What Gezeyra is not a Seyag?

Avodah
1

The invaluable Yad Peshuta commentary on Mishneh Torah (available on
AlHaTorah.org) by Rav Nachum Rabinowitz, zt"l, helped me with my problem.

Gezeyra is a general term for a rabbinical prohibition. A Gezeyra that is
a precaution against inadvertantly trespassing a Torah prohibition is a
Siyag kind of Gezeyra. An example is the rabbinical prohibition against
cooking milk together with poultry, on the grounds that people confuse
poultry with meat. Another example is the prohibition against doing
business on Shabbos, on the grounds that habit would lead one to transgress
the Torah prohibition against writing. These are called Gezeyros that are
Siyagim, and if their practice spread throughout Jewry, no Beis Din can
permanantly revoke them.

However, there are also rabbinical prohibitions that are not precautions
against transgressing Torah prohibitions, but strategies to enhance Torah
life, or to prevent denigrating it. They are like Takanos, but in the form
of a prohibition. An example is the decree against preparing things on
Shabbos for Yom Tov, or vice versa. These Gezeyros are not called Siyagim,
and they are technically revokable by a Beis Din that is greater than the
prohibiting one b'chochma u'b'minyan, even if their practice spread
throughout Jewry.

Zvi Lampel

2

I am glad that helped you, but I didn't say anything that wasn't in
my notes from R Sacks' shiur.

(I mentioned this primarily for personal reasons: RZL and I live in
the same neighborhood as R Yonasan Sachs, and RZL might appreciate
attending more of his shiurim if he knew more about their quality.)

The invaluable Yad Peshuta commentary on Mishneh Torah (available on
AlHaTorah.org) by Rav Nachum Rabinowitz, zt"l, helped me with my problem.

Gezeyra is a general term for a rabbinical prohibition. A Gezeyra that is
a precaution against inadvertantly trespassing a Torah prohibition is a
Siyag kind of Gezeyra....

This is from the Tif'eres Yisrael, according to my notes.

However, there are also rabbinical prohibitions that are not precautions
against transgressing Torah prohibitions, but strategies to enhance Torah
life, or to prevent denigrating it. They are like Takanos...

R Sacks called these issurim and taqanos "Dinim deRabbanan", but otherwise
this maps straight to what he said.

See also his application to the distinction between dinim derabbanan
and taqanos to understanding shehiyah vs chazarah in bishul beShabbos.
My notes, after the list of categories, a note about dinim derabbanan
vs gezeiros:

   The distinction between the second and third categories is subtle. In
   order to be a din (or issur, or melakhah) deRabanan, the prohibited
   action is one that is similar in purpose to the permitted one.

   In contrast, a gezeira does not even require an action. In the example
   I gave, it was inaction, leaving the pot where it is, that is
   prohibited. Second, the category includes things that are similar in
   means to the prohibited act, and will therefore cause confusion about
   what is and what isn't okay; and things which will allow people to be
   caught up in habit, and forget about the prohibition.
   
   Only a gezeira may defy an actual Divine law (although a pesaq will
   often define one), and even so only under specific circumstances. All
   of the following must be satisfied:

     * The law being protected is more stringent than the one being
       violated. This determination isn't easy.

     * The law is being violated only through inaction. No one is being
       told to actively violate G-d's commandment.

     * According to the Ta"z, the law being violated will still be
       applicable in most situations. It still must exist in some form.
       (Not every acharon agrees with this requirement.)

   In another way, a gezeira is less powerful than a normal rabbinic law
   in that it cannot be compounded. One may not make a "fence" for the
   express purpose of protecting another "fence".

Tir'u baTov!
-Micha