Two towering figures of twentieth-century Jewish thought—Mordecai Kaplan and Abraham Joshua Heschel—maintained a complex relationship that embodied the fundamental tensions within American Judaism between rationalist and mystical approaches to religious life. Drawing from Kaplan's extensive personal journals spanning 1942-1956, along with correspondence and institutional records, this analysis reveals how their intellectual partnership at the Jewish Theological Seminary evolved from initial mutual respect to measured discord. Kaplan's early admiration for Heschel's scholarship, particularly his "Analysis of Piety," inspired him to transform Heschel's ideas into liturgical poetry for his controversial 1945 prayer book. However, as Heschel's mystical traditionalism gained increasing influence among students, challenging Kaplan's rational reconstructionist approach, their philosophical differences became more pronounced. Through close textual analysis of Kaplan's diary entries, the research documents his growing frustration as students gravitated toward Heschel's inspirational teaching style over his own analytical methodology. Yet despite their divergent theological positions—Kaplan advocating for rational reconstruction of Jewish practice while Heschel emphasized traditional piety and ineffable spiritual experience—both scholars maintained professional respect throughout their careers, exemplifying the complexity of theological discourse in mid-twentieth-century American Jewish intellectual life.