Critical thinking

1. CRITICAL THINKING
• EVALUATING ARGUMENTS: TESTING FOR “VALIDITY” AND FOR “TRUTH”

2. IMPORTANT NOTE!
• For the best learning experience with this PowerPoint presentation, do the
following:
• FIRST: Click the “Slideshow” tab above, and Click “From Beginning.
• This is helpful, because you’ll have the option to work through this presentation’s short
examples and exercises BEFORE seeing the answer; and then click to see the answer after
you’ve done the short examples and exercises on your own
• SECOND: ALSO please watch my lecture video working through this PowerPoint.
• That video contains extra, more detailed explanations of the ideas here that are essential to
fully understanding this reading.

3. WHY WOULD WE WANT TO “EVALUATE”
ARGUMENTS?
• So far, we’ve mostly been talking about “analyzing”
arguments.
• In other words, we’ve been practicing how to find
the different PARTS of an argument
• Remember, an argument has TWO PARTS.
• Do you remember what the two parts of an
argument are?
• Take a moment to think of them, and then click
when you’re ready.
• The two parts of an argument are:
• The conclusion: the main point someone is trying to
convince you of
• … and the premises: the supporting reasons someone
gives to try to prove the conclusion is true
• But ultimately, we really want to do MORE. We don’t
want to just see the PARTS of other people’s arguments.
• We want to be able to tell when those arguments are
GOOD and BAD! We want to know when we should
BELIEVE another person’s arguments … and when we
should NOT believe their arguments!
• In other words, we need to “evaluate” arguments—
analyzing whether arguments are “good” or “bad”

4. BRIEF OVERVIEW OF “EVALUATING ARGUMENTS”
• There are two basic “rules” that every argument needs to follow to be a good
one.
• An argument needs to be “valid”
• It must have a good structure, in which the premises are relevant to proving the
conclusion
• In other words, if the premises turn out to be true, they should therefore support and
prove the conclusion is true.
• An argument needs true premises—it must have “truth”
• In other words, its premises must actually be true
• In this unit I’ll talk a LOT about both, especially validity (which is a little trickier).

5. ISN’T IT ALL SUBJECTIVE? HOW CAN YOU EVER
TELL IF AN ARGUMENT IS GOOD OR BAD?
• OK, so you might be saying … “Wait, isn’t it all subjective? Maybe YOU think an
argument is good, and I think an argument is bad—maybe it’s different for
everyone!”
• OK, but here’s the thing. You don’t actually think that.
• Actually, it’s obvious that SOME arguments are good ones and SOME arguments are
bad ones.
• And there are RULES for what make arguments good or bad! We’ll talk about those
rules a lot this time.
• The rules are called “validity” and “truth”

6. LET’S START WITH VALIDITY

7. INTRODUCING VALIDITY
• Here’s an example. This will show that some
arguments are good, and some are bad—and
EVERYONE can tell the difference if they can think
about it in the right way.
• Here’s an argument. Tell me if it’s good or bad.
My son Walt likes to eat bananas. Also,
chimpanzees like to eat bananas. Therefore, my
son must be a chimpanzee.
• Is this argument a good one, or a bad one?
• Think a moment, and continue when you’re ready.

8. GOOD ARGUMENT OR BAD ARGUMENT?
• So you probably said that that was NOT a good
argument.
• And you’re right! That was a really BAD argument!
• Just because my son Walt likes bananas doesn’t
make him a chimpanzee. Lots of things like bananas.
• You can start to glimpse that there are RULES about
what makes an argument good or bad. And it’s clear
that this argument “breaks” the rules.
• Let’s talk about why.
• Let’s look at the argument again, and see why it’s so
bad. In other words, let’s “evaluate” the argument.
• To evaluate an argument as “good” vs. “bad,” you
FIRST have to find its different parts.
• Here’s the argument again:
• My son Walt likes to eat bananas. Also, chimpanzees
like to eat bananas. Therefore, my son Walt must be a
chimpanzee.
• Can you find the conclusion and premises?
• Take a moment, and click forward when you’re ready.

9. WHY IS THIS A BAD ARGUMENT?
• Here are the premises and conclusion
Conclusion: My son Walt must be a chimpanzee.
Premise 1: My son Walt likes to eat bananas.
Premise 2: Chimpanzees like to eat bananas.
• OK, we’re ready to evaluate. What went wrong in this
argument? Why is this a BAD argument?
• The first thing to see is that the premises—the supporting
reasons—are actually all TRUE. My son Walt actually loves
bananas. And chimpanzees really love bananas too.
• So the problem isn’t that the premises themselves are
wrong or false.
• So WHAT THE HECK IS GOING ON HERE?
• The problem isn’t with the premises being false. The
problem is with the STRUCTURE OF THE ARGUMENT ITSELF.
• The problem is that the premises aren’t relevant enough to
prove the conclusion. In other words, those premises
don’t actually prove what they claim to prove—they
can’t prove that Walt is a chimpanzee.
• Here’s what I mean.
• Just because Walt likes bananas (premise 1), and
chimpanzees like bananas (premise 2), doesn’t prove Walt IS
a chimp!
• Lots of things eat bananas. Not ONLY chimps. Therefore,
this is a bad argument.
• The technical word for this problem is: this argument lacks
“validity”. It is “invalid.” Let’s talk about what that is.

