Sufficient/Necessary

[mcrittell]

So I have the M guide for LR, and although I (re)read the section on these concepts, I feel like I don't have it down enough. Can you explain the main differences and how the LSAT tests them? (Vague, I know. Sorry. If you have a previous thread that explains this, that would be sufficient (lol))

First time poster!

[chike_eze]

Conditionals or Assumption family?

Conditional:

If (sufficient condition) then (necessary claim) i.e. X --> Y

sufficient condition guarantees necessary claim. However, this does not mean that other factors could not also guarantee the necessary claim. All we can infer is that if the sufficient condition is true, then the necessary claim must follow. (and the contrapositive -Y --> -X )

Assumption family:

I still struggle with this -- sufficient vs. necessary assumption. I know that the sufficient assumption guarantees that the argument is valid (complete), and the necessary assumption is required for the argument to be valid, but a necessary assumption does not necessarily guarantee validity. (a mouthful)

I've noticed that more sufficient assumptions (than necessary assumptions) come in the form of conditional statements. I think this is because a sufficient assumption must complete the argument (i.e., guarantee that the argument is valid), therefore sufficient assumptions are usually broader statements or rules.

So, if there is a gap in an argument, a sufficient assumption must entirely fill the gap (sometimes it even over-fills the gap). On the other hand, a necessary assumption does not have to fill the entire gap in logic (it could, but this is not required). Instead, a necessary assumption is such that if you did not assume it, the entire deck of cards, i.e. the argument, would come falling down, would crash and burn-- would not hold!

To reiterate: An argument cannot hold without a necessary assumption, and a sufficient assumption guarantees that the argument is valid.