#16 - Theorist: To be capable of planned locomotion, an

[Administrator]

Question #16: Flaw, SN. The correct answer choice is (A)

The theorist begins by telling us that to be capable of planned or intentional movement, an organism must be able to form a mental picture of its environment and send messages to its muscles to control movements.

This is conditional, as follows:

Capable planned locomotion Mental image environment AND Control muscle movements

Don’t worry if your abbreviations don’t match those above. Just be sure you (1) spotted the conditional reasoning, and (2) put the pieces in the right places.

The next sentence goes from this to state that such an organism must have a central nervous system (CNS).

We can connect that to the relationship in the first sentence:

Capable planned locomotion Mental image environment AND Control muscle movements CNS

From that we can draw our own inference that any organism capable of planned locomotion must have a central nervous system:

Capable planned locomotion CNS

The author attempts to do something similar, taking that chain and tying together the first and last pieces, Capable planned locomotion and CNS, respectively. The mistake the theorist makes is that the connection is a Mistaken Negation, where the lack of planned locomotion capacity is incorrectly thought to prove a lack of a CNS.

Here’s the theorist’s conclusion diagrammed:

Not Capable planned locomotion Not CNS

Can you see how that’s simply the diagram we showed above, but with both terms negated? Classic Mistaken Negation error, and classic conditional reasoning flaw. Let’s find an answer that describes it.

Answer choice (A): This is the correct answer choice. Sure enough, this answer gives a great description of a Mistaken Negation, stating that the author has confused a necessary condition (CNS) with a sufficient condition, meaning if you incorrectly reverse our inference diagram above to be CNS Capable planned locomotion, then the conclusion would be the contrapositive of that and correct. Of course, reversing that inference is an error and that’s why the conclusion here doesn’t work.

Note too that this could also describe a Mistaken Reversal, another common conditional flaw. That’s to be expected since Mistaken Negations and Mistaken Reversals are really the same error presented in two different ways (both treat sufficient conditions as necessary, and vice versa).

Answer choice (B): The stimulus does not take this for granted. In fact it never says these things at all—nowhere does the stimulus tells us that the central nervous system is how organisms send messages to their muscles, nor does it just assume that that’s sufficient for locomotion (it’s actually one half of what’s necessary for locomotion).

Answer choice (C): Like (B), nowhere in the theorist’s argument is it suggested that planned locomotion is the only biologically useful reason for an organism to form a mental image of its surroundings. This answer can be ruled out by simply cross-referencing the facts in the stimulus.

Answer choice (D): This is the third straight answer that presents something that never occurred in the argument at hand. The theorist never implies that biologically useful adaptations (a concept entirely unmentioned here) had to originally be for that purpose. Once again, we can dismiss this on fact alone.

Answer choice (E): Somewhat surprisingly, albeit in a good way, we have a fourth description of the stimulus that fails on facts. The theorist at no point discusses any connection between forming an internal representation of an organism’s environment and that creature possessing “a rudimentary nervous system” (a phrase not used in this argument). All that is discussed is a central nervous system, however rudimentary or advanced.

[lita236]

For answer b), if the author took for granted that.... wouldn't it mean they assumed if organisms have a central nervous system and can send messages to muscles then they are capable of locomotion. If they assumed this conditional,that would have made their conclusion valid. But because this conditional was unfounded (taken for granted) then they were led to the wrong conclusion and so this is the flaw in their reasoning. Essentially I don't understand how this answer (b) could not be interpreted as the reverse conditional that their conclusion was based upon.

[Jeremy Press]

Hi lita,

A couple things about your read of answer choice B: it's not actually true that the answer as stated would validate the conclusion. The contrapositive of what's stated in answer choice B is: "If an organism is not capable of locomotion, then it's not capable of sending messages from its central nervous system to its muscles." This doesn't validate the conclusion in the stimulus, because the stimulus conclusion is a broader statement that the organism doesn't have a central nervous system, not merely that it can't send messages from a central nervous system elsewhere.

The second thing is that even if the answer choice as stated could potentially have rendered the conclusion valid, that wouldn't mean the author actually assumed what the answer choice states. We have to compare the answer against the content of the argument to be sure the author actually did assume it. And here, there's good evidence the author didn't assume what answer choice B states. First, the author actually states two necessary conditions for planned locomotion: ability to form an internal representation of the environment AND sending message to muscles to control movements. Thus, even if the author is assuming a mistaken reversal of that conditional relationship, the mistaken reversal would have to be that organisms capable of sending message to muscles to control movements AND also capable of forming an internal representation of the environment are also capable of planned locomotion. Second, the author makes no assumption about HOW an organism sends messages to its muscles. The author assumes a central nervous system is necessary for sending messages. But it's possible (consistent with the argument) that the central nervous system simply enables the sending of messages from some other source to the muscles. We just don't know (and can't know, from the stimulus alone) what the author thinks about how the messages are sent.

I hope this helps!

Jeremy

[Mmjd12]

My diagramming and prephrase was identical to the one in the explanation above. I was looking for an answer describing a mistaken negation. (A) seemed to me to be describing a mistaken reversal, so I didn't pick it and instead toiled with the other choices. How can you tell (A) is describing a mistaken negation and not a mistaken reversal?

[Chandler H]

My diagramming and prephrase was identical to the one in the explanation above. I was looking for an answer describing a mistaken negation. (A) seemed to me to be describing a mistaken reversal, so I didn't pick it and instead toiled with the other choices. How can you tell (A) is describing a mistaken negation and not a mistaken reversal?

Hi Mmjd12,

You may be confused because this is, in a way, both an MN and an MR. As you know, the correct inference from the stimulus is:

CPL CNS

From this, the author concludes:

CPL CNS

The contrapositive of this incorrect conclusion would be CNS CPL. Since this is the contrapositive, it is the exact same logical relationship. Do you see how that is a Mistaken Reversal?

[Mmjd12]

Hi Chandler,

Yes, I see it now thank you. I guess I was looking for some literal negating language in the answer choice, but I can see now that its still describing a mistaken negation as they are logical equivalents. Glad I came across this question and will keep an eye out for that on future PTs.