Contrapositives are killing me

Prepare for the LSAT or discuss it with others in this forum.

Posts: 71
Joined: Sun Dec 02, 2012 7:12 pm

Contrapositives are killing me

I just don't understand them.

I'm working through prep test 19, game 3 and I have the rule:

If F is assigned to boat 1, then G is assigned to boat 1 (only two boats and all variables get used)

So I diagram it to:

F1--->G1

And contrapositive of

G2--->F2

How the fuck is that different than

F<--->G ???

dingbat

Posts: 4975
Joined: Wed Jan 11, 2012 9:12 pm

Re: Contrapositives are killing me

^I don't know what this means

However, your possibilities for boat 1 include:

F+G
G, but not F
Neither F nor G

crazyrobin

Posts: 442
Joined: Thu Dec 13, 2012 2:52 am

Re: Contrapositives are killing me

In order to have your conclusion of F<---->G you have to have G1---->F1 or F2---->G2 (contrapositive of previous one)

But clearly you don't have this extrapolation.

Just as dingbat said, F1---->G1 leaves the possibilities for boat 1 are
F+G
G
Or neither

Note, F and G doesn't nessisarily have to be together.

dingbat

Posts: 4975
Joined: Wed Jan 11, 2012 9:12 pm

Re: Contrapositives are killing me

crazyrobin wrote:In order to have your conclusion of F<---->G you have to have G1---->F1 orand F2---->G2 (contrapositive of previous one)

But clearly you don't have this extrapolation.

Just as dingbat said, F1---->G1 leaves the possibilities for boat 1 are
F+G
G
Or neither

Note, F and G doesn't nessisarily have to be together.

I might not understand the terminology, but I know enough to fix this

crazyrobin

Posts: 442
Joined: Thu Dec 13, 2012 2:52 am

Re: Contrapositives are killing me

dingbat wrote:
crazyrobin wrote:In order to have your conclusion of F<---->G you have to have G1---->F1 orand F2---->G2 (contrapositive of previous one)

But clearly you don't have this extrapolation.

Just as dingbat said, F1---->G1 leaves the possibilities for boat 1 are
F+G
G
Or neither

Note, F and G doesn't nessisarily have to be together.

I might not understand the terminology, but I know enough to fix this

Haha right, when I type "or" what I actually mean is that one of these rules will lead to conclusion F and G has to be together. But logically is "and"

Posts: 394
Joined: Tue Mar 08, 2011 12:19 am

Re: Contrapositives are killing me

rvadog wrote:I just don't understand them.

I'm working through prep test 19, game 3 and I have the rule:

If F is assigned to boat 1, then G is assigned to boat 1 (only two boats and all variables get used)

So I diagram it to:

F1--->G1

And contrapositive of

G2--->F2

How the fuck is that different than

F<--->G ???

Because there is still the possibility of G1 and F2, and oftentimes they will test you to see if you made the distinction without jumping to an "if and only if" conclusion.

Posts: 71
Joined: Sun Dec 02, 2012 7:12 pm

Re: Contrapositives are killing me

I kind of get it. It's honestly difficult for me to wrap my head around contrapositives. I guess I need to reread that chapter in LG Bible.

Posts: 394
Joined: Tue Mar 08, 2011 12:19 am

Re: Contrapositives are killing me

Ya its something that comes with time, and eventually becomes second nature.

Its good that you are taking the time to nip it in the bud right now though, and your question was a good one.

dingbat

Posts: 4975
Joined: Wed Jan 11, 2012 9:12 pm

Re: Contrapositives are killing me

rvadog wrote:I kind of get it. It's honestly difficult for me to wrap my head around contrapositives. I guess I need to reread that chapter in LG Bible.

I don't ever want to tell someone what to do or what not to do in terms of learning, but there's a chance you're too focused on the methodology and not on the puzzle. Try finding some logic puzzle books and completing those, see how you fare.

(don't time yourself, focus on the words and adhere to them)

lsatkid007

Posts: 144
Joined: Wed Oct 17, 2012 7:07 pm

Re: Contrapositives are killing me

Hey guys. How would you diagram this sentence?

If that were indeed the way the mind works, the sensei could transfer facts and rules so the machine, which would replace the sensei as drilmaster and coach.

Thanks

TERS

Posts: 161
Joined: Thu May 24, 2012 11:29 pm

Re: Contrapositives are killing me

The syntax is strange.

I'm certainly not an expert, but my first approach would be the following

If the mind worked that way, the sensei could transfer facts and rules to the machine; if the sensei could transfer facts and rules to the machine, the machine could replace the sensei as drillmaster and coach.

Mind works that way --> Sensei can transfer facts and rules to machine

Sensei can transfer facts and rules to machine --> machine can replace sensei as drillmaster and coach

Mind works that way --> Sensei can transfer facts and rules to machine --> machine can replace sensei as drillmaster and coach

Mind works that way --> machine can replace sensei as drillmaster and coach

dingbat

Posts: 4975
Joined: Wed Jan 11, 2012 9:12 pm

Re: Contrapositives are killing me

lsatkid007 wrote:Hey guys. How would you diagram this sentence?

If that were indeed the way the mind works, the sensei could transfer facts and rules so the machine, which would replace the sensei as drilmaster and coach.

Thanks
If A then B can C resulting in D.

03152016

Posts: 9187
Joined: Wed Dec 21, 2011 3:14 am

Re: Contrapositives are killing me

.
Last edited by 03152016 on Tue Mar 15, 2016 3:11 am, edited 1 time in total.

willwash

Posts: 308
Joined: Mon Jan 25, 2010 9:51 pm

Re: Contrapositives are killing me

contrapositives used to be a big weakness fr me...i practiced them so much that i dont even diagram them anyymore. I found that the extra diagramming was cluttering up my diagrams and slowing me down and i realized that i'd reached the point where just from diagramming a rule i was instantly and automatically cognizant of the contrapositive. hence diagramming the cp became a waste of time.

for op i recommend mlsat's contrapositive drills. you can also make up your own...its pretty easy to do, especially in an in/out game