...while you're busy making other plans

Was John Lennon quoting Schopenhauer when he said:

Life is what happens to you while you're busy making other plans¹.

According to the character Philip, a "philosophical therapist" in Irvin D. Yalom's novel, The Schopenhauer Cure, quite a few others have "quoted" Schopenhauer:

And not only Thomas Mann but many other great minds acknowledged their debt to Arthur Schopenhauer. Tolstoy called Schopenhauer the "genius par excellence among men." To Richard Wagner he was a "gift from Heaven." Nietzsche said his life was never the same after purchasing a tattered volume of Schopenhauer in a used-book store in Leipzig and, as he put it, "letting the dynamic, dismal genius work on my mind." Schopenhauer forever changed the intellectual map of the Western World, and without him we would have had a very different and weaker Freud, Nietzsche, Hardy, Wittgenstein, Beckett, Ibsen, Conrad. [p. 50]

The Schopenhauer quotation [on p. 91 of The Schopenhauer Cure] that reminded me of John Lennon's famous statement was:

When, at the end of their lives, most men look back they will find that they have lived throughout ad interim [for the intervening time, temporarily]. They will be surprised to see that the very thing they allowed to slip by unappreciated and unenjoyed was just their life. And so a man, having been duped by hope, dances into the arms of death.

    Lennon's is easier to take and more memorable, isn't it? Without that final sentence.

But we need that sentence, to be reminded not to let our lives just slip by unappreciated while we're busy making other plans.
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1. He might have said this in an interview with Rolling Stone Magazine about 1970. At any rate, a citation in a Google hit list included both the part of the quotation I searched on and a reference to Rolling Stone.