In our experience we are, perhaps, directly acquainted with the facts concerning our mental states, but the possibility that experiences are hallucinations proves that we cannot be directly acquainted with the facts concerning physical objects that, beyond our reckoning, may or may not be causes of our experiences. — javi2541997
I have two things to say: one about acquaintance and one about certainty.
I don't think 'impossibility of direct acquaintance' follows from 'possibility of hallucination'. It's possible that I'm hallucinating my cup of coffee. So I cannot be looking at a cup of coffee, as it doesn't exist. It's also possible that I'm not hallucinating. In that case, I'm as directly acquainted with my cup of coffee as I am with anything else I could be said to be directly acquainted with. Or, if I'm not, I need to see additional argument why not. Here is the objection in summary:
It's possible I'm hallucinating my cup.
If hallucinating, no cup.
If no cup, no acquaintance with cup. But also:
It's possible I'm not hallucinating my cup.
If not hallucinating, then cup.
If cup, then acquaintance with cup. Hello, cup.
There's a separate argument about certainty. If it's always possible that I'm hallucinating and it's never possible for me to be certain that I'm not hallucinating then (it's claimed) I can never be certain that I'm not hallucinating my cup. From which it follows (it's claimed) that I can never be certain that I'm looking at the cup. This is a trickier proposition than the first. One approach is to claim that we can no more be 'certain' about some things than we can reasonably doubt them. If I try to doubt, for example, whether I have a body, then it is hard to give sense to that doubt or say what it consists in or what would follow from it. Both doubt and certainty are off limits in such cases and in many others in everyday life.
Another approach -
@unenlightened 's above - is to claim that the concept 'hallucination' itself depends upon the possibility of distinguishing hallucination from reality, if not always reliably in our own case then at least in the case of the patients, students, tutors or other victims under our surveillance and care. We may only suppose ourselves to be hallucinating because we know what it would mean to
not be hallucinating. And if we know
that then the argument undermines itself by invoking a concept to prove that the concept may not be reliably invoked.
No approach will be persuasive to someone who is convinced that everything might be a hallucination. There is no knock-down argument.