Logical Reasoning

PT111 · S1 · Q22 No chordates are tracheophytes

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No chordates are tracheophytes, and all members of Pteropsida are tracheophytes.

Conclusion (So)

No members of Pteropsida belong to the family Hominidae.

Evidence

No chordates are tracheophytes. C → ~T All Pteropsida are tracheophytes. P → T

Evaluate

Since this argument has conditional logic (no, all, no) and has repeating symbols (tracheophytes and Pteropsida are each mentioned twice) we would probably be wise to look at this in conditional logic.

However, there are two alternatives:

1. Mentioned Twice? That'll suffice. This is the nickname for the guessing heuristic on linking problems. Things mentioned twice don't need to be in our answer, but stuff mentioned only once should be. We'd get rid of any answer dealing with T or P, and we'd look for answers dealing with C and H, since they were each only mentioned once. That would get us down to (B) and (D), one of which is the correct answer.

2. Ask yourself questions about the conclusion. We're trying to prove that no P's are H's. What do we know about P's or H's? we've been told nothing about H. What do we know about P? we know that all P's are T's. What do we know about T's? no T's are C's. Okay, so if you're P, you're T. And if you're T you're not C. What does that mean if we cancel out the middleman? if you're P, you're not C What are we trying to prove in the conclusion? if you're P, you're not H. What's the missing link? if you're not C, you're not H.

If we're comfortable with conditional logic, then we draw the Conclusion with a long arrow to indicate the logic path we will eventually be building, once we combine the premises with the correct answer.

Conclusion

P -------------------------------> ~H H -------------------------------> ~P

We wrote the conclusion both ways (original / contrapositive) so that we can figure out the better match for our evidence. When we look at what the evidence says, do we see "P --> ?" or "H --> ?". We only see the former. So we'll use the first version of the conclusion:

Conclusion

P -------------------------------> ~H

Evidence

P ---> T C ---> ~T

Those have a common ingredient; they both talk about T. Do they chain together? Let's contrapose the C/T rule to get, T ---> ~C.

Now they chain together.

22.

The conclusion above follows logically if which one of the following is assumed?

  1. All members of the family

    Unrelated to Goal

    This isn't giving us H --> C (or ~C --> ~H). From our evidence we have, P ---> T ---> ~C This answer says, H --> T. That doesn't chain onto our evidence at all, other than that we could write. H or P ---> T ---> ~C So have we derived the conclusion? P --------------> ~H No, not even close.

    3% picked this

  2. Correct

    All members of the family

    Why this is right

    This is what we were looking for: H → C (or ~C → ~H) From our evidence we had, P → T → ~C This answer says, ~C → ~H We can chain that onto our evidence to get P → T → ~C → ~H So have we derived the conclusion? P --------------> ~H Yes!

    Skill tested: Sufficient Assumption · how this choice captures the argument's function is the move to repeat next time.

    63% picked this

  3. All tracheophytes are members of

    Unrelated to Goal

    The answer has to talk about H, since H only appears in the conclusion. We can't derive a claim about H if we have no facts (i.e. premises + answer choice) that contain H in them. Any answer choice without H is immediately useless on this problem. Because T and P have already been mentioned multiple times in the argument, we would not think that they are part of the missing link. So if we were just guessing, we'd avoid answers with T or P. Ultimately every wrong answer on Sufficient Assumption is wrong for the same reason: it fails to prove the conclusion. From our evidence we knew, P ---> T ---> ~C This answer says, T --> P. That doesn't chain onto our evidence at all, other than that we could write. P <---> T ---> ~C Does that allow us to prove this conclusion? P --------------> ~H No, because our facts don't include H at all. Who's H? We can't derive a claim about H if we have zero facts (i.e. premises + answer choice) that talk about H.

    3% picked this

  4. No members of the family

    Trap

    13% picked this

  5. No chordates are members of

    Unrelated to Goal Already Known

    The answer has to talk about H, since H only appears in the conclusion. We can't derive a claim about H if we have no facts (i.e. premises + answer choice) that contain H in them. Any answer choice without H is immediately useless on this problem. This answer also repeats something we already knew. From our evidence we knew, P → T → ~C contrapositive, C → ~T → ~P This answer says, C → ~P. So this answer is just telling us about the beginning/end of the chain we got when we linked up the two premises. We can read our own chain, (E). We already knew that C → ~P. Since this answer provides no new information, it can't possibly be right.

    17% picked this

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