who decides what is "true"? What if I believe that it's TRUE that Santa Claus exists? Wouldn't it be "true for me"?

I'm not sure how to interpret the quotation marks in your first question; I'll assume they're inessential. Who decides what's true? No one, as far as I can see. One can recognize what's true, discover what's true, conclude that such-and-such is true, etc. But I don't think any of that amounts to deciding what's true. I'm not sure that even the president genuinely decides that it's true that so-and-so is pardoned; I think he decides to declare that so-and-so is pardoned, and his declaration then makes it true that so-and-so is pardoned. But he doesn't decide that his declaration makes it true.

I'm not sure how to interpret the capitalization in your second question; I'll assume it's inessential. If you believe that Santa Claus exists, then as far as you're concerned Santa Claus exists. If that's what you mean by "true for me," then it's just another way of saying that you believe that Santa Claus exists, which of course doesn't make your belief true. If it did, then the concept of a false belief would be incoherent and wouldn't play the important role in our conceptual scheme that it does play.

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