Joel Roth, in his book The Halakhic Process A Systemic Analysis, describes in detail the authority of the posek and various factors that inform a decision. In the Orthodox community the posek is an individual, and in the Reform and Conservative movements a committee of experts plays the same role. The book clarifies systemic principles of legitimate change within the halakhic system.
The authority of the decisor is extensive: In the most extreme instance this principle applies, peiamim she bittulah shel Torah zehu yesodah (sometimes the abrogation of Torah which is its foundation). As Roth writes,“when the ultimate goals of the Torah would be better served by its abrogation, even in its entirety, it is within the purview of the sages to take that step.” The circumstances that might warrant such action are never defined. In the final analysis, the determination of the need for such action lies with the sages themselves. As Moses rendered the decision on his own, so too must the sages make the decision on their own?
Another important systemic principle for making a change is et laasot la-donai heferu toratekha. Roth notes sages amended or abrogated norms in order to strengthen the Jew ’s commitment to the will of God.2! He cites Rashi on Berakhot 54a s.v.ve-omer. There are times when we abrogate(she-mevattelin) the words of the Torah in order to act for the Lord.... It is permissible to violate the Torah and to do what seems to be forbidden.”
The sages used medical and scientific sources to change the law. What counts is the specialist's expertise. It is a matter of record that the number of matters of law in the first sense stipulated in the talmudic sources and contradicted either by the expert scientific opinion of later ages or by the personal observation of later sages has produced many problems. How could itbe that the talmudic sages had been mistaken? Surely it was not reasonable to suppose that the talmudic sages had misperceived their own reality. It was more reasonable to surmise that the reality had changed, and once it became acceptable to make such a claim, medical and scientific sources that might result in the
abrogation of previously held legal norms could be introduced
without impugning the reliability or integrity of the talmudic sages. A new systemic principle referred to as shinnui ha- ittim(“a changed reality”) became the vehicle that enabled later sages to