When I was an undergraduate, I attended a talk by Peter Lax in Budapest. He had recently been awarded the Abel Prize, but attributed all his success to "integration by parts." It seems he has said this publicly a few timesIt seems he has said this publicly a few times.
Link to actual answer that mentions Lax and integration by parts (not just to the question which has ~70 answers to sift through)
Zach Teitler
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