conditional Logic statements

[Yit HanS103]

Hi, I’m currently studying with the 3 prep materials from manhattan.
Conditional Logic seems to be my weakness. I’m a bit confused with the way MH explains it. For example,
I have learned from pw. the "unless equation", which is everything that comes AFTER the “unless” is the necessary, and everything that comes BEFORE the “unless” is the sufficient and MUST be negated (positive or negative). Thats how I learned it, and I feel comfortable with it. However, in MH does not how they have it, most of the conditionals, whatever comes after the unless/except is the sufficient (the negation part of the sufficient remains the same as the one in pw) and whatever comes before the unless/except is the necessary, in other words, they have the reverse of what I have learned.
I wouldn’t have a problem if the outcome was the same, however it seems different too. For example in MH LR 5th edition pg 348 (conditional logic)-

“javier cannot be chosen for the position unless he prepares for the interview”

to me is : chosen—>he prepares interview
MH: — prepares interview—> — chosen (to me this will be the contrapositive of my conditional)

Another problem I have is "compound conditional statements”

to MH:
AND as an outcome : diagram split up
OR as a trigger: diagram split up
AND as a trigger: diagram together
OR as an outcome: diagram together

this confuses me, I’m used to diagram it all like a tree, and all together. Though, the outcome is still the same, the “and” and “or” changes.
for example: “ K is selected, only if neither M nor N is selected”
to me this means: k—> -M or -N
contrapostive: M and N —> -k
MH says the "neither/nor" is NOT the same as “or”. neither/ nor is translated to "not M AND not N” So this is where I get confused, and although I’m comfortable with my method, seeing this causes confusion on wether I’m doing it okay or not.
Also, the way I diagram all compound statements, (it doesn’t matter wether “and” on the outcome or trigger, as well for “or” on outcome or trigger) all together and in a form of a tree, is that okay?

Thank you very much!

[ohthatpatrick]

Good questions!

My biggest advice is just to use whatever seems to work best for you. There are lots of tricks / terms / moves / shortcuts, etc. in the LSAT universe. As you read different materials, you’ll be exposed to different ways of dealing with LSAT material.

It’s all just a means to an end.

Whichever method of thinking about conditional logic seems to take you the least time, use the least brainpower, and result in the fewest errors is your best bet.

I’ve taught “Unless” at least three different ways. What I use personally is none of those.

1. Unless = “if not” (this is the MPrep way you’re describing)
2. Unless = “un-left” (this is the Powerscore way you’re describing)
3. The two ideas in the “unless” statement are both right sides (this was another company’s method)

What I personally use is, “I’ll take whichever idea is easier to negate and put that idea, negated, on the left side”.

If I had a statement like,
“Paul is 5th and Sue is 3rd unless Dave is 1st”
I would negate the 2nd idea.

If it said
“Dave is 1st unless Paul is 5th and Sue is 3rd”
I would negate the 1st idea.

The “Unless = if not” is also a handy thought when you come across the rare nested conditional:
If A, then B, unless C.

I deal with that like this:
“If A and ~C, then B”

The part where I got confused/concerned was when you said this:

I wouldn’t have a problem if the outcome was the same, however it seems different too. For example in MH LR 5th edition pg 348 (conditional logic)- 

“javier cannot be chosen for the position unless he prepares for the interview”

to me is : chosen—>he prepares interview
MH: — prepares interview—> — chosen (to me this will be the contrapositive of my conditional)

Those ARE the same outcome. I’m not sure what you think a contrapositive is, but it’s the same thing as the original statement. There’s no such thing, in fact, as the “original” vs. “the contrapositive”. Whichever one you happen to write first is “the original” and then if you contrapose that you get “the contrapositive”.

That’s another reason why my personal method with “unless” is flexible. If I’m only planning to write the conditional one way (rather than write it AND its contrapositive) then I’ll think through which version of the conditional I want.

This would be motivated either by wanting to write the conditional in a way that lent itself to chaining onto another conditional, or it would be motivated by trying to match the Premise -> Conclusion move, or it would be motivated by setting up a provided principle in a form that felt most like “if you satisfy this criteria, you deserve this label”.

In terms of “and” / “or”, I’m with you.

I don’t split up any conditionals. I just write them as they’re written and switch and/or when I’m contraposing (if I’m bothering to even write the contrapositive).

Are you saying that if you have
“if A, then B and C”
that you would have A, with two arrows coming out diagonally? (kinda like an ordering rule that says ‘A is before B and C’)

Cool! I’ve seen that. I think it’s great for a lot of uses.

I just write them as is,
A --> B and C
~B or ~C --> ~A
but that’s mainly because I don’t get enough out of the tree habit to break my much-ingrained habits.

The one thing you seemed to be confused about is “neither nor”.

Everybody thinks that “neither/nor” means “not this AND not that”. That’s not an MPrep thing. That’s just the meaning of those words.

If I say, “your vegan friend can eat this … it has neither meat nor dairy in it”, I’m promising you that the meal has no meat AND no dairy.

Hopes this helps.

[Yit HanS103]

Thank you very much, this was very helpful.

[SJK493]

The “Unless = if not” is also a handy thought when you come across the rare nested conditional:
If A, then B, unless C.

I deal with that like this:
“If A and ~C, then B”

Patrick, can you clarify how you got from the first statement to the second statement? If it doesn't bother you, can I ask for further explanation on nested conditionals as well?

Ps. are there any more types of these nested conditional statements? What if it is 'If A, then B, if C' or something of the sort?
I think the practice tests sometimes feature conditional statements that are getting more complex, as with the case above, and would love to get more practice on this!

Thank you