“But how can you say what’s really true? Your truth is no better than my truth!”
I’m nearly positive that you can never be certain of anything. That does not mean that there’s no such thing as “truth”.
Truth exists. We can argue over the word itself all you want, but the symbol is not the symbolized. And at the end of the day, no matter what you call it or how much you insist otherwise, there is something that acts like truth.
No matter how hard you believe you will fly if you jump naked off of a cliff, you won’t. It doesn’t matter who you convince. It doesn’t matter if absolutely everyone agrees you’ll fly. Your ability to fly is not relative. It is not dependant upon the human cultural norm. Humans are not the arbiters of truth.
When you jump off that cliff, you’ll fall. There is something out there that distinguishes between what you believe is true and what actually happens.
You may label this arbitrating thingy with whatever word you like. I, for one, will be calling it truth.