While there are many reasons one could use, I simply use the existence of two universal, invariant, abstract entities know as the laws of morality and the laws of logic.
The laws of morality are universal and invariant? Now
that's news! Why don't you tell my great-great-grandfather Ebenezer Dickey Junkin, D.D. - the minister. The one that wrote the catechism "for the instruction of coloured persons." Last time I looked around Texas, at least, chattel slavery wasn't universally, invariantly moral - but it was fine in 1850!
™ Version 4.2