Rabbi Zadok ha-Kohen Rabinowitz of Lublin (in Hebrew: צדוק הכהן מלובלין) (Kreisburg, 1823 - Lublin, Poland, 1900), (or Tzadok Hakohen or Tzadok of Lublin), was a significant Jewish thinker and Hasidic leader.
He was born into a Lithuanian Rabbinic family and then became a follower of the Hasidic Rebbe, Rabbi Mordechai Yosef Leiner of Izbica, and of Yehudah Leib Eiger (grandson of the famed Rabbi Akiva Eiger and another student of Mordechai Leiner), whom he succeeded in 1888. He is a classic example of a Litvish Jew turned Chasidic.
As a young man he gained widespread acclaim as an illuy (a brilliant talmudist). Rabbi Zadok refused to accept any rabbinic post for most of his life. He eked out a living by his wife running a small used clothing store. Upon the death of Eiger in 1888, Zadok Hakohen agreed to take over the leadership of the Hasidim. It was then that he began to give his public classes that would take place on Shabbat, Holidays, Rosh Chodesh and special occasions. The transcriptions of those classes were compiled into his work known as Pri Tzadik.
Rabbi Zadok was a prolific writer in all areas of Judaism, halakhah, Hasidut, Kabbalah, angelology, ethics; he also wrote scholarly essays on astronomy, geometry, and algebra.
One of his lone surviving students was Rabbi Michael Mokotovsky, whose son was Rabbi Avraham Eliyahu Mokotovsky, better known by his penname Eliyahu Kitov.
Zadok HaKohen's radical philosophy of Judaism very much continues the thinking of his teacher Rabbi Mordechai Yosef Leiner. Zadok HaKohen was much more of a prolific writer than Leiner ever was. It is therefore difficult to determine where Rabbi Zadok's radicalism is a mere articulation of ideas left somewhat veiled (albeit possibly purposely) in the writings of Leiner and where Rabbi Zadok is actually breaking new ground.
You can learn a lot about a person from his dreams. What we dream is a reflection of who we are. It is the measure of our aspirations and goals, and of those values we hold dear and place above all else.