It’s been a while since I summarized my psak shiur, but I’ll try to catch up. In the previous post on this topic, we began dealing with the issue of poskaning for one’s self – when is one qualified, are there different standards for ruling for one’s self than there are for others, etc. The second question is whether even an unquestionably qualified posek should or must refrain from ruling for himself due to bias. In the past, when dealing with the benefits of having a Rebbe (here), we noted that practically it is often good to have an outsiders perspective, both to ensure that you are not too lenient and to ensure you are not to stringent on yourself. Is that a Halachic requirement?
The shiur and sources are available: here.
Most positions break down into two possibilities, though there is an outlying third. Tosafot rules that one can poskan for yourself. Commenting on a Gemara in which Yalta, the wife of R. Nachman, asked Niddah question to Rabbah bar bar Chanah, Tosafot argue that she did this either because 1) R. Nachman was not an expert or 2) she did not want him to be disgusted by the process of answering her Niddah questions. However, in general there is not a problem. His proof that you can poskan for yourself is that Mishna in Negaim says that you cannot poskan for yourself when it comes to Negaim, Nedarim, or Bechorot. However, the implication is that you can poskan for yourself in general. In Eruvin we find that Talmidei Chachamim are allowed to poskan on the kashrut of their own shechita knives. This position in followed by the Shach, the Shulchan Aruch HaRav, R. Ovadiah Yosef, and many others. They note that in addition to this being the majority position in Rishonim, many poskim do poskan for themselves, and this should act as a proof that it is permitted. In general we take minhag seriously, and when we have a minhag of poskim, that has a tremendous impact on Halacha.
As for why the three cases in the Mishna are exceptions, some argue that they are all cases with a particular bias, or they are all cases where halachic realities are created by what you say – i.e. they are either practically or fundamentally different than all other cases of psak.
The Rash, on the other hand, rules that you are only allowed to poskan for yourself when there is no chezkat issur – meaning you are just clarifying the halacha but not removing a presumption of issur. This could be understood fundamentally – that only an outside party can create halachic reality, but not a biased person, parallel to how many understand the limitation on single witnesses (a comparison he makes). However, the Taz, who rules like the Rash, seems to take it much more pragmatically – one is less trusted when he has to overturn a presumption of prohibition. The Chayei Adam rules like this as well, thus ruling that you cannot poskan on tevilah questions for your wife (as in that case she is a Niddah and must remove the status). R. Feivel Cohen (in conversation with R. Ezra Schwartz) argues that according to this logic, one should not rule on one’s wife’s hefsek tahara. (The same would be true for a Yoetzet Halacha ruling for herself.)
Most poskim follow the first position, though many follow the second. The Meiri is a lone voice in presenting a third position – that you can never poskan for yourself.
Remember – just because it is mutar, does not always make it a good idea. If one knows he is biased, he should get an outside opinion.
Another issue that emerges is whether one is trusted to quote a previously unknown position to answer a question that one has a vested interest in. A Gemara in Yevamot says not, unless he issued his psak before he became personally involved. Rashi assumes that we are worried the posek in question will lie. The Ritva assumes that we are worried he will make a mistake unwittingly because of his bias. The Rishonim limit this Halacha in many ways, and Shulchan Aruch records many of these limitations. First, this rule only applies when the posek is quoting a tradition – I heard from Rabbi X – and that carried weight. If he makes an argument, since other poskim will be able to analyze whether it is right, and are therefore not relying on his tradition alone, there is not a problem. Similarly, if what he says is obvious, there is not a problem relying on it. The Shach notes that if it clear in the Gemara or some other Halachic source, this would be considered obvious.
Again, a certain level of common sense is needed here to determine whether the posek in question is too biased to argue honestly. However, fundamentally, here too there is no barrier to him being involved in a Halachic question that he has a personal stake in.