The Gemara then cites the Beraisa that states, "If people regard certain
permitted practices as prohibited, one is not allowed to perform those
practices [in that place] in order to reject the conduct of those who
prohibit them." (The Gemara earlier (15a) cites the same Halachah with a
slightly different wording: "If practices are permitted but some people
regard them as prohibited, one is not allowed *to permit those practices* in
front of them.")
The Gemara explains that this Halachah is in accordance with the view of
Raban Gamliel, that even a statement -- or practice, in this case -- that is
not a true Neder requires Heter or Hafarah.
(a) From the fact that the Gemara compares Minhagei Isur with Nedarim that
do not take effect, it seems that just like those Nedarim are Asur
mid'Rabanan, so, too, Minhagei Isur are Asur only mid'Rabanan and require
Hatarah only mid'Rabanan. (It is interesting to note, however, that although
the rule of "practices that are permitted..." is the Halachah, the ruling of
Raban Gamliel is *not* the Halachah, indicating that they are not totally
The BIRKEI YOSEF (YD 214) indeed rules that it is mid'Rabanan, supporting
his ruling with the fact that mid'Oraisa a Neder needs to be expressed
verbally (Shevuos 26b). He infers this from the wording of the Shulchan
Aruch as well, who writes that "it is *as if* he made a Neder," but not that
it is considered to be an actual Neder.
It is not clear why the Birkei Yosef does not cite the Gemara earlier in
Nedarim (15a) that says explicitly that this Isur of "Lo Yachel Devaro" in
this case is only mid'Rabanan.
(b) This Halachah includes both personal customs (Minhagim which a person
accepted upon himself privately, such as fasting on certain days), and
public prohibitions or "Minhagei Isur" (stringencies which a community took
upon themselves; for example, the Minhag not to work on Erev Pesach). In
both of these cases, one needs a Heter Chacham to discontinue the Minhag.
Some maintain that one needs a Heter only if he was aware at the outset that
this practice was merely an extra stringency and not Halachah. If one
practiced an Isur because he mistakenly thought that it was really Asur, he
does not need a Heter Chacham (SHULCHAN ARUCH YD 214:1, first opinion).
Others, however, maintain, on the contrary, that only if the practice was
mistakenly accepted may the Chacham be Matir it. When one accepted the
stringent practice knowing that it was permitted by Halachah, one may *not*
be Matir it (TESHUVOS HA'RASHBA 1:98, and RAN in Pesachim, Perek 4, second
opinion, based on the Yerushalmi).
The VILNA GA'ON (Shulchan Aruch, ibid.) cites the RAN in Pesachim who points
out that the fact that our Gemara says that the source for this Halachah is
from the verse "Lo Yachel Devaro" supports the first opinion that it is
similar to a Neder, where a person knows that something is permitted and he
consciously tries to prohibit it.
The Gemara in Pesachim (51a) cites the same Beraisa as our Gemara, ""If
practices are permitted but some people regard them as prohibited, one is
not allowed to permit those practices in front of them." Rav Chisda
qualifies this rule and says that it is referring only to permitting the
practice in front of "Kuta'i" (unlearned people who follow practices out of
ignorance), because when they see us acting leniently, they will take Isurim
lightly, since they see that what they think is prohibited is really
permitted. Why, then, does the Shulchan Aruch cite this rule as applying to
all people? The VILNA GA'ON (Shulchan Aruch, ibid.) answers that this single
statement teaches two different Halachos (as the BEIS YOSEF points out). The
first Halachah is that one is not allowed to permit the practice without a
Hatarah; this Halachah applies to everyone. The second Halachah is that one
should not *conduct himself* leniently in front of others who conduct
themselves stringently, lest they come to deride his own Minhag. The latter
Halachah is the subject of the Gemara in Pesachim which applies only in
front of "Kuta'im."