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Unstated Assumption In Critical Thinking

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The purpose of this article is to clearly bring out the difference between the Inference and Assumption so that any source of confusion can be eliminated.

Some students are confused between inference and Assumption because the phrase “Must be true” is used in both question types. We have also seen that some students do face problems in identifying one from the other. Let’s begin with the definitions of both the terms: Inference and Assumption.

What is inference in Critical Reasoning

 

An inference is a piece of information which can be logically deducted from the given set of statements. In GMAT, an inference could be a logical deduction from a single statement or from a combination from two or more statements.

Let’s take an example:

Everyone who reads this article will be more informed about Inference and Assumption. Rahul read this article.

Inference: Rahul is more informed about Inference and Assumption

This is an example of a combination inference where two or more statements are combined to produce the correct answer choice. In this specific case, the first statement lays out a general rule of the form: If X, then Y i.e. If one reads this article, then he will be more informed about Inference and Assumption.

The next statement about Rahul states that ‘X’ statement is satisfied in case of Rahul. Therefore, combining these statements, we can make an inference that Rahul is more informed about Inference and Assumption.

What is assumption in Critical Reasoning

An assumption is a hidden or an unstated premise. There are two keywords in this definition: “hidden” and “premise”.

  • Being a premise means that the assumption must be true for the conclusion to hold true
  • “Hidden” means it cannot be logically derived from existing information i.e. it must present some new information. This is because there is no need to assume a thing, which can be logically derived from existing information.

Everyone who reads this article will be more informed about Inference and Assumption. Rahul received this article in his email. Hence, Rahul is more informed about Inference and Assumption.

Assumption: Rahul has read every mail that he has received so far.

Using the same terminology we used in the previous example: In this case, we are given “If X, then Y” and that Rahul received this article in his email. These two statements are, then, taken to conclude that Y holds for Rahul.

But within the context of the passage, we know that to conclude Y, X must hold. Therefore, Rahul must have read the article. However, we are only given that Rahul received this article in his email.

Therefore, the underlying assumption is that Rahul has read every mail that he has received so far.

Inference vs Assumption: point of confusion

We can see from the definitions above that there are some critical differences between an inference and an assumption:

Inference

Assumption

Question stem Structure – If the statements above are true, which of the following must be true?Which of the following is an assumption on which the argument depends?
Inference is a statement which must be true, if the given information is trueAssumption is a statement which must be true, for the given information to be true; in other words, for the conclusion to hold true.
An inference can always be logically deducted from the given informationAn assumption can never be logically deducted from the given information – It contains some new information
It’s very rare for an inference question to have a conclusion in the passage or the question stemAn assumption question necessarily needs to have a conclusion in the passage or the question stem

 

If we focus on the second point of difference above, we can see that an inference can never be same as an assumption. In other words, they are mutually exclusive sets. There is no overlap.

When people give arguments sometimes certain assumptions are left implicit. Example :

Homosexuality is wrong because it is unnatural.

This argument as it stands is not valid. Someone who gives such an argument presumably has in mind the hidden assumption that whatever that is unnatural is wrong. When this assumption is added, the argument does become valid.

But once this is pointed out, we can ask what this assumption really means and whether it is justified. There are plenty of things that are presumably "unnatural" but are not usually regarded as wrong, such as wearing sunglasses or having surgery. So anyone who accepts the argument above will have to either give up the argument, or come up with a different hidden premise. So trying to identify the hidden assumption in an argument can help us think more deeply.

In everyday life, the arguments we normally encounter are often arguments where important assumptions are not made explicit. It is an important part of critical thinking that we should be able to identify such hidden assumptions or implicit assumptions.

So how should we go about identifying hidden assumptions? There are two main steps involved. First, determine whether the argument is valid or not. If the argument is valid, the conclusion does indeed follow from the premises, and so the premises have shown explicitly the assumptions needed to derive the conclusion. There are then no hidden assumptions involved. But if the argument is not valid, you should check carefully what additional premises should be added to the argument that would make it valid. Those would be the hidden assumptions. You can then ask questions such as : (a) what do these assumptions mean? (b) Why would the proponent of the argument accept such assumptions? (c) Should these assumptions be accepted?

This technique of revealing hidden assumptions is also useful in identifying hidden or neglected factors in causal explanations of empirical phenomena. Suppose someone lights a match and there was an explosion. The lighting of the match is an essential part in explaining why there was an explosion, but it is not a causally sufficient condition for the explosion since there are plenty of situations where someone lights a match and there is no explosion. To come up with a more complete explanation, we need to identify factors which together are sufficient for the occurrence of the explosion, or at least show that it has a high probability of happening. This might include factors such as the presence of a high level of oxygen in the environment.

Identify the likely hidden assumptions in these arguments:

  1. We should reduce the penalty for drunken driving, as a milder penalty would mean more convictions. answer
  2. Moby Dick is a whale. So Moby Dick is a mammal. answer
  3. Giving students a fail grade will damage their self-confidence. Therefore, we should not fail students. answer
  4. It should not be illegal for adults to smoke pot. After all, it does not harm anyone. answer
  5. There is nothing wrong talking on a mobile phone during lectures. Other students do it all the time. answer
  6. Killing an innocent person is wrong. Therefore, abortion is wrong. answer
  7. Traces of ammonia have been found in Mars' atmosphere. So there must be life on Mars. answer
  8. There cannot be more than one God. Otherwise, there would be two Gods equally powerful, or one is more powerful than the other. answer

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