10. WHAT IS “VALIDITY”, AND WHY MUST AN
ARGUMENT BE “VALID”?
• Validity means having premises that—if true—could
actually support and prove the conclusion.
• In other words: a valid argument is one in which “if
the premises (reasons) are true, then the conclusion
would have to be true as well.” (Chaffee 2017, p. 432)
• An invalid argument is one in which “the reasons
do not support the conclusion so that the reasons
could be true but the conclusion could be false.”
• This just means: valid arguments have a GOOD
STRUCTURE. The premises, if true, “link up with” and
prove the conclusion is true.
• It’s helpful to think about this using a visual example.
• Think about arguments like a temple that looks like
this.
• In a GOOD argument, the pillars should connect to, and
“support,” the roof.
• In other words, the premises should support the
conclusion
Conclusion
Premises

11. IN OTHER WORDS
• Think about our example argument.
• Walt likes bananas; chimpanzees like bananas;
therefore, Walt is a chimpanzee.
• The problem is that the premises just AREN’T
RELEVANT to proving the conclusion. They can’t
prove what they claim to prove.
• So this is a building that looks like this:
Conclusion
Premises
• In other words, the premises—the “pillars”—don’t
connect up with, and “support,” the conclusion—the
roof.
• That’s what we mean when we say invalid
arguments do not have a good structure.

12. OK, SO HOW WOULD BE MAKE THIS ARGUMENT A
“VALID” ONE?
• Remember, a “valid” argument is one in which the
premises are actually relevant for proving—they
“support”—the conclusion.
• So how could we make this argument about Walt and
chimpanzees a VALID one?
• Take a moment to think about your own answer. Click
ahead when you’re ready.
• The answer might look like this.
• We can make this a valid argument IF WE CHANGE THE
CONCLUSION
• “Walt likes bananas. Chimpanzees like bananas.
Therefore, Walt and chimpanzees have something in
common.”
• Now the argument looks like this:
• Conclusion: “Walt and chimpanzees have something in
common.”
• Premise 1: Walt likes bananas.
• Premise II: Chimpanzees like bananas.
• Now, the premises ARE relevant for proving the conclusion!
Now the roof (the conclusion) IS supported by the pillars
(the premises)!
Conclusion
Premises

13. SOME MORE TO REMEMBER.
• A valid argument has NOTHING to do with
whether or not the premises are true.
• When you’re asking about validity, you’re
ASSUMING the premises are true.
• You test the truth of the premises LATER. You
test the validity of an argument FIRST.
• All you’re asking is: “IF the premises turn out to
be true … does the argument have a good
structure? Are its premises relevant to proving
the conclusion? In other words, if they’re true,
would the premises actually prove that the
conclusion is true?”
• Here’s an example.
• Is it valid? Yes or no?
All fish are purple; and Jerry is a fish.
Therefore, Jerry is purple
Where are the premises? Where is the
conclusion?
Why is this valid?
Try this out for yourself. Click ahead when you’re
ready.

14. THIS IS A VALID ARGUMENT.
• Of course, the premises aren’t TRUE.
NOT all fish are purple.
• But that DOESN’T MATTER AT ALL when
it comes to “validity.”
• The structure of this argument looks like
this:
• Remember, a valid argument has NOTHING to do with
whether or not the premises are true.
• Here’s how we’d analyze this argument.
• Conclusion: Jerry is purple.
• Premise I: All fish are purple.
• Premise II: Jerry is a fish.
• Remember, to test for “validity”—to test the “structure” of
an argument—you first ASSUME all the premises are true.
• ASSUME all fish are purple (premise 1); and ASSUME Jerry
is actually a fish
• That would be enough to prove that Jerry is purple!
Those premises are relevant to proving the conclusion is
true!
Conclusion
Premises

15. LET’S DO SOME MORE EXAMPLES TOGETHER

16. This politician makes me feel good.
This politician makes me laugh.
Therefore, this politician is telling
the truth.
• Find conclusion and premises.
• Then, see if it’s valid.
• Click ahead when you’re ready.
The defendant is obviously guilty. We have no
idea where she was on the night of the murder,
and plus, she’s homeless.
Conclusion: This politician is telling the truth.
Premise I: This politician makes me feel good
Premise II: This politician makes me laugh.
Valid or invalid?
This argument is INVALID. Assume the premises
are true. Assume this politician DOES make me
feel good (prem. 1). Assume this politician DOES
make me laugh (prem. 2). But that’s STILL not
enough to prove that this politician is telling the
truth.
EXAMPLE 1
EXAMPLE 2
• Find conclusion and premises.
• Then, see if it’s valid.
• Click ahead when you’re ready.
Conclusion: The defendant is obviously guilty.
Premise I: We have no idea where she was on the
night of the murder.
Premise II: Plus, she’s homeless.
Valid or invalid?
This argument is INVALID. Assume the premises are true. Even
if we don’t know where the defendant was on the night in question;
and even if she IS homeless—this is NOT enough to prove that
the defendant committed murder.

