- Fundamentals: Introduction to Critical Thinking
- Introduction to Critical Thinking, Part 1
- Introduction to Critical Thinking, Part 2
- Fundamentals: Deductive Arguments
- Deductive Arguments
- Fundamentals: Abductive Arguments
- Necessary and Sufficient Conditions
- Instrumental vs. Intrinsic Value
- Implicit Premise
- Justification and Explanation
- Normative and Descriptive Claims
- Fundamentals: Validity
- Fundamentals: Truth and Validity
- Fundamentals: Soundness
- Fundamentals: Bayes' Theorem
- Fundamentals: Correlation and Causation
Paul Henne discusses the philosophical concept of validity. After reviewing the structure of an argument, he defines validity: an argument is valid if and only if its premises guarantee the conclusion. He reviews a few examples of validity and invalidity, and he leaves you with one example to figure out on your own.
Speaker: Paul Henne, Duke University.
Want to join the conversation?
- The last argument he gives is invalid, correct?(21 votes)
- Its invalid because the argument is "all fruit is a chair" but that does not mean "all chair is a fruit", there could me some chairs that are not fruit.(4 votes)
- Can an ampliative argument be valid? it is always invalid? Maybe we can not say annything about the validity of an ampliative argument...?(8 votes)
- Valid arguments are ones such that if the premises are true, then it is 100% certain that the conclusion is true.
Ampliative arguments are ones such that if the premises are true, then it is probable (less than 100% certainty) that the conclusion is true.
So, ampliative arguments are always invalid.
It is important to remember the invalid nature of ampliative arguments because it tells us to be cautious of the truth of the conclusion.(6 votes)
- Just to make sure that I'm getting it correctly, is it true that 'deductive arguments must be valid'?(5 votes)
- A deductive argument is one that claims the conclusion must be true, if the premises are true. If this claim is true, the argument is deductively valid. If this claim is false, the argument is invalid i.e. the conclusion can be false even if the premises are true.(3 votes)
- I'm having a hard time understanding validity....
But basically, if the argument is good (the premises supports the conclusion) then it is also valid, correct? An argument cannot be bad (premises does not support the conclusion) and still valid, right?(4 votes)
- Validity is part of what makes a good argument, but it is not the only thing. I think you're looking for the soundness of an argument, in which case the premise would need to be true.
For validity, it's not so much a matter of supporting the conclusion as it is of the conclusion never being false when the premise is true.
Remember that an argument can be valid even if the premise is false. And that can lead to some pretty ridiculous, but nonetheless valid arguments:
Either I am Jesus or I am a hamburger.
I am not a hamburger.
Therefore, I am Jesus.
This is perfectly valid, but obviously not a very good argument. Why? Because the premise is false. Now let's look at a sound argument:
No felons are eligible voters.
Some professional athletes are felons.
Therefore, some professional athletes are not eligible voters.
That's a good, sound argument because it is valid and its premises are true. Now let's look at one final case to see why validity still matters:
Grass is green
Paris is the capital of France
Therefore, a poodle is a dog
All of the premises are true, but this is an invalid argument. Even if poodles were horribly mutated squirrels, the premises would remain true--meaning they don't really connect. It's totally disconnected, in ways that even my crazy Jesus-hamburger example is still connected.(4 votes)
- So if I take P1 as sufficient to P2 and it is not, then I have an invalid argument? Analogously, if I take P2 as necessary to P1, but it is not, then I have an invalid argument too?(5 votes)
- I have 2 questions. First, is validity related to sufficiency? Throughout this video, I expected Paul to reference sufficiency, but he didn't. Second, at4:06Paul says to consider an argument with all false premises. P2 is that Irish Murdoch is a human. Iris Murdoch was, in fact, human. Does this mean that mixing true and false premises still produces a valid argument?(3 votes)
- 1)Validity deals with internal the proprieties of arguments. For a valid argument we supposing that if the premises are true - the conclusion must be true. For a non valid argument the truth of its conclusion does not follows from the truth of its premises.
As example lets take two arguments:
a)All cats are black. Tom is a cat. Thus, Tom is black.
b)All cats are black. Tom has a black tail. Thus, Tom is a cat.
An argument a) is valid, because, if we assume that it premises are truth, automatically the conclusion becomes truth also. In other hand, argument b) is non valid, because, even if it premises are truth, the truth of conclusion cant be derived from them certainly. Tom, ad example, can be a dog or a monkey.
The fact that Tom has a black tail is necessary but does not sufficient for being a cat.
Necessity and sufficiency both referring to the proprieties, conditions and relations between bodies, symbols, phenomenons, events e.t.c. Ad example for some process to happen now or before or in this time(rain) it is necessary or(and) sufficient to have some conditions in the past(clouds, low atmospheric pressure, wind, gradient of temperatures).
