Only. Only if

[ca_teran1]

Hello,
Whenever a statement has the word only or only if, I find myself placing the whole clause after only as the sufficient.
I was taught to look at only as sufficient and only if as necessary.
For example:
That angle can be kept fairly stable only by the gravitational influence of Earth's large moon.

I put moon----> angle stable but the correct way is
angle--->moon.

I did it like that because it says "only by."


Another example,
Journalists are the only ones who will be permitted to ask questions at the press conference.

I wrote J--->ask questions correct answer is Ask questions---->journalists.

I think I have trained my brain to see these absolute words and work mechanically instead of letting the logic processing flow.
How do I cure this?

Thanks a lot

[demetri.blaisdell]

Thanks for posting, ca_teran1. Conditional logic is one of the hardest things on the LSAT and I get lots of questions about it in class and on the forums. My first piece of advice is to watch the "Advanced Conditionals Lab" which you can find in the student center. It does a great job of explaining conditional logic and how to decode statements. That should help with the conceptual understanding of conditional logic.

Once you have a handle on the reasoning, you can also take a look at the LSAT Arcade "If/Then" for more practice on the mechanical application of what you've learned. Hopefully, by test day you will be able to reason through the logic if you have to but will have internalized the standard forms of conditional logic so you will be able to do most of it without thinking.

Let me turn to your examples, which are good ones to help explain the concept.

Example 1:

"That angle can be kept fairly stable only by the gravitational influence of Earth's large moon." To simplify the language: we will have the stable angle only if we have the moon. In other words, the moon is essential/necessary/required to keep the angle. Another way to think of it is: what happens if we take the moon away?

~moon --> ~angle

Take the contrapositive and you have: angle --> moon. The guarantee is if we have the angle, we know we must have the moon because it is the only way (only if) we can have the stable angle.

Example 2:

Journalists are the only ones who will be permitted to ask questions at the press conference.

Here, consider this question: will every journalist be allowed to ask a question? Of course not! That would take all day. The point of the announcement is to restrict the people who can ask questions to only journalists, not to guarantee that the person speaking will answer every single question. In other words, being a journalist is necessary in order to ask a question. So, if you're not a journalist, you can't ask a question: ~journalist --> ~question

Contrapositive: ask question --> journalist. (if you asked a question, you must have been a journalist).

I hope this helps. Please use the resources above. Feel free to post if you have any more questions. This stuff is very tricky.

Demetri

[wj097]

Can anyone explain logical meaning of "if only"?? = only if??

Ex) One substance can be more natural than another if only one is wholly derived from natural substances


Thx

[patrice.antoine]

I was under the impression that "IF" before the "only" introduces the sufficient and "IF" after the only, the necessary.

IF only = sufficient

only IF = necessary

They are not to be treated as one in the same.

P.S. "THE only" also introduces the sufficient.

[pjkoroma]

I am having an issue with this as well. How do I relate "Only if" and "if only" to the concept of Trigger --> Outcome for conditional logic. So is "only if" a trigger or outcome indicator? And similarly, is "if only" a trigger or outcome indicator? Thanks!

[JadeB304]

Can anyone explain logical meaning of "if only"?? = only if??

Ex) One substance can be more natural than another if only one is wholly derived from natural substances


Thx

I think it's about if time can happen and things can go differently

[Laura Damone]

Lucky for everyone on this thread, "If only" isn't a concept that the LSAT tests. It tests "the only", which an earlier poster correctly noted introduces a Sufficient condition. But "if only" is better suited to romantic yearning than it is to conditional logic: If only he had liked my picture!

[KendallC395]

When going through conditional logic drills that are concerned with finding answers that contain statements that are equivalent to the original given statement, I am having difficulty with understanding the different ways that are acceptable to switch between "If" and "Only if" statements.

I understand that "Only if" establishes the inverse relationship of "if" thus:
Max wears a hat if its sunny outside (S-->H)
Max wears a hat only if its sunny outside (H-->S)

But later when things start to get a bit murky is when the question introduces an "if, and only if, ___"

An example question that I struggle with follows as such:

Choose all of the answers that are equivalent to: "Only if the car is new is it in good shape" (N-->G)
A) The car is in good shape if, and only if, it is new
B) The car is new if, and only if, it is in good shape
C) If the car is in good shape, then it is new (G-->N)
D) If the car is new, then it is in good shape (N-->G)

I understand how C is one of the correct answers as it is the reverse relationship since it is simply an "if" statement, and D is incorrect because it is an illegal reversal in the absence of an "only if", but Im having a hard time diagramming/understanding why A and B are wrong.

Thanks so much for any help anyone might be able to provide!!

[Laura Damone]

Hi there!

"If and only if" is actually what's called a biconditional: a conditional relationship that moves in both directions. So, if I said "Max wears a hat if and only if it is sunny outside", that implies "if sunny, then hat" and also "if hat, then sunny." A biconditional can only be equivalent to another biconditional. So, in the example you posed, A and B are not equivalent to the original statement. They are, however, equivalent to one another. Since biconditional relationships move in both directions, "X, if and only if Y" is the same as "Y, if and only if X."

One more thing: Your initial diagram of "Only if the car is new is it in good shape" is reversed. You said N-->G, but it should be G --> N. The phrase "only if" introduces the necessary condition. You can think about drawing the arrow through the phrase, if that's a helpful visual. C is correct because that's another phrase that would diagram out to "G --> N". It's a simpler "if/then" statement, but it expresses the same relationship. D is incorrect because it is the illegal reversal. Illegal reversals are not equivalent.

Hope this helps!