Why can't I wrap my head around this one?

[joshmercer80]

T won’t be selected only if G is selected.

it must be true that...

- T → G

In the strategy guide, an example is Y is not selected if X is selected.

Given: X -> -Y

How are these different?

[ManhattanPrepLSAT1]

Really good question! Once you learn this, you're on your way to understanding the LSAT. This is at the core of conditional logic. And think about... The names of the sections are Logic Games, Logical Reasoning, and Reading Comprehension.

Two of the three sections overtly say that they're testing logic.

The key to the difference in your question is that "if" implies a sufficient condition, whereas "only if" implies a necessary condition.

Here are some words that introduce sufficient conditions (left side of the arrow):
if, if only, the only, all, any each, every, when, whenever, whoever, wherever, and no*

Here are some words that introduce necessary conditions (right side of the arrow):
only, only if, requires, depends on, relies on, needs, must , unless*, until*, cannot ... without*

* means that the other condition is negated

ex: No A's are B's
A ---> ~B

Does that help clear things up?

[nazu.s.shaikh]

Is the necessary condition always on the right side of the arrow? If so
then for this question in the arcade game

Z is in only if Y is not.

The way I diagrammed it was different it from the way it was presented in the answer choice. I understood that after the "only if" would be the necessary and therefore diagrammed it as such.

Z --> -Y

Contrapositive:

- Z --> Y

However the answer choices reflected something different

-Y ---> Z

Y---> -Z

Why is "Y" on the left side of the arrow? Isn't that the necessary component?

[ManhattanPrepLSAT1]

You had the original exactly right.

Z is in only if Y is not.

The way I diagrammed it was different it from the way it was presented in the answer choice. I understood that after the "only if" would be the necessary and therefore diagrammed it as such.

Z --> -Y

However, your contrapositive was actually the negation. It should have been

Y --> ~Z

That would mean that the second answer choice was the correct answer.

Hope this helps!

[sweranoor21]

Thank you for the explanation. But does that mean the answer on the game isn't correct, based on your diagram?

[ohthatpatrick]

I'm not sure how to interpret this question, five years after the last addition this thread.

What is the answer on the game that you're referring to? What did Matt say that contradicted it?

In the original post, the poster wanted to know how to symbolize two ideas:

T won’t be selected only if G is selected.

and

Y is not selected if X is selected.

According to Matt's post / rules / summary...

For the first one,
"T won’t be selected only if G is selected."
we need to see the conditional trigger only if, which we then remember indicates the necessary idea.

So "G is selected" is the necessary idea.

---> G

That means we put "T won't be selected" as the sufficient idea.

~T --> G

For the other one,
"Y is not selected if X is selected."
we need to see the conditional trigger if, which we then remember indicates the sufficient idea.

So "X is selected" is the sufficient idea.

X -->

That means we put "Y is not selected" as the necessary idea.

X --> ~Y

Let me know if you have trouble with either of those translations.