Talk:Necessity and sufficiency

message: I removed these sentences from the sufficient condition section "Necessary and sufficient conditions are therefore related. P is a necessary condition for Q just in case Q is a sufficient condition for P."  Does this make sense to anyone?
--Jeiki Rebirth 12:13, 9 June 2006 (UTC)
replies: text: I also find "just in case" to be a bit obfuscating.  "Push in case of emergency" means "If an emergency happens, then push".  "Just in case" is taking on the meaning "iff".  The way I under stand it, "(P sufficient for Q) iff (Q is necessary for P)"  Try it out with Q=thunder and P=lightning.--Rschwieb 17:04, 11 July 2006 (UTC)
thread_title:  "just in case", what? 

message: I removed the sentences mentioning causality because I felt that at their former location they interfered with the explanation, and I couldn't decide where to move it.  If someone really feels it's a vital part of the explanation, might I recommend them to retrieve the info from before my July edits and organize them in an independent section about "causality and it's relationship with necessary/sufficient conditions?--Rschwieb 01:57, 19 July 2006 (UTC)
thread_title:  Role (or lack thereof) of causality in Necessary/Sufficient conditions 

message: I've (Shawn Fitzgibbons 19:40, 25 November 2006 (UTC)
replies: text: I've removed the prod tag. If you want this deleted you should take this to AfD. Paul August ☎ 02:39, 26 November 2006 (UTC)

text: I think this is an important article.  Maybe the articles you cite here could link to this article, rather than redescribe necessary/sufficient? --Andy Fugard (talk) 07:13, 30 September 2008 (UTC)
thread_title:  Deletion tag 

message: Why does unnessesary link here? Shouldn't there be a page on it? —The preceding unsigned comment was added by Ootmc (talk • contribs) 23:26, 14 February 2007 (UTC)
thread_title:  Why is there not a page on unnessesary? 

message: I totally agree that this article is confusing.  I found this much
better description in a philosophy page from hong kong...(apparently open source)[ref]http://philosophy.hku.hk/think/meaning/nsc.php[/ref]:
The concepts of necessary and sufficient conditions help us understand and explain the different kinds of connections between concepts, and how different states of affairs are related to each other.
§ M06.1 Necessary conditions
To say that X is a necessary condition for Y is to say that it is impossible to have Y without X. In other words, the absence of X guarantees the absence of Y. A necessary condition is sometimes also called "an essential condition". Some examples :
Having four sides is necessary for being a square.
Being brave is a necessary condition for being a good soldier.
Not being divisible by four is essential for being a prime number.
To show that X is not a necessary condition for Y, we simply find a situation where Y is present but X is not. Examples :
Being rich is not necessary for being happy, since a poor person can be happy too.
Being Chinese is not necessary for being a Hong Kong permanent resident, since a non-Chinese can becoming a permanent resident if he or she has lived in Hong Kong for seven years.
Additional remarks about necessary conditions :
We invoke the notion of a necessary condition very often in our daily life, even though we might be using different terms. For example, when we say things like "life requires oxygen", this is equivalent to saying that the presence of oxygen is a necessary condition for the existence of life.
A certain state of affairs might have more than one necessary condition. For example, to be a good concert pianist, having good finger techniques is a necessary condition. But this is not enough. Another necessary condition is being good at interpreting piano pieces.
§ M06.2 Sufficient conditions
To say that X is a sufficient condition for Y is to say that the presence of X guarantees the presence of Y. In other words, it is impossible to have X without Y. If X is present, then Y must also be present. Again, some examples :
Being a square is sufficient for having four sides.
Being divisible by 4 is sufficient for being an even number.
To show that X is not sufficient for Y, we come up with cases where X is present but Y is not. Examples :
Loving someone is not sufficient for being loved. A person who loves someone might not be loved by anyone perhaps because she is a very nasty person.
Loyalty is not sufficient for honesty because one might have to lie in order to protect the person one is loyal to.
Additional remarks about sufficient conditions :
Expressions such as "If X then Y", or "X is enough for Y", can also be understood as saying that X is a sufficient condition for Y.
Some state of affairs can have more than one sufficient condition. Being blue is sufficient for being colored, but of course being green, being red are also sufficient for being coloured.
§ M06.3 Four possibilities
Given two conditions X and Y, there are four ways in which they might be related to each other:
X is necessary but not sufficient for Y.
X is sufficient but not necessary for Y.
X is both necessary and sufficient for Y. (or "jointly necessary and sufficient")
X is neither necessary nor sufficient for Y.
This classification is very useful in when we want to clarify how two concepts are related to each other. Here are some examples :
Having four sides is necessary but not sufficient for being a square (since a rectangle has four sides but it is not a square).
Having a son is sufficient but not necessary for being a parent (a parent can have only one daughter).
Being an unmarried man is both necessary and sufficient for being a bachelor.
Being a tall person is neither necessary nor sufficient for being a successful person.
Much clearer, maybe some ideas can be incorporated into this
article...
200.83.29.226 (talk) 14:50, 19 February 2014 (UTC)
thread_title: ==================

