What would be the best response to a person who rejects whatever you say, with no reasoning and by just citing the fact that, nothing is absolute in this world?

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To demonstrate something absolute. This is difficult, but not impossible.

My go-to solution for this is mathematics, in particular whole number arithmetic. 1 + 1 = 2 in every single context you can think of it in. Changing the words used to denote the concepts does not change the actual equation.

You can make this statement about most maths, but whole number arithmetic is the easiest go-to.