Talk:Necessity and sufficiency

In the previous version, the section on sufficient conditions had reversed which is the condition and which is the thing obtained. If you do it this way, it's hard to tell at a glance the main difference between necessary and sufficient: necessary is obtained-->condition, while sufficient is condition-->obtained. So I've switched Q and P so that one can see that. (this comment is hard to understand, but look back at the old version and you'll see) Motorneuron 21:08, 30 April 2006 (UTC)

It appears to me that you're rigidly applying "P is always a condition" and "Q is always the obtained". I think I understand this change, but I think this method is confusing also. If one misses the beginning phrase of the final statment in both sections, everything after looks identical and it's unclear what's going on. I favor preserving the order of the letters used (A sufficient for B, A necessary for B), and in that way one can see how the implication reverses direction between the two of them (B-->A, for necessity, A-->B for sufficiency).

The way I've heard the description of necessary/sufficient goes something like this: (A necessary for B) iff (Not A implies Not B). Note that this is logically equivalent to (B implies A)

Also, (A sufficient for B) iff (A implies B).

By preserving the order of A and B in the statements like this, when we get to "A is necessary and sufficient for B" it's immediately clear that "A-->B and B-->A". I plan on making this change in the article soon.--Rschwieb 17:37, 11 July 2006 (UTC)

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is this original work? for such a complex subject, that was sure rattled off quickly. had you already written it somewhere else? Kingturtle 03:12 Apr 18, 2003 (UTC)

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Previous version said:

[necessary and sufficient] should therefore not be too quickly conflated with iff

Correct me if I'm wrong, but I don't think this is correct. Necessary and sufficient is the same as iff, though neither are the same as logical equivalence. Evercat 23:40, 16 Nov 2003 (UTC)

I agree with Evercat. Can somebody explain that sentence? Otherwise I'm going to delete it. Timtzeptel 17:56, 14 Apr 2004 (UTC)

I agree as long as we're restricing this to mathematics and logic, where causality is not an issue. I'm slightly less certain in cases involving events occurring rather than propositions being true. But this locution "neither are the same", rather than "neither is the same" is weird. Michael Hardy 18:16, 14 Apr 2004 (UTC)

Examples used

Can someone rework this article with better examples? In the article itself are three different comments about how a particular example is confusing, and the smoke/fire example requires a footnote. I'm bad about thinking up "real-life" examples, but I'll give it a shot in a few days if no one else comes up with anything. Pagrashtak 5 July 2005 20:12 (UTC)

How about clouds and rain? 217.132.211.9 6 July 2005 05:40 (UTC)

I rewrote the example using thunder and ligthening. This may also be slightly confusing, but I understand the intent of using the confusing example of smoke and fire. It is to emphasize that causation and time ordering are not part of the definition of "necessary". I made this more explicit. Removed the stupid footnote. Let me know what you think of these changes.--Jeiki Rebirth 10:44, 9 June 2006 (UTC)

The thunder and lightening is a very poor example. Thunder and lightening are effects of the same physical phenomenon. To say one causes the other in any ordering will require a paragraph expalining eletrical current running through the air and how the two effects are produced. This example only serves to complicate the logical concept.

The Fourth of July example is logically flawed -- I would suggest that it is necessary but not sufficient that it be the Fourth of July for it to be Independence Day in the United States. If the year is 1730, for instance, there was a Fourth of July but not an Independence Day. Similarly, if an act of the United States Congress were to change the date for the celebration of Independence Day to, say, July 2 (as originally proposed by John Adams), then some future July Fourth could be July Fourth but not Independence Day. K95 18:05, 6 September 2006 (UTC)

Well then so is the thunder and lightning. If lightning strikes on another celestial body or in Earth's past/future where there is no atmosphere, there will not be any thunder. Similarly for card-based examples later - historically, or today in other countries, different suits were used.

