#25 - Each major earthquake in this region has been preceded

[Aspiring-Logicl-Rsnr]

Hi Amber!

Thanks for your help. What confuses me about B is the past tense phrasing. Answer B does not say "This river overflows in every spring thaw following a winter with high snowfall." Instead, it says "This river has overflowed in every spring thaw following a winter with high snowfall." To me, this reads as an observation of past events, not a logical statement. That's why it makes sense to me when Robert Carroll, above, affirms that B is flawed. I'm just struggling to figure out how B is flawed differently from E and the stimulus (unless, again, the river-disease outbreak distinction is relevant).

What, exactly, ensures that B can be read as a sufficient condition? How is the "every" in B different from the "all" in E or the "each" in the stimulus? All three words seem to me to be indicating universality. But I also feel like each of the three words, in its context, indicates a universality of past events—not a logical truth.

Sorry that I'm still stuck on this one.

[Jeff Wren]

Hi avalc,

When it comes to conditional reasoning, there is no set time relationship between the sufficient and necessary condition. In other words, the sufficient condition can occur before the necessary condition, after the necessary condition, or at the exact same time as the necessary condition.

Here, the word "each" indicates that when there was a major earthquake, then there was a series of tremors that preceded it. This means that, (at least in the past), a major earthquake indicated/guaranteed preceding tremors. This does not mean that every time there were tremors, that they always preceded a major earthquake. That idea would have been worded "each series of tremors preceded a major earthquake."

Even though the tremors precede the earthquakes for a time standpoint, this does not determine whether they are considered sufficient or necessary.

For example, in the conditional statement, "To do well on the LSAT, one must study hard," the necessary term "study hard" actually takes places before the sufficient term "do well on the LSAT."

[Jeff Wren]

Hi Aspiring,

I agree with you and Robert that Answer B does contain a time shift flaw.

While the term "every" in Answer B is a sufficient condition indicator (and is equivalent to the terms "all" and "each"), the term is modifying "every (past) spring following a winter with high snowfall." In other words, if we knew that a particular past spring (say the spring of 1980) followed a winter with high snowfall, then we would know for certain that the river overflowed that year.

For the reasons that you and Robert mention, this does not guarantee that this will continue to happen in the future and assuming that the future will never change from the past is a time shift flaw. If the conclusion had been worded "the river will probably overflow" or "the river will likely overflow," that would be a much more modest/reasonable claim.

The stimulus and Answer E also have time shift flaws, but they also have Mistaken Reversals. In other words, these arguments use the fact that an event that always accompanied another event (i.e. the necessary term) occurs to then predict that the other event (i.e. the sufficient term) will occur.

For example, in the stimulus, (at least in the past), a major earthquake indicated/guaranteed preceding tremors. This does not mean that every time there were tremors, that they always preceded a major earthquake. Since the argument uses the fact that there have been recent tremors to conclude that there will be a major earthquake, the argument makes two different flaws. Not only can we not make certain predictions about the future based on past events (the time shift flaw), but we also don't know how often tremors precede major earthquakes (the Mistaken Reversal).

Answer E contains both of these flaws, which is why it is correct.

[Aspiring-Logicl-Rsnr]

Hi Jeff,

Thank you for writing this out for me; it is helpful. I'm still, however, a bit confused. How does B not contain a mistaken reversal? B says that river overflows -> winter with high snowfall and then predicts that winter with high snowfall -> river overflows.

Meanwhile, the stimulus says that each major earthquake -> preceding tremors and then predicts that preceding tremors -> major earthquake.

And E says that disease outbreak -> high infection rates detected and then predicts that high infection rates detected -> disease outbreak.

Where am I going wrong?

[Jeff Wren]

Hi Aspiring,

Answer B does not contain a Mistaken Reversal.

Unfortunately, your diagram for this answer (specifically for the premise) is incorrect.

You wrote:

"B says that river overflows -> winter with high snowfall"

Actually, Answer B says that:

Winter with high snowfall -> river overflows

Answer B states: "This river has overflowed in every spring thaw following a winter with high snowfall" (my emphasis). The word "every" is a sufficient indicator, and it is modifying "spring thaw following a winter with high snowfall," so that term is the sufficient term.

You could rewrite the sentence as:

Every time there was a spring thaw following a winter with high snowfall, the river overflowed.

You could also rewrite the sentence as:

If there was a spring thaw following a winter with high snowfall, then the river overflowed.

The sentence is not saying:

Every time the river overflowed, there was a spring thaw following a winter with high snowfall.

Remember that the sufficient condition can appear at the beginning of a conditional sentence or at the end. To determine which term is sufficient and which is necessary, focus on the meaning of the sentence and how the terms are related in the sentence. Familiarizing yourself with specific sufficient (such as "every") and necessary indicator words will help with this.