3.15 - Necessary Assumption

Necessary Assumptions take a relatively simple concept—necessity—and complicate it with wonky English and bad arguments.

They tend to look something like this:

• Which one of the following is an assumption required by the argument?
• The argument relies on assuming which one of the following?
• Which one of the following is an assumption on which the argument depends?

These questions are all about what an argument requires, i.e. all the other stuff that must be true for the conclusion to hold any water.

Note, these questions can be harder to predict on the nose, but prediction nonetheless helps us wrestle with and understand the argument.

Let's dive into some examples.

Examples

June 2007 PrepTest, Section 3, Question 9

Here's Question 9 from Test J, Section 3:

Naturalist: The recent claims that the Tasmanian tiger is not extinct are false. The Tasmanian tiger's natural habitat was taken over by sheep farming decades ago, resulting in the animal's systematic elimination from the area. Since then naturalists working in the region have discovered no hard evidence of its survival, such as carcasses or tracks. In spite of alleged sightings of the animal, the Tasmanian tiger no longer exists.

We're asked to find an assumption the argument depends on.

Start with the conclusion: The Tasmanian tiger is extinct. Now, its support: There have been recent, alleged sightings. But, decades ago, the tiger's natural habitat was overtaken by sheep farming. Since, naturalists have found no hard evidence in the area.

First, what if the tigers simply migrated? Sure, we haven't found hard evidence in their natural habitat, but what about nearby habitats that weren't the tiger's original habitat? If we find even one tiger in a new environment, this conclusion falls apart.

Second, this conclusion leans pretty hard on the competence of the naturalists in the region. That is, we're assuming that if the Tasmanian tiger had left traces of its existence, naturalists operating in the region would have found it.

Our first prediction is a near perfect match to answer choice D, which reads, "The Tasmanian tiger did not move and adapt to a different region in response to the loss of habitat."

Exactly! If the tigers had moved, and even one is left out there somewhere, then this conclusion falls apart.

June 2007 PrepTest, Section 3, Question 11

Let's run through another, this time Question 11 from the same section:

Feathers recently taken from seabirds stuffed and preserved in the 1880s have been found to contain only half as much mercury as feathers recently taken from living birds of the same species. Since mercury that accumulates in a seabird's feathers as the feathers grow is derived from fish eaten by the bird, these results indicate that mercury levels in saltwater fish are higher now than they were 100 years ago.

Let's start with the conclusion again: Mercury levels in saltwater fish are higher now than the were 100 years ago. And, why? Because the stuffed seabirds from the 1880s have half the mercury in their feathers of modern seabirds and mercury accumulates in their feathers as they eat fish throughout their lives.

Here's a what-if for you. What if seabirds have dramatically increased the amount of saltwater fish they consume over the past 100 years? If they used to eat freshwater salmon, too, but the salmon are all gone now, then maybe the increase in mercury is simply from an increase in saltwater fish consumption. That means the author would need to believe the birds' dietary habits are relatively the same now as they were way back when .

Here's another. What if the mercury in the stuffed birds' feathers decays / evaporates over time? If the mercury levels drop significantly over 100 years, then of course the modern birds have more in their feathers. In other words, the author would have to assume that the mercury levels stay relatively constant in the stuffed birds feathers.

These are just two ideas arming me against bad answer choices, and hopefully, helping me identify the right one.

E turns out to be the answer and it's not too far gone from my second prediction. It says "[the argument assumes that] the process used to preserve birds in the 1880s did not substantially decrease the amount of mercury in the birds' feathers." Bingo.

Try removing the word not from the answer choice and thinking about the implication on the argument—you'll hear this is called the Negation Test. In other words, if the old time preservation process did substantially decrease mercury levels in the feathers, then the author's conclusion makes no sense. This is just another way of thinking about how to solve Necessary Assumptions.

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That's it for Necessary Assumptions. Join us next time as we tackle Sufficient Assumptions. I'll see you there.