In 1286, Emperor Rudolf I had instituted a new persecution of the Jews, and Rabbi Meir left Germany, but was captured and imprisoned. The ROSH raised a ransom for his release, but Rabbi Meir refused it, for fear of encouraging the imprisonment of other rabbis. Thereafter Rosh assumed Rabbi Meir's position in Worms. He was, however, forced to emigrate (in all likelihood, a victim of blackmail by the government, aimed at acquiring his fortune). After leaving Germany, he first settled in southern France, and then in Toledo, Spain, where he became rabbi on the recommendation of Rabbi Solomon ben Aderet (RaShBA). Rabbenu Asher's son Judah testified to the fact that he died in poverty.
Rabbenu Asher possessed "methodical and systematic" Talmudic knowledge, and was distinguished for his ability to adumbrate long Talmudic discussions. The ROSH, influenced by his teacher Rabbi Meir, was averse to lenient decisions in halakha, even when theoretically justified. (Several of his rulings which may appear lenient, are actually strictures: his decision against praying more than three times a day is, in fact, limiting. Similarly, his assertion that the phrase halacha le-Moshe me-Sinai - "an oral law revealed to Moses on Sinai" - does not always bear a literal meaning but often signifies a universally adopted custom, is not usually taken as a liberal interpretation.) The ROSH was, however, known for his independent legal reasoning: "We must not be guided in our decisions by the admiration of great men, and in the event of a law not being clearly stated in the Talmud, we are not bound to accept it, even if it be based on the works of the Geonim." (For instance, the ROSH ruled that the liturgy of the Geonim was not subject to the Talmudic rule against change in the prayers.)
Rabbenu Asher was opposed to the study of secular knowledge, especially philosophy. He held that philosophy is based on critical research, whereas religion is based on tradition and the two are thus "incapable of harmonization". He said that "none that go unto her may return" - in fact, he thanked God for having saved him from its influence, and boasted of possessing no knowledge outside the Torah. He led the anti-Maimonists in Spain; he also attempted to issue a decree against the study of non-Jewish learning. One effect of this attitude was to limit his influence on secular Spanish Jewry. At the same time, within rabbinic circles, "he transplanted the strict and narrow Talmudic spirit from Germany to Spain", and this, in some measure, turned Spanish Jews from secular research to the study of the Talmud.
This work resembles the Hilchot of the Rif (Rabbi Isaac Alfasi) - also an adumbration - but differs in quoting later authorities: Maimonides, the Tosafists and Alfasi himself. Rabbenu Asher's work superseded Alfasi's within a short time and has been printed with almost every edition of the Talmud since its publication. This work was so important in Jewish law that Yosef Karo included the ROSH together with Maimonides and Isaac Alfasi as one of the three major poskim (decisors) considered in determining the final ruling in his Shulkhan Arukh.
Rabbi Asher also wrote: