Necessary Assumption questions provide an argument in the stimulus that is invalid in some way, then ask us to identify an unstated premise – i.e. an assumption – that absolutely must be true for the argument to stand a chance. That is, if you take the correct answer and tell the argument “you’re not allowed to have this,” the argument will completely fail.

These questions feature an extraordinary range of argument styles – they can exhibit common flaws, causal reasoning, analogies, cost-benefit analysis, chains of conditional claims, and a thousand things besides. It’s critical, therefore, to approach the stimulus with an open mind. All we know is that the argument somehow falls short of complete validity – the rest we’ll have to discover as we go.

In approaching the answer choices, remember that weak claims are much more likely to be necessary than strong ones. You need to get at least 1 hour of sleep on at least some nights to be well rested. You do not need to get 100 hours of sleep every night.

At its core, this argument’s conclusion (which is sentence 1, as indicated by the word “For” opening sentence 2) is about sufficiency: You can’t say [stuff] is okay based on [this reason] alone. [This reason] alone does not suffice to show [stuff] is okay.

Undermining this conclusion means suggesting [this reason] does suffice to show [stuff] is okay. Put that in your back pocket for now.

Let’s bring in the argument’s other claims, and tone down the abstraction a bit:

Claim 1: When people freely choose to do stuff, it’s okay to pay them to do stuff.
Claim 2: When people don’t freely choose to do stuff, it’s not okay to pay them to do stuff.
Claim 3: The procedures involve test subjects who are volunteers, who know about the risks, and who are paid for the risks.
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Conclusion: Claim 3 doesn’t make the procedures okay.

The support claims establish freedom as the critical difference-maker. Yes freedom? The procedures are okay. No freedom? The procedures are not okay.

The weak point in this argument comes with the question of how the characteristics described in Claim 3 relate to the concept of freedom. Do volunteers who know the risks count as free choosers? If the answer is no, the author is elated: if these test subjects aren’t free, the procedures are not okay.

If the answer is yes, the author is screwed: The procedures use volunteers who know the risks, which makes them free choosers, which suffices to prove the procedures are totally okay.

That’s the necessary assumption here: the author needs volunteering, knowledge, and being paid to NOT count as freedom.