People often say of an argument that "No one could accept this argument who did not already accept the conclusion". In general the idea, of course, is that the argument is circular, which can be a perfectly legitimate claim to make: no one could accept the premises who did not already accept the conclusion. I'm interested here more in the phrase, though, than the intent, because I think the phrase is potentially misleading, and I'm very sure that in conversation I've come across non-circular arguments treated as if they were circular precisely because the phrase is misleading. The basic point is this:
Accepting an argument always presupposes accepting the conclusion.
This actually just follows directly from the nature of validity. Suppose you have a valid argument with seemingly true premises and a conclusion that is seemingly false. Do you accept the argument? No, you look to see where the premises went wrong. In order to accept the argument you have to have evaluated the whole argument as good, which requires already accepting the conclusion. What is true of valid arguments is true a fortiori of defeasible arguments.
In practice we tend not to distinguish the argument from the premises; and it is true that if you can only accept the premises by presupposing the conclusion, you have a circular argument. (It is often forgotten, of course, that this is something that should be itself provable, and thus can reasonably be expected to be proven rather than simply asserted; but this is a distinct issue.) But accepting the argument requires more than accepting the premises: it requires accepting the whole inference, which includes the conclusion as well as the premises. Accepting the premises may lead us to the conclusion; but accepting the argument requires also accepting tha tthe premises lead us to the conclusion.
A small matter, one that rarely makes a big difference. But as I've said, I've come across arguments in conversation where I'm certain that the problem was that people were confusing the two, and treating arguments as circular explicitly just because someone could reject the conclusion and therefore (in so doing) at least one of the premises, and not because the premises actually required the conclusion. It's an interesting mistake, and I wonder if anyone has come across it before?