17. VALIDITY, A REMINDER …
• To tell whether or not an argument is VALID is all about the structure of the
argument itself
• Assume the premises are true; do they lead us to the conclusion?
• To test validity, you do NOT need to know ANYTHING
• You do NOT need to do RESEARCH AT ALL.
• You do NOT need to be an EXPERT
• … because you are NOT judging whether or not the facts are true!

18. VALIDITY, A REMINDER …
• Let’s do more examples …

19. All actors are robots, and Tom
Cruise is an actor. So Tom Cruise
must be a robot.
• Find conclusion and premises.
• Then, see if it’s valid.
• Click ahead when you’re ready.
All actors are robots, and Tom Cruise is a
robot. So Tom Cruise must be an actor, too.
Conclusion:
Premise I:
Premise II:
Valid or invalid?
EXAMPLE 1
EXAMPLE 2
• Find conclusion and premises.
• Then, see if it’s valid.
• Click ahead when you’re ready.
Conclusion:
Premise I:
Premise II:
Valid or invalid?

20. The Eagles are one of just a few of
the best team in the history of
football. That’s because they have
an astonishing quarterback, and
they are Super Bowl champions.
• Find conclusion and premises.
• Then, see if it’s valid.
• Click ahead when you’re ready.
Bosses should always answer to their
employees. That’s one reason why employees
should vote for all of their bosses. And what’s
more, bosses should also always be ready to
step down whenever they can’t perform their
job well.
Conclusion:
Premise I:
Premise II:
Valid or invalid?
EXAMPLE 1
EXAMPLE 2
• Find conclusion and premises.
• Then, see if it’s valid.
• Click ahead when you’re ready.
Conclusion:
Premise I:
Premise II:
Valid or invalid?

21. The soul of humans can’t be seen,
touched, or smelled. It is noumenal,
and therefore completely unlike
anything physical. In addition, nothing
noumenal can die. Moreover, the soul
is not from this physical realm, but
rather was called into being by an
eternal, all-loving God that would never
let the soul perish. Therefore, the soul
cannot die.
Anything that is not a threat to our health
should be illegal. Marijuana, for example, is
not a threat to our health. So it is a threat to
some of our most basic freedoms as
Americans to make marijuana illegal.
Conclusion:
Premise:
Premise:
Premise
Valid or invalid?
EXAMPLE 1
EXAMPLE 2
• Find conclusion and premises.
• Then, see if it’s valid.
• Click ahead when you’re ready.
Conclusion:
Premise I:
Premise II:
Valid or invalid?

22. OK, LET’S TALK ABOUT “TRUTH”
• The first rule for evaluating an argument: an argument must have “validity.”
• The SECOND rule for evaluating an argument:
• The premises must actually be true.
• Are the premises ACTUALLY TRUE? Do they get the facts right?

23. OK, LET’S TALK ABOUT “TRUTH”
• Example:
All fish are purple; Jerry is a fish; therefore, Jerry is purple.
• Is this a TRUE argument?
• Answer for yourself; click ahead when you’re ready.
• NO. Not all the premises are true. Premise I is false. All fish are NOT purple.
• Therefore, this argument lacks all true premises!

24. TO TEST “TRUTH”
• To test the truth of an argument, GO OUT AND FIND THE FACTS FOR YOURSELF!
• Here’s an example.
• Someone says to you:
You know, you shouldn’t vote for X. That’s because they’re violently racist and
homophobic.
• Let’s work on this example together.

25. You know, you shouldn’t vote for X.
That’s because they’re violently racist
and homophobic.
• Find conclusion and premises.
• Then, see if it’s valid.
• Click ahead when you’re ready.
Conclusion: You shouldn’t vote for X.
Premise I: They’re violently racist and homophobic.
Validity
Valid or invalid?
This argument is VALID. Assume the premise is
true—assume that X really is racist and
homophobic.
It’s a natural conclusion that we should not vote for
X.
EXAMPLE 1
Truth
OK … but is the premise actually true?
Well, we don’t know yet!
We need to go out and research on our own!
Find reliable facts to see if the premises are
true!

26. SUMMARY SO FAR …
Validity
• All about how good the
“structure” of the argument is
• IF the premises are all true,
would the conclusion therefore
be true?
Truth
• Not about the structure
• All about whether or not the
premises are true
• Are they accurate? Do they have
enough information? Are they well-
researched and reliable?

27. A “SOUND” ARGUMENT
• Arguments must have BOTH validity AND truth
• When they do, we call them “sound” arguments.
• But we should reject arguments if they lack EVEN ONE.
• So for example, if an argument is valid, but has false premises, we REJECT the
argument as “unsound.”
• And if an argument is invalid, but has true premises, we REJECT the argument as
“unsound”

28. SUMMING UP
• In this unit we covered:
• Why it’s important to be able to evaluate others’ arguments
• What it means for an argument to be “valid” or “invalid”
• What it means for premises to be “true” or “false”
• What it means for an argument to be “sound” or “unsound”