If you post something on the internet and don't sign it, you have no claim to being the originator.
Discussion
Of the post yes, but even then ypu cannot claim the message itself is authentic.
By authentic I think you mean original, yeah, and without timestamp attestation (like opentimestamp) you can't provably claim that you are the first to say it.
No, I mean, the message could always be a copy. Timestamp attestation will allow you to claim an event at a certain moment in time, but you're attesting the message-carrier, not the message. The message could be a duplicate where the original is 100s of years old.
Yes, you can also claim to be the originator and have a form of proof that others won't have. The 100 year old original cannot make a provable claim to being original. It has to be carbon dated or whatever but you have to trust whoever did the carbon dating to not be lying. I think this gets important as the ability to memory hole and change facts to support new narratives Orwell-style becomes easier in the digital age.
Even then, signing does not imply that you are the only possible signer.
I was pointing out the same thing. You can only prove that the content-carrier (event in nostr) originates from the owner of the keypair, and even then we assume it is only owned by one person.