If one means one then it just ain't two
And it can't be love cause it's only one
1 = 1. 1 does not = 2. Even though 1 + 1 = 2, when Frank and ‘Girl’ are together or added don’t equal two. They equal one because they are both physically one person each when apart, but when together their love makes them both into one (symbolic) item. This is weird because it proves the 1 + 1 = 2 equation wrong, but their love together equaling one item again, is symbolic and not mathematically correct which is why he says “and it can’t be”.
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