that any of us grind our own medicines today), we may find that the balance of conflicting values shifted. But the din itself, to be implemented no conflict exists, stays on the books.
The exception to (1c) is where the motivation is codified in the din. For example, YD 1:1, women don’t shecht because it would make them queasy. If the pesaq was that queasy people who would be unable to maintain the settled mind necessary for good shechitah should shecht, eg women, then a veteran of a MASH unit may very well be allowed to shecht even as decided before employing her. However, if the pesaq is that women may not shecht, and the pesaq made because of assumptions about women, then we do not have such latitude.
This is a subcategory of #1 because we aren’t giving a new pesaq to an old situation. We are recognizing the fact that the current situation is actually a different one than what the precedent was set for.
2- “Halachic technology” advanced. Someone thought of this once-new concept of a heter mechirah (selling the land to a non-Jew so that the land may be worked during shemittah), or of a heter iska (configuring something similar to a loan with interest, but structured as a permissible investment), etc… If a poseiq feels that this invention is obvious enough that if it were valid, someone would have utilized it by now, then he would find that lack of usage to be a proof that it must be flawed. However, if it’s not obvious, then pruzbul or a rider for the kesuvah to protect the bride in case the groom might someday refuse to give her a get, etc… have not been ruled out. And so, some such proposals are actually shinuyim, and others are healthy new growth, chiddushim.
Here we aren’t changing the pesaq, we are orchestrating a new situation in order to be able to be subject to a different pesaq.
3- The realia don’t change, our knowledge of them does. The advance of science. But what to do when the realia change is a huge dispute with three basic approaches:
Rav Kook allowed changes in scientific theory to make rulings more stringent, not more lenient. This is based in an idea earlier expressed by the Vilna Gaon, that for every known reason for a law, there could be many undocumented reasons. Therefore a change in science might remove a cause for a stringency, but other, unknown, causes may still exist. However, if it removes a single explanation for leniency, then we have sufficient grounds to be strict.
Alternatively, one could argue that changes in theory do not warrant such change. Either on the grounds that we lack the pro forma authority to make changes (we lack the legal authority of Chazal), or because of a skepticism about scientific theory. Why change the law when the theory is bound to change eventually anyway?
A third possibility is to handle each one on a case-by-case basis. Often in effect this is means a resistance to change in halakhah, but in each case a distinct reason is found to support the original ruling. See for example this earlier essay in which I discuss my recollection of Rav Dovid Lifshitz explananing the kashrus of maggots.
Here, in the United States, around the 1980s, it became more common to have concern about the bugs on vegetables we eat. Any bug that is large enough to be seen by the naked eye may not be eaten. However, one need not use a magnifying glass or microscope to find tiny insects. My rebbe, R. Dovid Lifshitz, used a similar idea to explain a different problem. The Gemara explains that maggots found inside a piece of meat are kosher. (I presume that the case if where someone ate them accidentally and now wants to know whether he must bring a sacrifice.) The reason given is that they were born from the meat, and idea known in the history of science as spontaneous generation.” Therefore, halachah treats the maggots identically to the meat.
Spontaneous generation has since been disproven. Maggots come from microscopic eggs. Now that we know that the underlying science is wrong, does this mean that the halachic ruling is also wrong?
Rav Dovid taught that the halachic ruling is still correct. The microscopic eggs and maggot larvae are not within the realm of human experience. The only cause for the current presence of maggots that we can see is the meat. In terms of human experience, the meat is the source of the maggots.
Thus Rav Dovid manages to save the ruling, but does so by finding reasoning specific to the case. I do not know if Rav Dovid had an article of faith that such reasoning would always be found, and if not, what he would do when it couldn’t — change the halakhah or not?
The subject of chiddush vs shinui will therefore be omitted in this case; it depends on how one believes the halakhah should change — or not. Does precedent matter, or was the earlier ruling simply in error (technically: a valid ruling about a situation that doesn’t arise)?