Personal stringencies in Jewish practice are private matters but they can also provide insight into the nature of Jewish law. In an undated responsum, R. Moshe Feinstein answered a question about the complex topic of eruvin with a brief musing on the effect of halakhic decision-making on contemporary religious life.
The Shulchan Arukh (Orach Chaim 362:10) quotes the Rambam as ruling that “doorpost” eruvin, the common string connecting poles, are ineffective unless the majority of the wall is solid. According to this position of the Rambam, nearly all contemporary eruvin are invalid because they utilize the “doorpost” model across most, maybe even all, the perimeter of the eruv. While the broad consensus of halakhic authorities rejects this ruling of the Rambam, some people adopt the stringency of refusing to utilize eruvin in deference to the Rambam.
An unidentified questioner asked R. Moshe Feinstein whether someone who followed this position of the Rambam may change his practice and adopt the mainstream position allowing “doorpost” eruvin. I imagine a recently married yeshiva student who easily refrained from using an eruv while single but now, with wife and baby in tow, finds rejecting the eruv overly burdensome. R. Feinstein (Iggeros Moshe, OC 2:83 – link) responded that it depends why he acted strictly. If it was because he followed a ruling that the Rambam was correct, then he may not change his practice. While no one in our generation, R. Feinstein stated, has the authority to rule either according to or against the Rambam in this debate, he may come from a community that has a longstanding ruling. Someone who comes from a community that follows the Rambam on this issue is as if he received a personal halakhic ruling according to the Rambam and he may not change his practice.
This last step is very important. Why does it matter whether he received a halakhic ruling? If the Rambam’s position is a stringency for most people, it should also be for someone who received a halakhic ruling on the subject. R. Feinstein directs readers to the Chayei Adam without specifying where. I believe he means ch. 127 par. 10, where the Chayei Adam follows the lead of the Peri Chadash (Orach Chaim 567) in a fascinating discussion of the nature of custom and halakhah. A custom, a minhag, retains the force of a vow. While this means that a personal custom is binding, it may be undone through the process of hataras nedarim, nullifying the vow.
A halakhic ruling, however, is not a matter of a vow. By asking a halakhic question, you are not merely implicitly accepting to follow a rabbi’s guidance on the matter you bring before him. When a rabbi rules for a questioner on an halakhic matter, his ruling shapes the questioner’s Torah obligation, creating a new halakhic reality for him. Such is the power of the halakhic decisor. The rabbi not only teaches the law but creates it. According to this approach, halakhah can be different for different people. For one person, the Rambam’s view is merely a stringency. For another, it is Torah law.
This personal nature of Jewish law is at once jarring and reassuring. The discovery that halakhah is not a static body of law, an objective corpus of knowledge that we need only access, but a vibrant tree with many branches can be misleading. If there are multiple options, can I choose the law I want just I like choose the pizza topping that fits my taste and mood? R. Feinstein tells us that we may not. We are bound by the religion of our ancestors, the practices of our birthplaces and the guidance of our teachers. The circumstances of our lives, the experiences that formed us and the people who molded us, determine the details of our religious lives.