Q5 - Landscape architect: If the screen

[bbirdwell]

Can you help me understand the reason why C is wrong? Thanks!

One great strategy on problems like this is to boil them down to conditional logic and just use x's and y's rather than the real elements of the argument. Then you can just substitute elements from the choices to see if there's a good match. Remember, all the pieces have to fit!

Here we have:
hedge --> hemlock or cypress
NOT cypress
therefore: hedge --> hemlock

A --> B or C
Not B
therefore: A --> C

(C) subsoil --> clay or shale [A --> B or C]
if one shale --> all shale [??]

This is not a match! It's missing the critical "Not B" element! The presence of "C" in one spot does not allow us to infer that "B" is totally ruled out of the entire area!

[timmydoeslsat]

(C) subsoil --> clay or shale [A --> B or C]
if one shale --> all shale [??]

The presence of "C" in one spot does not allow us to infer that "B" is totally ruled out of the entire area!

I am under the impression that this answer choice does allow us to rule out B, if C is present.

No doubt answer choice C is not parallel, however, I believe it operates like this:

Shale is either completely A or completely B in this area.

Revelation of a test hole in this area shows A. Therefore the shale in the entire area is A.

That is valid. However, what makes this incorrect is that is does not conclude "If test hole reveals A ---> No B in the area.

I believe answer choice C is a totally valid argument, simply not parallel to the stimulus.

[jlz1202]

"if ...to be a hedge, that hedge must be of either H or L"

there's no "but not both", therefore, is there in total three options?

hedge--> H / L / H + L

the issue of purely "either...or..." and "either...or...but not both" always bothers me.

Could any one plesae help confirm whether the diagram above correct?

Thanks in advance!

[timmydoeslsat]

"if ...to be a hedge, that hedge must be of either H or L"

there's no "but not both", therefore, is there in total three options?

hedge--> H / L / H + L

the issue of purely "either...or..." and "either...or...but not both" always bothers me.

Could any one plesae help confirm whether the diagram above correct?

Thanks in advance!

Well don't let it bother you! When somebody states, "Either A or B"...That totally gives us the right to have both.

The "or" in LSAT use is not disjunctive, it is always inclusive of both possibilities being listed.

The only time it does not is when you are directly told "not both."

Either A or B
Translation: Has to be at least one of A and B, could be both.

Either A or B but not both
Translation: One must selected, the other must not be selected.

Either A or B and not both
Translation: And/But are logical equivalents on the LSAT, so this is the same statement as above.

In this scenario (#5) we are given

If hedge ---> Hem or Ley

The stimulus gives us information that directly tells us that both types could not be used. So the possibility of both is gone.

We now have: If hedge ---> Hem

This is because we know that Ley is out of contention as we read from the stimulus.

[irenaj]

Really appreciate your comprehensive cover, timmydoeslsat! Thanks a lot!

[JeremyK686]

Breakdown:
Either H or L
not-L
H

Thoughts:
This is a strong disjunctive. The difference between this disjunction and the previous disjunction is that this focuses on a single subject and two disjuncts, whereas the other focuses on two subjects and each their own respective either/or disjuncts.

Components of Disjunctive Syllogism:
All disjunctive syllogisms consist of a major premise presenting the disjunction; a minor premise affirming or denying one of the disjuncts; a conclusion that acts inversely with its minor premise.

Four Moods of Disjunctive Syllogism:
There are four possible moods to disjunctive syllogisms (weak or strong). Two moods consist of a minor premise affirming a disjunct and two moods consist of a minor premise denying a disjunct (conclusion acting inversely).

Either P or Q
P
not-Q

Either P or Q
not-P
Q

Either P or Q
Q
not-P

Either P or Q
not-Q
P

This argument is an example of mood-4. If an argument is a weak disjunction, only moods 2 and 4 (where the minor is a denial) are valid. When an argument is a strong disjunction, all four moods are valid.

For the sake of the ITEM, weak or strong is irrelevant, because mood-4 is applicable to both strong and weak disjunctions.

Well don't let it bother you! When somebody states, "Either A or B"...That totally gives us the right to have both.

The "or" in LSAT use is not disjunctive, it is always inclusive of both possibilities being listed.

The only time it does not is when you are directly told "not both."

To your first and third points here...
Not necessarily. If the relationship between P and Q are exclusive and collectively exhaustive, then there’s no possible way you can ‘get the right to have both (irrespective of whether the statement says ‘not both’). For instance…

Either the window is open or closed.
The window is open (closed).
The window is not-closed (not-open).

The window can’t be both open and closed at the same time, so you can’t have both. This is because the disjuncts are mutually exclusive and collectively exhaustive of all alternative possibilities. Also, ‘not both’ was not used.

To your second point...
Using ‘or’ is the basis of disjunctive syllogism. In traditional logic: a proposition is a disjunction only if the major premise consists of an ‘either/or’ concept. If the major premise doesn’t consist of an ‘either/or’ concept, then the syllogism is something other than disjunctive.