Necessary And Sufficient Conditions

[steve_bank]

Philosophical jargon usually goes in one ear and out the other for me, but someone is using the terms regularly.

Seems straightforward enough.

All Zogs are red and are five feet tall. Then it is a necessary condition to be red to be a Zog, but not a sufficient condition. There are other qualifiers.

Correct or am I missing something?

Confusion of Necessary with a Sufficient Condition

Confusion of Necessary with a Sufficient Condition

A causal fallacy you commit this fallacy when you assume that a necessary condition of an event is sufficient for the event to occur. A necessary condition is a condition that must be present for an event to occur. A sufficient condition is a condition or set of conditions that will produce the event. A necessary condition must be there, but it alone does not provide sufficient cause for the occurrence of the event. Only the sufficient grounds can do this. In other words, all of the necessary elements must be there.

Examples:

Juan: "How do you think you'll do on our philosophy exam tomorrow?" Monique: "Great, I read all the books." Juan: "Yeah but do you understand this stuff?" Monique: "I said I read all the books, didn't I?"

Don't let all the talk about the necessity of exercise to a long life mislead you. Jim was a jock all his very short life.

Who said food keeps us alive? Tom died a few days ago and he was not short of good food.

I don't know why the car won't run; I just filled the gas tank.

Why don't you want to spend your life with me? I love you, and am I not good to you?

The counselor told me that if I wanted to graduate I must have at least 128 credit hours. \The job description said that they were looking for someone with a Master's degree. I've got my MA, but I cannot understand why they did not hire me.

My high school English teacher told me successful people have extensive vocabularies. So, I've tried to learn a new vocabulary word every day since then. I should be a successful person soon.

 

[pood]

Right, all those examples show necessary conditions have met but not sufficient ones.

As to Zogs, either red or five feet tall are necessary but not sufficient. Both must be present.

But there is a further problem. Suppose you observe a thousand black swans and conclude all swans are black — that being black is a necessary but not sufficient condition for being a swan. Then one day you spot a white swan. What — it’s not a swan? Hume’s Problem of Induction.

 

[Swammerdami]

The logic statement

• If A then B

is equivalent to

• A is a sufficient condition for B

and is also equivalent to

• Not_A is a necessary condition for Not_B

However

All Zogs are red and are five feet tall. Then it is a necessary condition to be red to be a Zog, but not a sufficient condition.

Correct or am I missing something?

What you are missing is that the reddened clause is AMBIGUOUS. It isn't clear whether you intend
* "Then a necessary condition to be red is to be a Zog."; or
* "Then being red is a necessary condition to be a Zog."
I am NOT trying to be pedantic. Looking at the reddened clause in isolation I wouldn't be sure which you intend.

 

[steve_bank]

Right, all those examples show necessary conditions have met but not sufficient ones.

As to Zogs, either red or five feet tall are necessary but not sufficient. Both must be present.

But there is a further problem. Suppose you observe a thousand black swans and conclude all swans are black — that being black is a necessary but not sufficient condition for being a swan. Then one day you spot a white swan. What — it’s not a swan? Hume’s Problem of Induction.

I don't see it as a problem in logic per se. My response to that is a logically valid argument, no logical fallacies, does not guarantee physical truth.

One would preface the concussion by saying something like based on my survey all swans appear to be black.