#22 - No chordates are tracheophytes, and all members of

[Dave Killoran]

Complete Question Explanation


Justify the Conclusion—Formal Logic. The correct answer choice is (B)

This stimulus contains a conclusion. Therefore, argumentation is
present and the chances of a Must Be True question appearing are diminished. Indeed, a Justify the
Conclusion question is presented, and when combined with the complex relationship in the stimulus,
the result is a challenging problem. The stimulus can be diagrammed as follows:

          C = chordates
          T = tracheophytes
          P = member of Pteropsida
          H = member of family Hominidae

LR1.PNG

Given that we need to justify the conclusion, try to solve the problem mechanistically:

     1. The conclusion contains the new element H, and thus the correct answer should contain H.
     This analysis eliminates answer choices (C) and (E) from contention.

     2. The element P is common to both the conclusion and the premises, and the correct answer
     should not contain P. Unfortunately, this has no impact on the three answer choices in
     contention.

     3. The premises contain elements C and T that are unconnected to the conclusion. Again,
     answer choices (A), (B), and (D) contain one or the other.

Based on the analysis above, we should expect the correct answer to either link H and C or to link H
and T. Given that C would involve a longer chain of deduction, you should suspect that the correct
answer will link H and C. The question becomes what is the exact relationship that will connect the
premises to the conclusion? The first relationship that comes to mind is H T. But the only
answer that involves H and T is answer choice (A), and the relationship there is the opposite of what
is needed. Answer choice (A) would not lead to the conclusion and is therefore incorrect.

The elimination of answer choice (A) leaves two answers that involve H and C. Answer choice (B)
stipulates that all Hs are Cs, and answer choice (D) stipulates that no Hs are Cs. Keep in mind that
with a Justify question you can simply add the answer choice under consideration to the premise
diagram and then check to see if one of the inferences that follows matches the conclusion. If
so, then that answer is correct. If not, then the answer is incorrect. Let’s use that method with the
remaining two answer choices:

The addition of answer choice (B) to the premises leads to the following diagram:

LR2.PNG

Because no inference can be made from H to T, the diagram above does not allow for an inference to
be made between H and P. Therefore, this answer choice is incorrect.

This is an extremely challenging question because it contains a Formal Logic structure that is
complex. The addition of the Justify question forces you to find the missing link, but by quickly
eliminating answer choices that do not match the Justify paradigm, you can narrow the field to two
contenders and then make a decision. But, as you can see, you must have complete control of the
diagrams and you must be able to make inferences quickly, accurately, and confidently. This is not
for the faint-hearted or lazy!

[srcline@noctrl.edu]

Hello

I do not understand the linkage in this problem. I had originally diagrammed this problem as:

Premise 1: not C T
Premise 2 : P T
Conclusion: not P H

Thankyou
Sarah

[Robert Carroll]

Sarah,

The first premise is not diagrammed correctly. As you have it, it says that anything not a chordate must be a tracheophyte. The conditional is actually:

chordate tracheophyte

The conclusion has the same problem. It should be:

Pteropsida Hominidae

"No A are B" statements have the negation on the necessary condition side.

Hope this helps!

Robert Carroll

[cboles]

Confused how you come to answer choice B.

I set it up correctly but am unsure of how you come to the conclusion that all Hs are Cs.

This was my setup:

C->T (with line negating)
P->T
P->H (with line negating)

[Nikki Siclunov]

Hi cboles,

It's imperative to identify each conditional relationship as a premise or a conclusion before linking them up. So:

• Premise: C T
  Premise: P T
  ===========
  Conclu.: P H

The premises, when combined, produce the following chain:

• Premise (1) + (2): P T C

(note that we're using the contrapositive of Premise 1)

To justify the conclusion, we need to establish that:

• Justify: C H
  
  Contrapositive: H C

Thus, if we "add" this additional premise to the two premises we already have, the conclusion will be properly justified:

• Premise (1) + (2) + Justify: P T C H
  
  Therefore, P H

This validates answer choice (B).

Hope this helps! Let me know.

Thanks,

[avengingangel]

hi! so, i hope this'll make sense:

i correctly diagrammed the two premises and conclusion, but instead of seeing h c (or it's contrapositive) as the missing link, i saw t h.

i then went to look for that or its contrapositive in the answer choices, and after not seeing it, i realized b works also, but seemingly in a longer chain/combination of conditionals ?

my question is, 1) is that correct, that the other link that i found was missing could have been a correct answer choice as well, and 2) isn't it a more direct link that answer choice B ???

thanks!!!

[Adam Tyson]

In short, angel - yes! If we had an answer that told us that no tracheophytes are Hominidae, that too would have justified the conclusion. Would it be "more direct", as you said? I suppose if by that you mean we could "skip" one of the terms (chordates), then yes, it would be more direct. Don't expect that too often on the LSAT, though, as the authors like to be more remote much of the time. It's easier to be more direct, and easy is not their goal, right? Making us tie back to the first sentence, with the confusion that its structure often causes, is more their style.

You can often Justify more way than one, just as there are frequently multiple good prephrases to strengthen or weaken, and there may be many assumptions in a stimulus. Your goal isn't always to find the best one or the easiest one, but instead to find the best one of the five that are given to you. Cast a wider net, step back and look at the big picture, don't lose the forest for the trees, and some other cliche bits of advice may also apply here.

Keep at it, you're doing just fine.

[Adam354]

Using strikethrough's on H's is sometimes not readable.

P in -> T in -> C out -> H out
contrapositive is
H in -> C in -> T out -> P out

[Dave Killoran]

Using strikethrough's on H's is sometimes not readable.

P in -> T in -> C out -> H out
contrapositive is
H in -> C in -> T out -> P out

Hi Adam,

That really only applies to the forum, where the strikethrough we are forced to use appears similar to a dash ( - ) across the letter, as in H.

When actually diagramming them yourself, the symbol should be more like a slash: / . Then it is far less likely you'd make an error since it's such a clear symbol that doesn't get confused with the horizontal crossbar of the H. You see this in how we represent that in our books etc. On the forum, we are limited to the basic character tools, so it's a different situation entirely

Thanks!

[Adam354]

Using strikethrough's on H's is sometimes not readable.

P in -> T in -> C out -> H out
contrapositive is
H in -> C in -> T out -> P out

Hi Adam,

That really only applies to the forum, where the strikethrough we are forced to use appears similar to a dash ( - ) across the letter, as in H.

When actually diagramming them yourself, the symbol should be more like a slash: / . Then it is far less likely you'd make an error since it's such a clear symbol that doesn't get confused with the horizontal crossbar of the H. You see this in how we represent that in our books etc. On the forum, we are limited to the basic character tools, so it's a different situation entirely

Thanks!

Haha yes for sure. I meant on the forum, but it only took a couple minutes to figure out that was the case. I agree about the strikethrough technique on paper.