Whenever we argue about whether a thing can be proved, we should distinguish five different questions about that thing:

1. Does it really exist or not? "To be or not to be, that is the question."
2. If it does exist, do we know that it exists? A thing can obviously exist without our knowing it.
3. If we know that it exists, can we be certain of this knowledge? Our knowledge might be true but uncertain; it might be "right opinion."
4. If it is certain, is there a logical proof, a demonstration of why we have a right to be certain? There may be some certainties that are not logically demonstrable (e.g. my own existence, or the law of non-contradiction).
5. If there is a proof, is it a scientific one in the modern sense of 'scientific'? Is it publicly verifiable by formal logic and/or empirical observation? There may be other valid kinds of proof besides proofs by the scientific method.