LR Inferences: Additive and Inherent Inferences Forum

[sjlawand88]

All,

I am studying LR Bible and ran into two questions that I could not figure it using their answer key.

1. Some A’s are B’s.
No B’s are C’s.
All C’s are D’s.
Diagram: A some B <-I-> C---> D
Inferences:
A some -C
D some -B

They only state those 2 inferences. Why can't we infer that:
1) A some -D
2)D some -A

6. Some N’s are O’s.
No O’s are P’s.
No P’s are Q’s.
All Q’s are R’s.
Diagram: N some O<--I--> P<--I--> Q---> R
Inferences:
N some -P
R some -P

They only state those 2 inferences. Why can't we infer that:
1) N some -Q
2) N some -R


I do not understand this.....

Thanks in advance....

[brickman]

read the formal logic section of LRB again.

I'll help you understand the first question better at least.

you cant have "D some -A" b/c in order to have a reversible, D/C relationship you must make it a some relationship and you can't have 2 some relationships combine to tell you anything.

You only know about A that some of them are -C, since C is sufficient for D, we can't assume to know anything about D from the absence of a sufficient.

follow similar thinking/understanding for your later questions.

[TheLuckyOne]

This diagram may clarify the first issue:

A some B --> -C
C--> D

^^^ This is exactly why you can't link A and D. You know what's NOT C.

Same goes to the second one:

N some O --> -P
P--> -Q
Q--> R

"Some" groups may not overlap, so we can't deduce your inferences.