Question 1

The last argument he gives is invalid, correct?

Accepted Answer

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.

Question 2

Can an ampliative argument be valid? it is always invalid? Maybe we can not say annything about the validity of an ampliative argument...?

Accepted Answer

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.

Question 3

Just to make sure that I'm getting it correctly, is it true that 'deductive arguments must be valid'?

Accepted Answer

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.

Question 4

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?

Accepted Answer

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.

Question 5

I have 2 questions. First, is validity related to sufficiency? Throughout this video, I expected Paul to reference sufficiency, but he didn't. Second, at 4:06 Paul 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?

Accepted Answer

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.

Question 6

This is very off topic but since youtube updated, this should also yet it is still the old youtube style. Why is that?

Accepted Answer

Because Khan Academy most likely did not update YouTube on it's side.

Question 7

Should we discourage our friends from saying "valid" in the colloquial sense?

Accepted Answer

I feel it doesn't do any harm to use it in the regular sense, just preface the philosophical use of "valid" by saying that's what you're going for and everyone gets what they want.

Course: Wireless Philosophy > Unit 1

Fundamentals: Validity

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