Tag: critical reasoning tips

Determining an argument’s necessary assumption, as we’ve just seen, is required to answer and Assumption question.  But it also is required to answer another common type of question: Strengthen or Weaken the argument.

One way to weaken an argument is to  break down a central piece of evidence.  Another way is to attack the validity of any assumptions the author has made.  The answer to many Weaken the Argument questions is the one that reveals an author’s assumption to be unreasonable; conversely, the answer to many Strengthen the Argument questions provides additional support by affirming the truth of an assumption or by presenting more persuasive evidence.

Let’s use the same stimulus as before but in the context of these other question types:

Allyson plays volleyball for Central High School

Therefore, Allyson must be over 6 feet tall.

Remember the assumption holding this argument together?  It was that all volleyball players for Central High are over 6 feet tall.  That’s the assumption that makes or breaks the argument.  So, if you’re asked to weaken the argument, you’d want to attack that assumption:

Which one of the following, if true, would most weaken the argument?

Answer:  Not all volleyball players at Central High School are over 6 feet tall.

We’ve called into doubt the author’s basic assumption, thus damaging the argument.  But what about strengthening the argument?  Again, the key is the necessary assumption:

Which one of the following, if true, would most strengthen the argument?

Answer:  All volleyball players at Central High School are over 6 feet tall.

Here, by confirming the author’s assumption, we’ve in effect bolstered the argument.

An assumption bridges the gap between an argument’s evidence and conclusion.  It’s a piece of support that isn’t explicitly stated, but that is required for the conclusion to remain valid.  When a question asks you to find an author’s assumption, it’s asking you to find the statement without which the argument falls apart

Denial Test

To test whether a statement is necessarily assumed by an author, we can employ the Denial Test.  Here’s how it works:  Simply deny or negate the statement and see if the argument falls apart.  If it does, that choice is a necessary assumption.  If, on the other hand, the argument is unaffected, the choice is wrong.  Consider this simple stimulus:

Allyson plays volleyball for Central High School.

Therefore, Allyson must be over 6 feet tall.

You should recognize the second sentence as the conclusion and the first sentence as the evidence for it.  But is the argument complete?  Obviously not.  The piece that’s missing-the unstated link between the evidence and conclusion-is the assumption, and you could probably prephrase this one pretty easily:

All volleyball players for Central High School are over 6 feet tall.

Is this an assumption really necessary to the argument?  Let’s negate it using the Denial Test.  What if it’s not true that all volleyball players for Central High are taller than 6 feet?  Can we still logically conclude that Allyson must be taller than 6 feet?  No, we can’t.  Sure, she might be, but she also might not be.  By denying the statement, then, the argument falls to pieces; it is no longer valid.  And that’s our conclusive proof that the statement above is a necessary assumption of this argument.

So, we can use the Denial Test to chick whether a statement is an assumption, but what if we haven’t a clue about what the assumption is?  Is there a way to track it down?  Sure enough, there is?

Compare the ideas in the evidence with those in the conclusion.  If the conclusion has an idea (an important word) but the evidence does not, then you’ve found an assumption.  A new idea cannot occur in the conclusion, so there must be an assumption about this idea.  Every idea in the conclusion need support-that is, evidence.  While it may not be quite clear what the assumption is, knowing something about it allows us to prephrase and eliminate choices.

As we’ve just seen, you can often prephrase the answer to an Assumption question.  By previewing the question stem, you’ll know what to look for.  And stimuli for Assumption questions just “feel” as if they’re missing something.  Often, the answer will jump out at you, as it did here.  But in more difficult Assumption questions, it might not be so obvious.  Either way, use the Denial Test to quickly check whichever choice seems correct.

Sample Question Stems

Assumption questions are worded in some of the following ways:

• Which one of the following is assumed by the author?
• Upon which one of the following assumptions does the author rely?
• The argument depends on the assumption that?
• Which of the following, if added to the passage, will make the conclusion logical?
• The validity of the argument depends on which one of the following?
• The argument presupposes which one of the following?

Critical Reasoning Question 2

In recent years, attacks by Dobermans on small children have risen dramatically. Last year saw 35 such attacks in the continental United States alone, an increase of almost 21% over the previous year’s total. Clearly, then, it is unsafe to keep dogs as pets if one has small children in the house.

The argument above depends upon which of the following assumptions?

A. No reasonable justification for these attacks by Dobermans on small children has been discovered.

B. Other household pets, such as cats, don’t display the same violent tendencies that dogs do.

C. The number of attacks by Dobermans on small children will continue to rise in the coming years.

D. A large percentage of the attacks by Dobermans on small children could have been prevented by proper training.

E. The behavior toward small children exhibited by Dobermans is representative of dogs in general.

The correct answer is E.

The evidence discusses attacks by Dobermans, but the conclusion is that the dogs-any dogs- are unsafe around little kids. This makes sense only if we assume (E): that Dobermans, in their behavior toward little kids, are generally representative of dogs. A good way of checking assumptions is to see what happens if we take their opposite: if the opposite of a statement weakens the argument, then that statement is assumed; if it doesn’t, it’s not. Here, if Dobermans’ behavior toward small children isn’t typical of dogs, the argument falls apart.

(A), whether the attacks were justified, is beside the point. Even if the kids were pulling the dogs’ tails, the author’s point that the dogs aren’t safe still holds. Other pets are beyond the scope, so (B)’s out. As for (C), the argument doesn’t deal with the future, so the author needn’t assume anything about it. And it certainly wouldn’t weaken the argument if, contrary to (D), many of the attack could not have been prevented, so (D)’s not assumed.