Excerpt from An Interview with Rabbi Hershel Schachter:
Does Halakhah change over time and, if so, in what ways? When, if ever, are halakhic innovations acceptable?
Halachah does not change over time. We believe “ani Hashem lo shanisi” (I am Hashem, I have not changed). Because the essence of God does not change, and we assume that the Torah is a description of E-lohus (Godliness), Halachah cannot change either. However, the world around us is ever-changing, and because of that, the way Halachah is practiced today is not exactly the same as it was one hundred years ago, fifty years ago, or even yesterday. There are so many changes taking place and the slightest one makes for a new she’eilah. In almost every siman in Shulchan Aruch, you have many se’ifim, not just one, so that under different conditions, you follow a different se’if. Every she’eilah has to be taken within the historical context in which it comes up and with the proper perspective. So the application of Halachah changes, even as Halachah itself does not.
In my introduction to Erets ha-Tsevi, I give the mashal (example) that Avraham Avinu, when there was a famine, left Erets Yisrael for Mitsrayim. Then, in the days of Yitschak, there was another famine, so he thought to go to Mitsrayim also. Yitschak was known for following the traditions of his father – he dug the same wells as his father had and gave them the same names, etc.
So the Zohar, quoted by Ha-Kesav ve-ha-Kabbalah, comments that Yitschak followed the whole masorah (tradition) of his father, and wanted to continue doing so by leaving for Egypt, but then the Ribbono shel Olam told him no – you are an olah temimah (a perfect offering) brought on the mizbeach (altar); you are not allowed to leave for Chuts la-Arets. He thought he was doing exactly the same thing as his father by deciding to leave, but Hashem informed him that the circumstances had changed.
Note: I heard this before but don’t agree this is what the Zohar means. Attributing blind “Mesorah” in ret-con to Yitzchak in seriousness requires stronger textual evidence. I cannot find this within now.
Similarly, the Gemara tells us that Moshe Rabbeinu made many charamim(bans) on cities Benei Yisrael fought against, so Yehoshua bin Nun made a cherem on Yericho thinking that he was doing the exact same thing as his rebbe had. The Ribbono shel Olam got angry with him for having done so, though, because the circumstances had changed: Moshe Rabbeinu made his charamim before Benei Yisrael crossed the Yarden, so there was no din of kol Yisrael arevim zeh ba-zeh (every Jew is mutually responsible for the next); Yehoshua, though, made his cherem after that din took effect, so all of Benei Yisrael would be responsible for the sins of those like Achan who violated the cherem, potentially endangering them all. He simply did not realize that he was living in a different generation and the she’eilah was a different she’eilah.
Oftentimes, people will say, “My father belonged to the Agudah, so I belong to the Agudah; my father belonged to the Mizrachi, so I belong to the Mizrachi,” without taking into consideration that today everything is totally different: that was before Hakkamas ha-Medinah (the establishment of the State) and before Milchemet Sheshet ha-Yamim (the Six Day War)! The Agudah today is not necessarily the same as the Agudah of 50 years ago. Everything is changing in the world. Halachah does not change but its application does.
Note: I mostly agree with all this.