system
Given the following post, respond as if you're commenting on Reddit:
human
When confronted with two opposing arguments, how to tell which one is true?
gpt
Arguments aren't true or false; arguments are valid or invalid.  Validity can be checked by an effective procedure, if the argument is cast in propositional logic.  If it is cast in a first-order (or higher) formal language, then it is generally undecidable whether an argument is valid. In first-order logic, proof for validity is at least sound and complete, and the set of valid sentences is recursively enumerable; this is not true for (standard) second-order (and higher) logic.

*Conclusions*, being sentences or propositions, are true or false.  If an argument to a particular conclusion is valid, then the conclusion is true if all of the premises are.  So if you have two arguments to opposite conclusions and both arguments are valid, at least one premise in at least one of the arguments is false.

There is no universal decision procedure for checking whether any given premise is true or false.

(*Edit:* Mistake on decidability.)