Philosophy Chapter 3 Quiz

Deductive argument

A logical structure where if the premises are true, the conclusion must be true

• aims for guaranteed certainty

Deductive argument example

All men are mortal.

Socrates is a man.

Therefore, Socrates is mortal.

Inductive argument

Premises provide probable evidence—rather than absolute certainty—for the truth of the conclusion

Inductive argument example

Almost all dogs have fleas.

Therefore, Bowser, my dog, probably has fleas.

Note about inductive arguments

Arguments inferring future events from past observations is an inductive prediction

Steps for judging arguments

Step 1. Find the argument’s conclusion and then its
premises.
Step 2. Ask: Is it the case that if the premises are true
the conclusion must be true?
Step 3. Ask: Is it the case that if the premises are true,
its conclusion is probably true?
Step 4. Ask: Is the argument intended to offer
conclusive or probable support for its conclusion but
fails to do so?

Implicit premise

an unstated assumption or hidden claim that an argument needs to be true so its conclusion can logically follow from its stated facts

Why do implicit premises matter?

• everyday arguments are rarely presented in neat, numbered form

• People often leave out “obvious” background assumptions

• To evaluate an argument fairly, you sometimes need to supply the missing piece

Implicit premise example

Argument: "John is a doctor, so he is well-educated."

• Implicit Premise: All doctors have a good education

How to find implicit premises

• Look for gaps: Ask yourself, “How does the conclusion
  actually follow from the premise(s) given?”

• Bridge the gap: Supply the most reasonable statement that
  connects the premise to the conclusion.

• Be cautious: Don’t add too much — only insert what is
  necessary for the argument to work

Modus ponens (“Affirming the antecedent”)

If p, then q.

p.

Therefore, q.

Modus ponens example

If the job is worth doing, then it’s worth doing well.
The job is worth doing.
Therefore, it’s worth doing well.

Modus tollens (“Denying the consequent”)

If p, then q.
Not q.
Therefore, not p.

Modus tollens example

If the restaurant were open today, the lights would be on.
The lights are not on.
Therefore, the restaurant is not open today.

Pure hypothetical syllogism

If p, then q.
If q, then r.
Therefore, if p, then r.

Pure hypothetical syllogism example

• If I finish my work early, I’ll go to the gym.

• If I go to the gym, I’ll sleep better tonight.

• Therefore, if I finish my work early, I’ll sleep better
  tonight.

Disjunctive syllogism

Either p or q.
Not q.
Therefore, p.

Disjunctive syllogism example

• Either the meeting is on Zoom or it’s in person.

• The meeting is not on Zoom.

• Therefore, the meeting is in person

Necessary conditions

something that must be true for a specific outcome to
occur

• key point: it is a prerequisite—if it isn’t met, the conclusion cannot be reached

Necessary conditions example

Oxygen is necessary for fire. Without oxygen, fire cannot exist.

Sufficient conditions

One that, if met, ensures the outcome happens

• key point: its presence guarantees the conclusion is true, even though it might not be the only way to achieve the outcome

Sufficient conditions example

being a twin is sufficient to have a sibling, but you do not have to be a twin to have a sibling

Sound argument

A deductively valid argument that has true premises.

Cogent argument

A strong inductive argument with all true premises.