That is indeed a valid argument-form, in basic classical logic. To illustrate this we can just change the labels to ones less likely to cause confusion:
Person X is a Foffler with respect to Y.
Things said about Y by persons who are Fofflers with respect to Y are Snarfly.
Person X said Z about Y.
Z is Snarfly.
The problem arises when instead of sticking a label on the set like “Snarfly” or “bulbous” or whatever you use a label such as “likely to be correct”, and people start trying to pull meaning out of that label and apply it to the argument they’ve just heard. Classical logic, barring specific elaborations, just doesn’t let you do that. Classical logic just wants you to treat each label as a meaningless and potentially interchangeable label.
In classical logic if you make up a set called “statements which are likely to be correct” then a statement is either a member of that set or it isn’t. (Barring paradoxical scenarios). If it’s a member of that set then it is always and forever a member of that set no matter what happens, and if it’s not a member then it is always and forever not a member. This is totally counterintuitive because that label makes you want to think that objects should be able to pop in and out of that set as the evidence changes. This is why you have to be incredibly careful in parsing classical-logic arguments that use such labels because it’s very easy to get confused about what is actually being claimed.
What’s actually being claimed by that argument in classical logical terms is “Z is ‘likely to be correct’, and Z always will be ‘likely to be correct’, and this is an eternal feature of the universe”. The argument for that conclusion is indeed valid, but once the conclusion is properly explicated it immediately becomes patently obvious that the second premise isn’t true and hence the argument is unsound.
Where the parent is simply mistaken in my view is in presenting the above as an instance of the argument from authority. It’s not, simply because the argument from authority as it’s usually construed contains the second premise only in implicit form and reaches a more definite conclusion. The argument from authority in the sense that it’s usually referred to just goes:
Person X has reputation for being an expert on Y.
Person X said Z about Y.
Z is true.
That is indeed an invalid argument.
You can turn it in to a valid argument by adding something like:
2a. Everything Person X says about Y is true.
...but then it wouldn’t be the canonical appeal to authority any more.
That is indeed a valid argument-form, in basic classical logic. To illustrate this we can just change the labels to ones less likely to cause confusion:
Person X is a Foffler with respect to Y.
Things said about Y by persons who are Fofflers with respect to Y are Snarfly.
Person X said Z about Y.
Z is Snarfly.
The problem arises when instead of sticking a label on the set like “Snarfly” or “bulbous” or whatever you use a label such as “likely to be correct”, and people start trying to pull meaning out of that label and apply it to the argument they’ve just heard. Classical logic, barring specific elaborations, just doesn’t let you do that. Classical logic just wants you to treat each label as a meaningless and potentially interchangeable label.
In classical logic if you make up a set called “statements which are likely to be correct” then a statement is either a member of that set or it isn’t. (Barring paradoxical scenarios). If it’s a member of that set then it is always and forever a member of that set no matter what happens, and if it’s not a member then it is always and forever not a member. This is totally counterintuitive because that label makes you want to think that objects should be able to pop in and out of that set as the evidence changes. This is why you have to be incredibly careful in parsing classical-logic arguments that use such labels because it’s very easy to get confused about what is actually being claimed.
What’s actually being claimed by that argument in classical logical terms is “Z is ‘likely to be correct’, and Z always will be ‘likely to be correct’, and this is an eternal feature of the universe”. The argument for that conclusion is indeed valid, but once the conclusion is properly explicated it immediately becomes patently obvious that the second premise isn’t true and hence the argument is unsound.
Where the parent is simply mistaken in my view is in presenting the above as an instance of the argument from authority. It’s not, simply because the argument from authority as it’s usually construed contains the second premise only in implicit form and reaches a more definite conclusion. The argument from authority in the sense that it’s usually referred to just goes:
Person X has reputation for being an expert on Y.
Person X said Z about Y.
Z is true.
That is indeed an invalid argument.
You can turn it in to a valid argument by adding something like:
2a. Everything Person X says about Y is true.
...but then it wouldn’t be the canonical appeal to authority any more.