The decision of a posek is known as a psak din or psak halakha ("ruling of law"; pl. piskei din, piskei halakha) or simply a "psak". In Hebrew, is the root implying to "stop" or "cease"-- the posek finalizes the process of legal debate. Piskei din are generally recorded in the responsa literature.
Formulating a ruling (psak din)
In formulating a ruling, a posek will base the psak din on a careful analysis of the relevant underlying legal principles, as well as a careful study of the application of these principles. A posek must therefore be thoroughly versed in rabbinic literature, from the Babylonian Talmud, through the major medieval codifications of the Law and up to recent decisions.
and finally, a close analysis of the works of the Acharonim (rabbinic authorities from about the 16th century onwards) discussing the Halakha as recorded in the literature of the Rishonim (and earlier Acharonim).
The ruling itself is an attempt to apply the precedents and principles of the tradition to the question being asked. One common goal of poskim in this regard is, as far as possible, to be consistent with the codified law, as well as with the maximal relevant legal precedents, generally being decisions recorded in the responsa literature.
In the Haredi world, each community will regard one of its poskim as its Posek HaDor ("Posek of the present Generation").
Hasidic Jews rely on the Rov in their community (sometimes but not always Rebbes also get the position as Rov) or leading posek recommended by their Rebbe. Yet there are some hassidim (Jews who are hasidic), but not part of a specific movement. These hassidim will vary in who they follow, sometimes following generic hassid-style poskim like Rav Shmuel Wosner.
Modern Orthodox Jews may select a posek on a more individual rather than a communal basis, although customs vary.
The approach taken here will, generally, be as above. Thus poskim will not overrule a specific law unless based on an earlier authority: a posek will generally extend a law to new situations but will not change the Halakhah; see the article on Orthodox Judaism.
Conservative Judaism approaches the idea of posek, and Halakha in general, somewhat differently, poskim here apply a relatively lower weighting to precedent, and will thus frequently re-interpret (or even change) a previous ruling through a formal argument; see Conservative Halakha. Although there are some "poskim" in the Conservative movement - e.g. Rabbis Louis Ginzberg, David Golinkin, Joel Roth, and Elliot Dorff - the rulings of any one individual rabbi are considered less authoritative than a consensus ruling. Thus, the Conservative movement's Rabbinical Assembly maintains a Committee on Jewish Law and Standards, whose decisions are accepted as authoritative within the American Conservative movement. At the same time, every Conservative rabbi has the right as mara d'atra to interpret Jewish law for his own community, regardless of the responsa of the Law Committee.
Although Reform stresses the individual autonomy of its membership, it never completely abandoned the field of responsa literature, if only to counter its rivals' demands. Even Classical Reformers such as Rabbi David Einhorn composed some. Rabbi Solomon Freehof, and his successor Rabbi Walter Jacob, attempted to create a concept of "Progressive Halacha", authoring numerous responsa based on a methodology laying great emphasis on current sensibilities and ethical ideals. Full text collections of Reform responsa are available on the website of the Central Conference of American Rabbis.
The Reconstructionist position is that if Jews would have formed cohesive communities again, their rulings would be binding, yet presently Judaism is in a "post-Halakhic state". Therefore, their basic policy is to allow tradition "a vote, not a veto" in communal and personal affairs.
List of poskim and major works
In chronological order, by the year of birth, and if needed, secondarily, by year of death and surname.
Poskim of past years
Toledot HaPoskim, History of the Jewish Codes, by Chaim Tchernowitz.