Identifying AssumptionsLesson 14

Assumption

-Noun.

A thing that is accepted as true or as certain to happen, without proof.

Assumption is information not stated in the argument that has to be true in order for the argument’s logic to hold. Simply put, an assumption is something the argument takes for granted in reaching its conclusion.

Example:

All kids are excited when they get new toys.

Therefore, Tim will be thrilled when he gets the new

radio-controlled car we bought him.

In this argument, the author says two things

explicitly:

• All kids are excited when they get new toy

• Tim will be thrilled when he gets the new radio-

controlled car we bought him.

Steps in identifying assumptions:

Identify the conclusion of the argument.

Identify what is stated in the author’s

reasoning.

Evidence

-Noun

The available body of facts or information

indicating whether a belief or proposition is true

or valid.

Evidence: All kids are excited when they get new toys.

Conclusion: Tim will be thrilled when he gets the new

radio-controlled car we bought him.

Author’s Assumption: Tim is a child.

Important Characteristics of Assumptions

• Adding the correct assumption to the argument will

strengthen the argument.

• Adding the negated assumption to the argument will

weaken the argument.

All kids are excited when they get new toys. Tim is a

child. Therefore, Tim will be thrilled when he gets

the new radio-controlled car

we got him.

• This argument is stronger

because the new premise

(assumption) eliminates the gap

between the evidence and the

conclusion. Now the evidence

(first premise) is about children,

and the second premise includes

Tim in that category, making the

evidence directly applicable to

Tim.

All kids are

excited when

they get new

toys. Tim is not a

child. Therefore,

Tim will be

thrilled when he

gets the new

radio-controlled

car we got him.

• The argument is weaker

because its conclusion has

become purely speculative.

You have no logical grounds

to predict anything about

Tim’s reaction, because the

second premise (assumption)

does not now connect the

first premise (evidence) to

the conclusion.

Remember!

• An assumption is information not stated in the argument that must be true for the argument’s conclusion to hold. An assumption is unstated vital information that the argument takes for granted.• When the correct assumption is added to the

argument, it will strengthen the argument. HOWEVER, not every answer choice that strengthens the argument must be assumed.• When the correct assumption is negated, adding it

will weaken the argument

• We should reduce the penalty for drunken driving,

as a milder penalty would mean more

convictions.

Answer: We should increase the number of

convictions for drunken driving.

• Moby Dick is a whale. So Moby Dick is a

mammal.

Answer: “Anything that is a whale is a mammal”, or

“If Moby Dick is a whale it is a mammal.”

• Giving students a fail grade will damage their

self-confidence. Therefore, we should not fail

students.

Answer: We should not damage students' self-

confidence.

• It should not be illegal for adults to smoke pot.

After all, it does not harm anyone.

Answer: Anything that does not cause harm should

not be made illegal.

• There is nothing wrong talking on a mobile

phone during lectures. Other students do it all

the time.

Answer: If an action is done by other students (or

people) all the time, then there is nothing wrong

with it.

• Killing an innocent person is wrong. Therefore,

abortion is wrong.

Answer: Abortion involves the killing of an

innocent person.

• Traces of ammonia have been found in Mars'

atmosphere. So there must be life on Mars.

Answer: "Only living things produce ammonia." (or

something similar)

• There cannot be more than one God. Otherwise,

there would be two Gods equally powerful, or one

is more powerful than the other.

Answer: If something is a God, nothing else can be

as powerful or more powerful than it.