In the meantime, I produce a sketch of my own presuppositionalism I have been developing. It goes like this.
My claim: Sye's mind is addled and his thinking unreliable because he was hit on the head by a rock.
Prove this is false Sye.
Try to, and I will say - "But your "proof" presupposes your mind is not addled and you can recognise a proof when you see it. So it fails."
Ask me to prove my claim and I will say: "But prove to me your mind is not addled, then, Sye". Which you won't be able to, for the above reason. I might then add, with a flourish - "So you see, it's proved by the impossibility of the contrary".
And of course I have a good explanation for why your brain is addled - you were hit by a rock.
Is my claim reasonable, then? Of course not. It's bullshit. I really can't see how
your position is any less of a bullshit position. Can you?