If you observe X then the thing you update on is “I observed X” and not just “X”. Just because you observed something doesn’t mean it was necessarily the case (you could be hallucinating etc.). So while you don’t assign probability 1 to “X” you do assign probability 1 to “I observed X”, which is fine.
If you observe X then the thing you update on is “I observed X” and not just “X”. Just because you observed something doesn’t mean it was necessarily the case (you could be hallucinating etc.). So while you don’t assign probability 1 to “X” you do assign probability 1 to “I observed X”, which is fine.