"_________ is all subjective."

Behold, the magical phrase that settles all arguments and solves all problems!

Or does it?

Two questions:
1. How do you know _______ is subjective? A coherent answer to this question presupposes that some things are not subjective. So,
2. What are those things--the non-subjective ones? (I'll refrain from using the o-word here, since it seems to scare the bejeezus out of most people, especially the ones who tend to throw around that aforementioned special mystical scrap of alchemistic phraseology which turns gold into lead and the need for critical thinking into easy answers.)

I debase myself by not throwing down the gauntlet whenever I hear that sentence. I guess this is something I'll have to work on.