In the introduction to his responsa, Rav Moshe Feinstein writes that the Sages of each generation are permitted and obligated to issue halachic rulings despite the fact that they may not “decide according to truth in heaven”. Applying the rules of Divine law to evolving human conditions is a most difficult task, and even the greatest of Sages may be misunderstanding G-d’s intent. All that can be asked of a posek is to examine the material as best he can, “with a heavy head and fear of G-d, the Blessed One”. When such is done, the ruling issued is true and becomes part of the corpus of Torah, even is such a ruling does not comport with G-d’s actual will—which, of course, we can never know.
Moreover, it matters little what G-d’s theoretical will might have been. G-d’s will is that man decides how the Torah is to be applied. Truth may be in heaven, but “Torah is not in heaven”. G-d gave the Torah to man, thereby relinquishing His say in the development of Torah. Not only is Torah “not in heaven”, Rav Moshe explains, but G-d purposely left much of the Torah vague for man to interpret, fully aware that different people will reach different conclusions. These different, even contradictory, conclusions “are both words of the living G-d” and reflect the Divine will. Heavenly truth and halachic truth are not the same, and that is how G-d willed it. While the former is fixed and unalterable, the latter is multifaceted and can vary from place to place and from time to time.
This may be frustrating to the legal or philosophical purist, but halachic truth makes no claim to be absolutely true. Halacha is true in practice, even if it may not be true in theory.
It is not only heavenly truth that may conflict with halachic truth. Halachic and scientific truth can also be at odds with each other. For religious Jews, it should be axiomatic that the G-d of nature and the G-d of history are one and the same (denial of this fundamental premise is, I would argue, the most basic form of heresy) and thus, it is impossible for there to be actual conflict between Torah and science. The G-d who created the world is the same G-d who gave us the Torah; Torah and science must work together harmoniously.
But that is a work in progress. With the explosion of scientific knowledge, many long-held religious beliefs have been challenged, and it does appear that there are discrepancies between these two manifestations of the Divine. But since such is impossible, it behooves us to re-evaluate our prior understanding of Torah, even as we are aware that scientific theories are refined and at times, abandoned as more research is done. But to deny one or the other of these truths is at best misguided, and at worst, heresy.
It was the inability of religious leaders—Jewish and not—to acknowledge the truths of science that led multitudes to abandon religion. Claiming that science is false may work to keep some in the fold, but it is the best way to drive the intellectually honest and curious far away from faith. Acknowledgment of the inherent compatibility of Torah and science does not mean we can (yet) explain all discrepancies. Our knowledge of both Torah and science is far from perfect and thus, it is inevitable that there will be tension between the two.
Much of the tension can be eased once we realize that the Torah is not a science book. It is not only futile, but often silly, to try to harmonize the two. While much ink has been spilled on reconciling the Torah’s account of creation with that of science, it may not be necessary, or even wise, to attempt to do so. The Torah’s creation narrative is meant as a moral and ethical charge, not as a factual, scientific description. Rather than trying—and very possibly failing—to resconcile the two, we might be better served trying to understand the Torah’s message.
A more practical problem arises when a practical law is based on scientific principles that we now know (or at least believe) to be incorrect. And here we come to the third chapter of masechet Chulin, which details the 18 treifot, 18 conditions listed in the opening Mishna (Chulin 42a) which would kill an animal within a year, rendering the animal a treifah.
The problem in learning this Mishna today is that parts of the Mishna are no longer scientifically accurate. For example, an animal clawed by a lion can live for many years. Moreover, there are other conditions that would kill an animal within a year but are not listed in the Mishna and are therefore considered to be kosher, whereas scientifically, they would be declared treif.
To my knowledge, the first serious treatment of this issue was undertaken by Rabbi (Dr.) Yitzchak Lampronti, in his 18th century halachic encyclopaedia, Pachad Yitzchak. His discussion centres on the Talmudic (Shabbbat 107b), but scientifically mistaken, notion that lice are spontaneously generated. Thus, the Sages permit their killing on Shabbat, as it would not be considered the taking of actual life. The Pachad Yitzchak suggests that with our understanding that no life is spontaneously generated, it should now be forbidden to kill them on Shabbat. Furthermore, he asserts that if the Talmudic Sages could hear the scientific proofs, they would acknowledge their correctness and would adjust the law to forbid the killing of lice on Shabbat.
Despite (at least in my mind) the cogency of his argument, this view has not been widely accepted. Perhaps surprisingly, many, actually most, halachic authorities permit the killing of lice on Shabbat even as they acknowledge that lice are not spontaneously generated. The Hazon Ish explains that Jewish law was fixed with the closing of the Talmud, and the role of latter-day authorities is to interpret and apply their words, but not to uproot them, even if the rules of science would require one to do so.
Yet despite the conservatism inherent—and necessary—in halacha (and in all serious legal systems), sometimes the science is so overwhelming and the consequences so important that halacha readily changes to deal with the new reality. The Talmud assumes that a baby born in the eighth month cannot survive and hence, it would be forbidden to violate Shabbat to save such a baby. Yet that is exactly what we do today—because we all know that a baby born in the eighth month can survive and flourish, and to not save its life would be a terrible sin.
It is one thing to ignore the findings of science when it comes to matters of kashrut—for both stringency and leniency—or even regarding the laws of Shabbat. It is quite another to do so when a life hangs in the balance.
 This is true of each generation of Torah scholars, too. Halacha k’batrai, the halacha follows the latter-day authorities, and even if Moshe Rabbeinu were to enter the Beit Midrash (see Menachot 29b), we would ignore his halachic view.
 And as alluded to above, halacha does not require absolute knowledge. Halacha is to be based on the best available knowledge at the time.
 This work heralded a new genre of rabbinic writing, namely an encyclopaedia of halachic topics arranged alphabetically—the most famous today being Encyclopaedia Talmudit.
 There are those who refuse to acknowledge the scientific view and are sure that eventually science will come around to the view expressed in the Talmud. I suspect very few reading this devar Torah fall into that camp. More common, though scientifically no more reasonable, is the view that nistanu hateva, nature has changed, and what was true in Talmudic times is no longer true today. A number of years ago, Dr. Norman Lamm—appearing on a panel we hosted (listen here)—mentioned in passing that nistanu hateva means our understanding of nature has changed. This, of course, has great resonance with many (and effectively validates the view of Rabbi Lampronti) but that is not how the term has been traditionally understood.