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I've been hearing the term "Mussar Movement" being used a lot more often as of late and I even saw some advertisements for it on other Jewish forums. I'm speaking to a specific movement, not just the concept of Mussar in Judaism.

What I gathered is it frames itself as a type of deeper spiritual movement like Kabbalah does but it also incorporates Reform-like philosophies. The history I read shows it came up in Lithuania in the 1800s which makes sense considering that was around the time Reform Judaism really came up.

I was just curious if there was a clear explanation as to what it is and how it relates to Jewish practice.

The musar movement is mostly associated with Rabbi Yisra'el Salanter and his students. There is a lot that could be said about it, but I will write mostly about the more controversial aspects and differences in Jewish practice, rather than, for example, psychological insights.

Rabbi Yisra'el advocated taking the time to learn works of musar. Already in his Letter of Musar (English), he says that intellectual knowledge isn't enough; he recommends visualizing "a sword between your legs and hell beneath you" (the Talmud says this about a judge; he explicitly generalized this statement to everyone). Learning musar was not just reading a book; after the intellectual understanding it had to be repeated "with an excited soul, a ready heart, a distraught voice, burning lips." But even apart from his special method, his advocating actually learning musar at set times and at set places (he required a special place for this learning), in addition to learning standard books of Torah, was a novelty. One of his students, Rabbi Simcha Zisel Ziv, opened up a Talmud Torah, i.e. a yeshiva, in which musar was one of the prominently studied subjects, not only more traditional subjects such as Talmud. This was controversial and sometimes unpopular even within; in another yeshiva the students actually revolted against this type of learning and forced it to split into two.

Another novelty with regard to Jewish practice was that ethical behavior is actually a law on par with other religious obligations. In his Letter of Musar he bemoans the fact that while people are careful to eat kosher food, they are not careful in their business dealings. The Torah counts both of them as commandments, and yet people who would never violate the commandment not to eat non-kosher food aren't careful not to cheat in business.

In other cases, Rabbi Yisra'el "discovered" unnoticed laws in the Talmud. One example would be his close reading of the law in certain cases (e.g. Bava Metsi'a 6:1), that if two parties deceived each other, the only claim they have against the other party is "a complaint." He learns from this that the right to have a complaint against someone is a legal right just like any other monetary claim. When the Torah allows you to have a complaint, you can have a complaint. Conversely, if you forgive the other party, you are no longer allowed to have a complaint against him, just like you wouldn't be allowed to demand his money if you had forgiven a monetary claim. (And, he continues, you can use this trick to stop fighting with a person: you can forgive him, which will legally forbid you to retain any anger against him.)

For more information about the movement, you might want to read Rabbi Dov Katz's five-volume Tenu'at Hamusar (unfortunately not online) and Pulmus Hamusar (about the controversy surrounding the movement). The primary source for Rabbi Yisra'el of Salant are his letters, nearly all published posthumously in Rabbi Yitschak Blazer's Or Yisra'el.