Basically any examples we come up with will have the assumption (whether or not stated) that the situation is present-day Earth (and maybe some more specifics). Defining the situation/environment is inevitably required to use logic in all but the most abstract of fashions. (And even then, you need to define your notation at the least). 128.232.228.174 (talk) 08:59, 20 May 2008 (UTC)

"just in case", what?

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)

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)

Role (or lack thereof) of causality in Necessary/Sufficient conditions

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)

Deletion tag

I've (Shawn Fitzgibbons 19:40, 25 November 2006 (UTC)) nominated this article for deletion because it is redundant. There are already several articles that cover this topic. Here is a short list:

propositional logic
material conditional
logical implication

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)

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)

Why is there not a page on unnessesary?

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).

This page could use a lot of improvement

I think that as is, this article is too confusing to be useful to someone who is trying to understand the concept of necessary and sufficient conditions. Norman Swartz's article (in the external links page) is much clearer and simpler. I like how he makes it clear right away that necessary conditions are not the same thing as sufficient conditions, and vice versa.

One serious problem is the use of the language of implication in the explanations. If someone doesn't know what N/S conditions are, they probably won't know what implication is either. You can teach someone N/S conditions without mentioning implication.

Sorry I'm too lazy to fix anything at the moment. I thought I should point out the problems though. If the spirit moves me I will come do some editing. Manderr (talk) 10:02, 27 March 2008 (UTC)

The mathmatics section is horribly unclear. I have no idea what it's supposed to mean, and I've taken some legit Philosophy department logic classes :(Floodo1 (talk) 08:23, 17 December 2008 (UTC)

Yes, this article is seriously important for ordinary usage of the terms, which is more in line with philosophy than the specialist maths examples. The value of an example is its ability to make something complex more specific and clear. Logic comes before Group Theory in pure mathematics, the examples are not really valuable even to mathematicians.

I hope the pictures and captions I added help. I think people will understand the usefulness of the ideas better if they see examples of each kind of condition, where it is important that the other kind of condition would not be the right description.

Mate, I'm swamped with other work, would you care to add some more simple, clear text to the article. Go right to the top and work on the lead. Don't delete anything, then no one can complain. But good simple clear text might lead others to have confidence the more tricky stuff isn't really needed.

There's some links that could help. Stanford Encyclopedia of Philosophy is excellent on topics like this. Without breaching copyright, following their example might give you plenty of ideas. Make this a holiday project mate! I'll cheer from the sidelines. ;) Just do it! Alastair Haines (talk) 23:08, 17 December 2008 (UTC)

I know this thread is a bit old, but I must agree with Manderr. I consider myself a reasonably intelligent person (although more on the liberal arts side), but I still have only a very foggy idea of the difference between a necessary and sufficient condition after reading this article several times.... Lithoderm 02:05, 18 September 2010 (UTC)

Title discussion

I don't believe that the recent title change really reflects anything helpful to the reader. "Necessity" and "sufficiency" are themselves used in many different contexts, and I think that without some sort of qualifier of logic, the purpose of the article is unclear. MSJapan (talk) 19:23, 11 February 2012 (UTC)

The article is about the distinct, but related, concepts of 'necessity' and 'sufficiency', and not about 'necessary and sufficient conditions', which is only one of the four basic types: necessary and sufficient, necessary but not sufficient, sufficient but not necessary, and neither necessary nor sufficient. So, although the current title uses a phrase which may be used in other contexts – I would argue, however, that the logical concept is the primary meaning – the former title was misleading.

As for concerns of clarity, I don't think that disambiguation is needed at this time since we don't seem to have another article with a similar title. We generally do not disambiguate preemptively (e.g., we have Leroy Shield and not Leroy Shield (composer)) and the article's lead – "In logic, ..." – makes clear the scope of the article for anyone who is unfamiliar with the topic. -- Black Falcon ^(talk) 19:58, 11 February 2012 (UTC)

Definitions and letters

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)