Born in Riga, Yeshayahu Leibowitz (brother of Nechama) was educated in Germany and Switzerland and immigrated to Eretz Yisrael in 1935. He joined the faculty of Hebrew University and taught chemistry, physiology, and history and philosophy of science. He authored many books and articles, lectured publicly, and was an editor of several volumes of the Encyclopedia Hebraica. Outspoken in his views on Judaism and Israel, he aroused a great deal of debate and antagonism among religious and non-religious circles. The decision in 1992 to award him the Israel Prize sparked much controversy, and Leibowitz declined to receive it. He died in Jerusalem in 1994.

Leibowitz's notion of Judaism focused entirely on the importance of Halacha. He held that the obligation to observe the commandments was an end in itself, and that religion therefore was not a means to a greater personal or social good. Because of his belief in the overriding value of the Law, Leibowitz advocated fresh Halachic deliberations that deal with situations and challenges of the modern world. He stressed nationalism's religious importance, but following the establishment of the State of Israel and its independence of Halachic norms, Leibowitz argued fiercely for the separation of religion from the state. He insisted that the state was not an ideal with an intrinsic significance, but was there to serve its citizens.

Leibowitz was also uncompromising in his political views. Although he had been active in various political groups, he disapproved of the system of party rule and the numerous political parties, including the religious parties. He labored publicly against government corruption and the proliferation of nuclear weapons. Following the Six Day War, he objected staunchly to retaining any Arab territory, arguing that occupation morally destroys the conqueror. He supported military conscientious objection to serving in the territories and in Lebanon, and it was largely his vociferous left-wing views that made him such a controversial figure.