message: There's a fifty-fifty chance the misunderstanding is my fault and not the articles, but why in this world is the statement called S when defining necessary conditions, and N when defining sufficient conditions, particularly when the conditions are called N and S respectively? It results in both definition evaluating to the same string 'S implies N' although they should have, I presume, entirely different meanings. Would it be unreasonable for the statement to be defined by a single, not-S/N letter in both cases? Darryl from Mars (talk) 01:08, 26 July 2012 (UTC)
thread_title:  Definitions and letters 

message: Seems to me that the N(S) Venn diagram in the Relationships... section illustrates only ONE of the possible relationships between S and N.  The caption makes it sound as if that is the ONLY relationship.  In fact, the text directly next to the image states "... sufficient but not necessary".
This is highly confusing.  Am I missing something?
ScottHW (talk) 14:07, 21 August 2013 (UTC)
replies: text: I'm not sure what you're complaining about.  The diagram seems to accurately display the generic relationship between a necessary condition and a sufficient condition.  — Arthur Rubin  (talk) 18:15, 21 August 2013 (UTC)

text: This confused me also.  After puzzling for some time, I understand that N and S in the diagram actually represent statements (eg N="x is a mammal", S="x is human") and do not actually represent the relationships "necessity" and "sufficiency".  I changed the caption to be clearer, I hope. Krubo (talk) 22:26, 7 October 2013 (UTC)
thread_title:  Venn diagram is NOT representative 

message: "Example 1: In order for it to be true that "John is a bachelor," it is necessary that it be also true that he is: unmarried, male, and an adult, since to state "John is a bachelor" implies John has each of those three additional predicates." Should the list include human? If John is an unmarried adult male dog, that doesn't make him a bachelor. Alternatively you could replace 'is adult, male, and a human' with 'is a man.'OckRaz talk 19:02, 3 February 2014 (UTC)
thread_title:  Bachelor Example 

message: It seems to me that the train example in the image caption is actually flawed "That a train runs on schedule can be a sufficient condition for arriving on time (if one boards the train and it departs on time, then one will arrive on time)".  This is probably a poor choice of example trains and rail networks are complex systems with many variables a train which departs on time may still fail to arrive on time due to breakdown of the vehicle or delays on the network for many reasons.  I'm struggling to think of a better everyday example right now but thought I'd mention it in case someone has an idea. MttJocy (talk) 00:19, 25 July 2015 (UTC)
thread_title:  Train Example 

message: As MttJocy wrote above, the train example is quite a confusing one. I found a simpler example from Khan Academy (which can be used as a source). Here's the example:
"Boiling potatoes is a sufficient condition for cooking potatoes but is NOT a necessary condition for cooking them. This is because boiling potatoes is enough to cook them which makes this method sufficient. However, boiling potatoes is not a necessary condition to cook potatoes because there are other ways such as frying, grilling and baking that can cook potatoes as well."  — Preceding unsigned comment added by Ragzputin (talk • contribs) 20:39, 14 June 2017 (UTC)
thread_title:  An Alternative to the Train Example 

message: there's a problem with character spacing when an italicized uppercase*** letter ends a statement in double quotation marks. the letter and the first mark get smooshed together so instead of "if S then N", it looks like "if S then N', which could cause some confusion if an already confused reader thinks there is some new entity N prime. 
i.e. 
A true necessary condition in a conditional statement makes the statement true (see "truth table" immediately below). In formal terms, a consequent N is a necessary condition for an antecedent S, in the conditional statement, , , or . In common words, we would also say  or . For example, it is necessary to be Named, to be called "Socrates".
(highlighted sections)
i added a single space in the middle of the following highlighted source markup, in all 20 instances of this issue.
"N if S becomes "N if S
resulting in, "N if S" becoming, "N if S "
i know it's not syntactically correct, so if this really rubs someone the wrong way and they want to revert, i did it all in a single edit. 
***oh, i also put the space in example 5 of Necessity_and_sufficiency#Necessity where it's a lowercase x.
sensorsweep 04:36, 3 March 2015 (UTC)
thread_title:  italicized uppercase letter & double quotes. 

