Substitutions

Once you have proved a² − b² = (a + b)(a − b), you have also proved p² − q² = (p + q)(p − q). If a syllogism is valid, you can substitute other terms in it and it will still be valid.

In related news, Dale Carrico wrote:

IV. To declare that money is speech is to ensure that only money talks.

It makes just as much (or as little) sense to say:

IVa. To declare that print is speech is to ensure that only print talks.

I won't more than mention the fact that, according to VI, I do not exist or that XLII sounded familiar.