By reasoning about ideas. Mathematics is a great example—you can gain knowledge with nothing more than first principles and numbers.

Reason is the precondition for knowledge. The real question is, how do you get knowledge without reason? It's not possible.

Yes, reason, which is the precondition for science itself.

Can you prove science is the only way to truth via the scientific method?

Uh oh…

So it's okay if we contradict ourselves? We shouldn't strive to have coherent paradigms?

Thinking rarely involves predictive modeling.

Do you believe in the law of non-contradiction?

Then what does "truth" even refer to in your worldview?

You're arguing that predictive modeling is the best/only real standard for truth. That's a truth claim. So did you come to this belief via predictive modeling? If not, it's an invalid claim on your own grounds.

That’s a truth claim. So what model did you use to construct it?

Determining what is and is not the most accurate picture of the world, along with what philosophy can and can’t justify, both presuppose some standard of truth and epistemic justification which anti-realism makes impossible.

Why should I trust chemical reactions to unfold in a way that “references truth”? Reason requires a rational agent, not a biorobot.

True, no empirical evidence is really possible for free will (as far as I know). However, we can show rationally how determinism leads to absurdity and the impossibility of knowledge. For example: if all of your thoughts are mere byproducts of electro-chemical reactions in the brain (which you yourself don't even understand), then so are the laws of logic that are preconditions for knowledge of any kind. Not only would these laws of logic be reduced to blind chemical reactions with no real reference to "truth" and no way to epistemically justify them, but your brain (and hence mind) could also be determined to believe false things outside of your control. Basically, determinism makes epistemology impossible. It's an absurd and self-contradictory belief.

I've been experimenting with myself since I was 8.

Evidence: the available body of facts or information indicating whether a belief or proposition is true or valid.

Scientific evidence is not the only form of evidence.