Friedrich Nietzsche, who in his younger years was one of Wagner's staunchest allies, wrote that, for him, "Tristan and Isolde is the real opus metaphysicum of all art. . . insatiable and sweet craving for the secrets of night and death. . . it is overpowering in its simple grandeur".
In a letter to his friend Erwin Rohde in October 1868, Nietzsche described his reaction to Tristan's Prelude: "I simply cannot bring myself to remain critically aloof from this music; every nerve in me is atwitch, and it has been a long time since I had such a lasting sense of ecstasy as with this overture".
Even after his break with Wagner, Nietzsche continued to consider Tristan a masterpiece: "Even now I am still in search of a work which exercises such a dangerous fascination, such a spine-tingling and blissful infinity as Tristan I have sought in vain, in every art."