A noted Jewish ascetic, he took a Nazirite vow at the outbreak of World War I. Cohen was born in Maišiagala, near Vilna (in modern Lithuania), the scion of a distinguished rabbinic family.

Even at that time, his restless and inquiring mind led him to extend his studies beyond the traditional subjects taught in the yeshivot.

His spiritual unrest and the desire to widen his intellectual horizon led him to enroll in the Academy for Jewish Studies established by Baron David Guenzburg, where one of his close fellow students was Zalman Shazar, later president of Israel.

It was then that he took upon himself a lifelong Nazirite vow, which involves complete abstention from cutting one's hair and partaking of any products of the vine.

However, his personal asceticism went further: he became vegetarian, eschewing not only meat but also any garment made of leather, and practised self-imposed silence vows (referred to in Hebrew as "Speech Fasts") on a regular basis.

Rabbi Cohen was fully aware of the day-to-day occurrences in Israel and did not hesitate to publicly express his views on important topical issues.

[citation needed] The turning point in his life came with his meeting, on 29 Av, with Rabbi Kook, who was then in St. Gallen in Switzerland (1915).