فإن كان بإزاء كل واحد من الأولى واحد من الثانية، كان الناقص كالزائد وهو محال
If every member of the first has a counterpart in the second, the shorter would equal the longer, which the classical proof calls impossible.
The historical inference assumes that a genuinely larger ordered collection cannot be equal in number to the collection formed by removing a member. On that assumption complete matching would erase the difference between the whole and the shortened whole.
Important qualification: Modern set theory allows a bijection between an infinite set and a proper subset. This step is therefore presented as a premise of the classical argument, not as a generally accepted result of modern mathematics.