Rabbi Yaacov Yosef of Polnoye is the primary first-hand source for the Torah of the Ba’al Shem Tov. His main work, Toldos Yaacov Yosef, is replete with direct quotes from the Ba’al Shem. It is the first major work which expounds Chassidism.
Reb Yaacov Yosef was a great talmid chochom and was the rabbi of Shargorod. When he embraced Chassidism, he was expelled from his position on a Friday afternoon. It is said that G-d sends the cure before the sickness. The rationalism of the enlightenment which undermined religion was met through Chassidism by an unprecedented yearning for spirituality.
In the name of the Ba’al Shem, Reb Yaacov Yosef characterized foreign thoughts during prayer as “kelipos” (shells), which contained divine sparks waiting to be redeemed and elevated. By engaging in this process, a Jew participates in the end of the exile of the shechinah. Reb Yaacov Yosef developed the concept of the tzadik as the conduit for spirituality to the masses and the interdependency of the two. Ironically, he did not personally enjoy a great following and Rabbi Dov Baer of Mezhirech, rather than he, became the successor to the Ba’al Shem Tov. However, his written words retain their power and influence.