LOGICAL REASONING: The Conclusion Cap
When you’re looking for a necessary assumption, remember that you’re looking for what must be true, and what must be true doesn’t necessarily “fill the
Barry's suit & briefcase disguise: necessary, but not sufficient
whole gap,” as we say. It could be a small piece of the puzzle, but a critical one. Imagine a bridge supported by several buttresses, each of which is necessary (you knock it down and the bridge falls) even though it could never support the bridge on its own. Therefore, the buttresses are necessary, but certainly not sufficient, for supporting the bridge.
For these reasons, you generally want to be wary of answer choices that seem sweeping—words like “always” and “never” are tip-offs. (This isn’t to say those answers are never right, though. If the argument is extreme, there can be a necessary assumption that is, too.)
A good rule of thumb is that the conclusion caps how “strong” the necessary assumption can be. If the conclusion is on the milder side of the spectrum
—“Jenn will probably choose cake over pie” or “Jim is likely to find the suit distasteful”—it wouldn’t make sense to choose answers that read, respectively, “Jenn will always choose cake over pie” and “Jim will definitely find the suit distasteful.” These wrong answers push the arguments beyond their scope, which has already been set.
This isn’t a new concept but simply another way of thinking about the same old idea. The conclusion caps off the argument, and at the cap, it stops assuming.