Example: “I have a fever, cough and I am feeling weak. Therefore, I most likely have the flu”.
Imagine a world with thousand different diseases, all of them comparably frequent, that all cause fever, cough, and weakness. Flu would be one of them.
In such world, this kind of reasoning would be clearly wrong. You have symptoms that correspond to thousand different diseases; that is no reason to suspect a specific one among them.
This reasoning is okay in our world, because we have the prior that among diseases that have these three symptoms, flu is most frequent. But this is an extra information that is not included in the quote. That means that the form of reasoning in the example will be sometimes correct and sometimes wrong, depending on circumstance, which means that it is unreliable in general.
Imagine a world with thousand different diseases, all of them comparably frequent, that all cause fever, cough, and weakness. Flu would be one of them.
In such world, this kind of reasoning would be clearly wrong. You have symptoms that correspond to thousand different diseases; that is no reason to suspect a specific one among them.
This reasoning is okay in our world, because we have the prior that among diseases that have these three symptoms, flu is most frequent. But this is an extra information that is not included in the quote. That means that the form of reasoning in the example will be sometimes correct and sometimes wrong, depending on circumstance, which means that it is unreliable in general.
...is my guess.