Logical reasoning PrepTest 143 · Section 3 · Question 16

Correct. Argument or Facts:
Argument

Valid or Flawed:
Flawed

Question Type:
Errors in Reasoning

Stimulus Summary:
Planned locomotion → Internal map of environment AND Control muscles
Internal map of environment AND Control muscles → Central nervous system
Therefore: not Planned locomotion → not Central nervous system

Answer Anticipation:
No matter how you diagrammed the sufficient condition of the second conditional ("Such an organism" calls back to the preceding phrase, which is how we got to where we did), you should have noted that the premises build together, and that the conclusion negates the initial sufficient condition to reach the negated necessary condition. No matter how you shake it, that's an illegal negation, so this argument has a conditional logic flaw.

Answer Explanation:
While this answer is more frequently used to describe an illegal reversal, it can also describe an illegal negation—the two are contrapositives of each other than thus are logically equivalent. Here, the central nervous system is a necessary condition for planned locomotion. The argument, however, concludes that it's sufficient—just take the contrapositive of the conclusion to see that! This answer correctly describes the flaw and is thus correct.

Key Takeaway:
When an Errors in Reasoning question has conditional logic, it's likely the flaw will be related to it. And even though illegal reversal and illegal negations tend to have certain answers associated with them, they're logically related and thus either answer can be associated with both.

Incorrect. Since the conclusion of the argument just negates the endpoints of the conditionals, we can't tell how it's dealing with the intermediate terms. As such, we can't infer that this connection is being assumed by the author.

Incorrect. There's no discussion of other potential purposes, so this answer choice is out of scope. Even if there are other reasons to develop internal representation of the environment, it could still serve the purpose noted here.

Incorrect. The argument doesn't talk about the development of these adaptations or of original purpose, so this answer is out of scope.

Incorrect. Having a central nervous system is, for all intents and purposes of this argument, a binary consideration—you either have one or you don't. How rudimentary it is doesn't factor into it.