2. 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.
3. 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.
4. Steps in identifying assumptions:
Identify the conclusion of the argument.
Identify what is stated in the author’s
reasoning.
6. 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.
7. 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.
8. 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.
9. 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.
10. 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
11.
12. • 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.”
13. • 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.
14. • 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.
15. • 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.
