Having trouble mapping out an "unless" rule. Forum

[SingleLadies]

I am trying to set up In-Out LG from the LSAT Blog and am having lots of difficulty with the following rule:

"Unless she votes against J, she will vote for I."

O.k. words like "then", "only", "only if", and "unless" introduce the necessary condition. Thus using the Sufficient --> Necessary mapping method this would become;

I --> not J.

However apparently this is not the correct way to map the rule:

The rule provided by Steve for dealing with "unless" states that you are to negate the sufficient condition, so in this case a "I" would become a "not I"? Then you have to work out the counter-positive which makes the accurate rule

"J --> I"

Is this correct? If so, could someone please flush out the method and reason of forgoing the standard Sufficient --> Necessary mapping method by adding on the steps of negating the sufficient condition and then using the counter-positive as the representation of the "unless" rule?

Any help here will prevent the abuse of several inanimate objects.

Thanks

[Elston Gunn]

Yes, TCR is J-->I.

I don't understand why you think I-->J is the standard way to do it. perhaps you could explain? The unless makes the rule mean, "If she doesn't not vote for J, she will vote for I." See the double negative? Not-not-J = J.

ETA: In this context, think of "unless" as meaning "if she doesn't."

[kaiser]

You can flip this sentence around to make it more clear:

"She will vote for I unless she votes against J"

Thus, she is going to vote for I, and the only thing that would stop her from voting for I is if she votes against J. Nothing aside from the phenomenon of "voting against J" will stop her from voting for I. So lets say she does NOT vote for I. What did we just say is the only thing that could have made that happen? She must have voted against J (i.e. it is NECESSARY that this have occurred, and is therefore the necessary condition):

Did NOT vote for I --> Voted AGAINST J

We can make the contrapositive of this as well:

Voted FOR J --> Voted FOR I

The above is true because the only thing that could stop her from voting for I would be a vote against J. And since that one possibility is negated in the sufficient condition above, we know that the path is clear to conclude that she necessarily voted for I.

Hope this demonstrates why the explanation provided by Steve is entirely accurate. And note that the condition which followed "unless" (voted against J) became the necessary condition of my initial conditional relationship. So the bottom line is that "unless" is a term introducing a necessary condition. Just remember to then flip around the language which will become the sufficient clause. Here is a simple example to illustrate what to do with unless:

I will go to the beach unless it rains

So if I do NOT go to the beach --> It must have rained

So what follows unless is the necessary condition, and the language before "unless" must be flipped a bit (i.e. how I changed going to the beach into NOT going to the beach). An unless phrase says that something WILL happen UNLESS something else happens. It tells you the one situation in which the first thing will NOT occur. So make that your starting point. What happens if that sufficient condition did not occur? Well, that means that the one obstacle (noted by "unless") got in its way.

[TrojanHopeful]

^TL:DR

Here's a simple way of figuring out the whole "unless" thing...

Whatever follows "unless" is the necessary condition; negate the other part of the statement and you have the sufficient.

Example:

Mike will not wipe his ass unless he takes a shit.

Sufficient: Mike wipes his ass.

Necessary: Mike took a shit.

Contrapositive:

Sufficient: Mike did not take a shit.

Necessary: Mike did not wipe his ass.

There you have it.

[kaiser]

^TL:DR

Here's a simple way of figuring out the whole "unless" thing...

Whatever follows "unless" is the necessary condition; negate the other part of the statement and you have the sufficient.

Example:

Mike will not wipe his ass unless he takes a shit.

Sufficient: Mike wipes his ass.

Necessary: Mike took a shit.

Contrapositive:

Sufficient: Mike did not take a shit.

Necessary: Mike did not wipe his ass.

There you have it.

Yup, this summarizes pretty much everything I said in one delightfully crude example

[Helicio]

^TL:DR

Here's a simple way of figuring out the whole "unless" thing...

Whatever follows "unless" is the necessary condition; negate the other part of the statement and you have the sufficient.

Example:

Mike will not wipe his ass unless he takes a shit.

Sufficient: Mike wipes his ass.

Necessary: Mike took a shit.

Contrapositive:

Sufficient: Mike did not take a shit.

Necessary: Mike did not wipe his ass.

There you have it.

That made me Lol .

[SingleLadies]

Thank you all for the answers they helped a lot!

[SanDiegoJake]

Here's an alternative way of thinking about symbolizing "only if" and "unless". These are the only common "weird" words that you have to pay attention to when diagramming these conditionals.

Step 1: Notice the weird word ("only" or "unless")
Step 2: Look at the action that comes directly after the weird word
Step 3: Negate that action.
Step 4: Brain it out. The goal of this method is that it makes sense to you. I detest the mechancial memorization of rules.

Example, "I'm only happy if it rains".
Step 0: rewrite so "only" and "if" are together. I'm happy only if it rains.
Step 1: Yep, there's the "only"
Step 2: It rains
Step 3: OK, what if it doesn't rain?
Step 4: Then I won't be happy. Diagram: ~r --> ~h; h --> r

Example 2 (yours): Unless she votes for J, she will vote for I.
Step 1: Yep, there's the "unless".
Step 2: She votes for J
Step 3: OK. What if she doesn't vote for J?
Step 4: Clearly, she will vote for I. Diagram: ~J --> I; ~I --> J

Good luck! This is important to understand.

[yipeng024]

.

[bp shinners]

Whatever follows "unless" is the necessary condition; negate the other part of the statement and you have the sufficient.

Another way is to cross out the "Unless" and replace it with "If not". That means "Unless" introduces the negation of the sufficient condition. That will give you the contrapositive of the above method, but I find it to be a little more straightforward.

Not everyone does, though, so I'm throwing this up there so people can choose between the two.

Note - this method also works with "Without", "Except", and "Until"

[kulshan]

Even easier might be this: unless = or.

So, 'unless she votes against J, she will vote for I' would be 'not J or I', which is equivalent to 'if J then I'.

Whatever makes most sense to you!