Sufficient and Necessary Assumptions

Sufficient
In answering sufficient assumption questions, you need to find a link between the stated premises and the conclusion. Try to determine from the explicit parts of the argument what logical work that link needs to do. Finally, look among the answer choices for one that can do that logical work and that, taken along with the explicit premises, allows the conclusion to be properly inferred.

In trying to figure out what logical work the link needs to do, don't get too specific. Answer choice (A) was not the only possible sufficient assumption. An equally acceptable sufficient assumption would have been "People cannot feel secure if they have vague limits on their freedom." So don't approach the answer choices with too specific a view of what you're looking for.

When trying to identify a sufficient assumption, keep in mind that the correct answer must, when added to the argument's explicit premises, result in a conclusive argument; that is, in an argument that fully establishes its conclusion (provided that the explicit premises and the added assumption are all true).

Sample question prompts
Which one of the following, if assumed, enables the conclusion of the argument to be properly drawn?

The conclusion follows logically from the premises if which one of the following is assumed?

Necessary
A necessary assumption is an indispensable link in the support for the conclusion of an argument. Therefore, an argument will be ineffective if a necessary assumption is deemed to be false. This points to a useful test: to see whether an answer choice is a necessary assumption, suppose that what is stated in that answer choice is false. If under those circumstances the premises of the argument fail to support the conclusion, the answer choice being evaluated is a necessary assumption.

Test for necessary assumptions by asking whether the argument would fail if your answer choice is false. 

Denying the answer choice (by negating it) - if it causes the argument to fail means that you have found a necessary assumption.

Some points to consider
1. Identifying necessary assumptions is a matter of logically analyzing the structure of an argument
2. Identifying an assumption is a matter of probing the structure of an argument and recognizing hidden parts of that structure
3. An argument may have more than one necessary assumption but only one of them will appear in the answer choices. But the one that does appear may not be one that occurred to you when you analyzed the argument. So it is a good idea not to prejudge what the correct answer will be. Instead, keep an open mind and examine each of the answer choices in turn.


Sample question prompts
The argument relies on assuming which one of the following?

The argument depends on the assumption that

Which one of the following is an assumption required by the argument?