message: "Being a male sibling is a necessary and sufficient condition for being a brother"
No it isn't.
It's a necessary condition, because, in order to be a brother, you must be a male sibling. The second in implied by the first.
It is not a sufficient condition, because you can be a male sibling and not be a brother. For example, father and son are male, sibling, non-brothers.  — Preceding unsigned comment added by 81.156.217.128 (talk) 03:22, 9 September 2015 (UTC)
replies: text: What is your definition of a sibling? Bill Cherowitzo  (talk) 03:31, 9 September 2015 (UTC)
thread_title:  Necessary AND sufficient? 

message: It is a bit of a lacuna in the article that these are not treated. — Charles Stewart (talk) 07:24, 2 May 2016 (UTC)
thread_title:  Non-classical and modal logics 

message: While the phrase "necessary or sufficient" is grammatically correct, it is only mathematically correct because we use the inclusive "or" in mathematics. There is a certain duality present in these terms, and while they can hold jointly, holding separately does not make much sense. Indeed, the statement that "a condition is either necessary or sufficient" conveys, at least to me, a state of befuddlement and not much more. I see absolutely nothing to be gained by using "or" in this caption and it is really stretching the English language to use it in this way. --Bill Cherowitzo  (talk) 22:23, 27 July 2017 (UTC)
thread_title:  "And" or "or" in the caption 

message: As an engineer I've frequently found myself driven to use this phrase when criticising poor ideas from colleagues. For instance, in order to get a car to go above 60mph, 'go faster' stripes painted on the side are unnecessary and insufficient. It's usually an indication that they've totally misunderstood the problem to be solved. However, I fear the concept may be a little too jocular to include in this article. Apostrostomper (talk) 05:50, 31 March 2018 (UTC)
thread_title:  Unnecessary and Insufficient 

message: I came to this page looking for the history of the idea of looking at necessary and sufficient conditions.  I assume that the first to invoke it would have been Aristotle, Plato, or Socrates, but which? I've someone knows, I'd be grateful for a "History" subsection Enfascination (talk) 18:02, 8 May 2018 (UTC)
thread_title:  History of this definition of definitions 

message: In both sections for Necessary and Sufficient, they each have been defined to have the exact same logical relation: "The logical relation between them is expressed as "if P, then Q" and denoted "P ⇒ Q"." Yet, we know they have a different logical meaning from each other, thus there appears to be a conflict here.  — Preceding unsigned comment added by 68.99.65.50 (talk) 13:37, 23 June 2018 (UTC)
replies: text: In the above quote, the "them" refers to the statements P and Q, not the terms necessary and sufficient. These terms are used in discussing the implication, they aren't the implication itself.--Bill Cherowitzo  (talk) 17:53, 23 June 2018 (UTC)
thread_title:  Both Necessary and Sufficient Have Been Given the Same Logical Relation 

message: :Unfortunately, in the opening section you only have P and Q and speak about sufficient and necessary. 
:But if you would talk about P, Q, both causing S then everything would be clear.
:Necessary means we need it, but we may need others.  
:Sufficent means we don't need anything else.
:1. P is necessary but not sufficient:  If (P AND Q) then R.  
::We need P. (so P is necessary).
::But that's not enough. For R we need Q too. So having P is not sufficient. 
:2. P is necessary AND sufficient:  If P then R. 
::We need P. And we only need P. 
:3. P is not necessary but it is sufficient: If (P OR Q) then S.
::We don't NEED P. We can get S from Q.
::But on the other hand if we DO have P, P is enough in itself to create S. No need for anything else.
:4. P is not necessary AND not sufficient: If (P OR Q) AND R then S. 
::P is not necessary. We can get S from Q even though we do not have P. 
::And P is not sufficient because even if we do have P we still need Q. 
:5. P is not necessary AND not sufficient: If (P AND Q) OR R then S.
::P is not necessary. We can get S from R without P. 
::P is not sufficient. Even if we DO HAVE P, we still need Q to complete S. 
:פשוט pashute ♫ (talk) 02:40, 28 April 2021 (UTC)
thread_title:  A bad example