Anyone who believes that he has the truth, does not possess it
Actually, you can have truth on something. Just because you can't have absolute truth about the totality of all data on something, doesn't mean you can't have truth about some aspect or particular. Believing you have truth when you don't is the issue for wars and conflicts, not that no one can actually have truth. Beliefs are not truths. A truth about something isn't believed, it's recognized as truth because it;s shown to be such in reality, demonstrably. That's why they are two different words that mean different things.
I recognize that. What I am saying is that, if you don't have all the information in the world, then you can not be certain that what you believe is "proven" is really true. You need to have access to the truth, as a whole, to be able to know with certainty if what you think you have demonstrated is true.
Indeed, I adhere to the fact that you can have the truth in something in particular. And in fact, I think that the human knows several of those particular truths.
However, despite the man knowing that truth, he can not be sure that he has it, because, as I said, he does not have access to all the information and it is impossible for him to know if something escapes him. From there to say, paraphrasing Socrates, the only thing we know with certainty is that we ignore.