Can a necessary assumption also be sufficient?

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Can a necessary assumption also be sufficient?

Postby theoretics » Tue May 19, 2015 4:08 pm

I understand that there is a difference between necessary assumptions and sufficient assumptions. There seem to be a great many possible necessary assumptions for an argument. However, in some arguments, is it appropriate to say that a necessary assumption is also a sufficient one? I ask because sometimes I will wrongly eliminate a necessary assumption because it seems too strong, like it would correct the argument or carry it to its conclusion.

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Christine (MLSAT)

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Re: Can a necessary assumption also be sufficient?

Postby Christine (MLSAT) » Tue May 19, 2015 4:12 pm

I don't have the time to be as long-winded on this as I want to be, but YES, assumptions can absolutely be both necessary and sufficient.

In fact, most assumption answers are both necessary and sufficient. I like to call those 'perfect assumptions'. They perfectly fill the gap - fill it completely, and they don't go too far.

You should only eliminate answers on a necessary assumption question because they are so strong that they go OVERBOARD - i.e., the overkill portion is NOT necessary. Never eliminate an answer just because it would perfectly complete the argument.

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Re: Can a necessary assumption also be sufficient?

Postby kray » Tue May 19, 2015 4:33 pm


Necessary means that it HAS TO be true in order for conclusion to follow. Sufficient means that it is ENOUGH for the conclusion to follow (i.e. if it's true, you need no more support). Something could both need to be true for the conclusion to follow, and be enough to confirm the conclusion.

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