Final answer:
No, it is false to always assume the missing premise that would make the syllogism valid for your opponent, because it might involve a false lemma which can weaken the argument.
Step-by-step explanation:
The question deals with the principles of constructing a valid argument in the context of philosophical reasoning, particularly within the framework of the classic syllogism. When engaging with an argument, it's false to say that one should always assume the missing premise to be the one that would make the syllogism valid for your opponent. This is because doing so might involve introducing a false lemma, which is a false premise that can lead to a true conclusion through invalid reasoning. The introduction of false premises can weaken an argument, as identified by Gilbert Harman's added condition to the Justified True Belief (JTB) account: a belief justified only if not inferred from any falsehoods. Therefore, rather than assuming a premise that makes an argument valid, critical analysis should focus on the truth and relevance of premises to ensure valid and sound reasoning.