2)For better explanation I let myself to quote the definition of validity again: for a valid argument the truth of premises entails the truth of the conclusion. So even if the statements of premises are totally nonsense(all humans are immortal) holding them as truth and getting from the logical chain between premises and conclusion necessity of truth of the conclusion - yes, your argument is still valid.(1 vote)
- Can an argument have false premises and still be valid?(2 votes)
- This is very off topic but since youtube updated, this should also yet it is still the old youtube style. Why is that?(1 vote)
- Because Khan Academy most likely did not update YouTube on it's side.(3 votes)
- im cool look: Yes, you are correct that it is INVALID, because the two premises do make the conclusion true. Let me demonstrate with a similar example: P1) All dogs are mammals. P2) Charlie is a mammal. C) Charlie is a dog. This is obviously not valid, because Charlie could be ANY mammal. He does NOT have to be a dog. There are lots of things that are mammals, but are not dogs. The same is true for the example above. All fruit is a chair and square is a chair, but there could be other things besides fruit that are also chairs, like maybe vegetables and windows are also chairs. So, you cannot conclude that square is a fruit. Square could be a vegetable or a window or anything else that is a chair. Hope this helps. Good luck.(2 votes)
- The last example is invalid, correct?
But is this valid:
P1: All chair is fruit
P2: Square is a chair
C: Therefore, square is a fruit(1 vote)
- I believe that would be valid, since the truth of the premises would mean the truth of the conclusion.(2 votes)
(intro music) Hello, I'm Paul Henne and I'm a philosophy graduate student at Duke University. And in this video I'm going to discuss validity, an important tool for evaluating deductive arguments. You've probably heard someone say "that's a valid point," or maybe in an argument you've heard a friend say something like "that's valid, but..." In these everyday uses of the term "valid" or "validity," people often mean to convey something like "that's a good point," or "that statement's true." But I won't be talking, in this video at least, about those usages. Instead, I'll be discussing the technical philosophical notion of validity, as in "a valid argument." You already know that an argument is a set of statements, and that one or more of these statements is offered in support of some other statements. The statements doing the supporting are called "premises," and the statements being supported are called "conclusions." Validity, in the sense that I'm talking about it, applies to deductive arguments. So an argument is valid or invalid. Validity, then, isn't a property of statements or anything of the like. So, what exactly is a valid argument then? Well, suppose that you make the following argument, and here I'll use "P"s to stand for "premises" and I'll use a "C" to stand for the conclusion. (P1): All humans are mortal. (P2): Iris Murdoch is a human. (C): Therefore, Iris Murdoch is mortal. Suppose that I say that your argument is valid. Do I mean to say that your argument is good? Do I mean to say that your conclusion, or that all of the premises and the conclusion, are true? While this might sound like what I'm saying, validity has nothing to do with the truth of the conclusion or with how good the argument is in general. So, let's define it. An argument is valid if and only if the truth of its premises guarantees the truth of its conclusion. That is, validity is a property of arguments, such that if the premises of the arguments are true, then the conclusion must be true. So it's impossible for a valid argument to have all true premises unless the conclusion is also true. When an argument is valid in this sense, we say that the premises entail the conclusion. So, let's back up for a second. An argument is composed of statements. Statements can be true or false, like the statement "this square is orange." Arguments cannot be true or false. They can, however, be valid or invalid, as well as other things. And, if an argument is valid, then if its premises are true, its conclusion is true. Notice that I have not said that a valid argument has true or false premises or a true and false conclusion. I have said something conditional. That is, if the argument is valid, then the truth of its conclusion follows from the truth of its premises. Conversely, if the truth of the premises entails the conclusion, then the argument is valid. Now, this all sounds very abstract, so let's return to some examples. Let's look at our previous example. I have said that the argument about the British philosopher, Iris Murdoch is valid. Am I right? Yes! If the premises of the argument are true, then the conclusion must be true, in this case. Remember, it doesn't matter if our premises are true or false. Consider, for example, an argument with all false premises in it. (P1): All humans are immortal. Premise (2): Iris Murdoch is a human. Conclusion: Therefore, Iris Murdoch is immortal. This argument is also valid, just like the first argument. The truth of the premises entails the truth of the conclusion, right? If it is the case that all humans are immortal, and it is the case that Iris Murdoch is one of these humans, then it's necessarily the case that Iris Murdoch is immortal. Let's try an example with premises of which we don't know the truth. (P1): All aliens speak English. (P2): Splock is an alien. Conclusion: Therefore, Splock speaks English. We don't know if there are aliens, let alone ones that can speak at all. We don't know if they speak English. It could be the case, or it couldn't be the case. But this argument, nonetheless, is valid. If premise one and two are true, then the conclusion must be true. We could even use undefined terms. (P1): All sliff are splat. (P2): Sniff is a sliff. Conclusion: Therefore, sniff is a splat. Again, although the truth of the premises is undefined, we have a valid argument. This is just one type of valid argument form, and you can learn about others in upcoming videos. Note now what it means for an argument to be invalid. The truth of the argument's premises does not entail the truth of the conclusion. For instance: (P1): All dogs have fur. (P2): Claire has a lot of fur. Conclusion: Therefore, Claire is a dog. Now, it could be the case that all of the premises in this argument are true, but the conclusion false. The truth of this conclusion, in other words, does not follow from the premises, right? Because cats also have a lot of fur. So this is an invalid argument. You may wonder why validity matters at all, if the truth of the premises doesn't matter. This is a good question to ask, and it deserves a long discussion. But the short answer is that validity is used to determine whether or not an argument obeys valid inference rules, the laws of deductive logic. That is, we are ensuring that inferences in the argument are good inferences to make. I'll leave you with one last example, and ask you to determine its validity or invalidity. (P1): All fruit is a chair. (P2): Square is a chair. Conclusion: Therefore, square is a fruit. What do you think? Subtitles by the Amara.org community