I was speaking of translations in general, which is why I also mentioned religious texts and philosophy. They all have, inherently, the same problems when being translated.
Actually, I do.
First, if a doctor promises something like that, beware. If it's a terminal illness and they say it will be a success, they are probably not wise.
Second, if it is a terminal illness, it can be known that it exists, based on symptoms and lab-results. However, diagnosing is not an exact science. Tons of disorders, diseases, and conditions share many overlapping and similar symptoms. The very act of going to a doctor means you have faith in their confidence to properly diagnose you and choose what will hopefully be an effective treatment option, because many people also put faith in the assumption nothing bad will go wrong when they are put on a medication. But, for a surgeon, we don't have to have as much faith in a surgeon because their work is of a line that can speak for itself. If a surgeon is a patent holder with specialized treatments, is ranked highly by credible sources, is frequently cited, makes frequent speaking appearances, then you know you're dealing with someone who has proven they are capable. But even then there are still mistakes that can happen and, as you have to acknowledge any time you are put under, "unforeseen circumstances" may complicate things. But if the surgeon has few reviews, isn't well known, and seems to be just another surgeon, then you are placing an elevated amount of faith in them.
And there are things that you are more likely to survive, such as being diagnosed with early melanoma, which is generally very survivable, compared to being diagnosed with pancreas cancer, which only has about 50% six-month survival rate, which drops to 20% by one-year. It's not usually an "I don't know if I'll survive," but rather varying degrees of "likely" and "not likely." And doctors don't make promises, but rather "do everything they can" and "do they best they can." (which is how you know it's not looking likely).