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What is the logical difference between something being necessary in order for something else to be true; as opposed to something being sufficient to make something else true. i.e.

Fuel is sufficient to make an internal combustion engine run.

vs

Fuel is necessary to make an internal combustion engine run.

and what are some subtle examples of how the difference between these two things can greatly impact the meaning of a sentence, discussion, or conclusion.

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    Isn't this really a language question for english.SE? – Lennart Regebro Jun 7 '11 at 20:51
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    The difference between these concepts is important to logical thinking; it would be good if someone could offer a nice clear answer to this. – eMansipater Jun 7 '11 at 20:52
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    In my experience of people learning about philosophy, this topic invariably comes up; and it is necessary to have a clear and concise answer to it which seemed precisely like the What is a "straw man" argument? example question from the commitment phase. The vast majority of people do not understand the logical difference between these two concepts at all. – eMansipater Jun 7 '11 at 21:30
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    I surprised to find this closed as "off-topic". It's a fairly fundamental question to the philosophy of science. Perhaps it would be a better question if it explored some of the edge case. The question that inspired this one (philosophy.stackexchange.com/questions/3/…), for instance, does explore the question of what count for a scientific explanation. – Jon Ericson Jun 10 '11 at 19:11
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    The question is fairly basic and the example for necessary conditions is poor. But, the distinction between necessary and sufficient conditions is one that most philosophy students get exposed to early on and with good reason. It is clearly on topic; I voted to reopen. – vanden Jun 11 '11 at 6:10
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The difference between "necessary" and "sufficient" is the direction of the logical arrow.

If you have A is sufficient for B it means that every time you have A you will have B, without exception:

A ⇒ B

If you have A is necessary for B it means that every time you have B you will have A, without exception

A ⇐ B

So as an example of A being sufficient for B, it is correct to say that every time you (successfully) kill someone, they will be dead, and the assertion that "Person X being killed is sufficient for Person X being dead" would be true. By contrast, it is not correct to say that every time someone is dead, it is because they have been killed. They could have died of natural causes, or there could have been some sort of accident. So the assertion "Person X being killed is necessary for Person X being dead" would be false.

This page has an excellent example of how the difference between these two concepts can change your conclusion. In his answer to the famous Seven Bridges of Königsberg problem, Euler demonstrated that in order to walk across each bridge exactly once it is necessary that the number of places with an odd number of bridges is either 0 or 2. Or put another way, it is necessary that a graph have either 0 or 2 nodes with an odd number of edges for you to be able to draw it without lifting your pen from the paper. However, this is not sufficient to ensure that such a walk or drawing is possible:

Either 0 or 2 places have an odd # of bridges ⇐ You can walk across each bridge once is true

but

Either 0 or 2 places have an odd # of bridges ⇒ You can walk across each bridge once is NOT!

So the difference between the two matters very much. Can you think why? Hint.

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    "So in the first example, it is correct to say that every time you kill someone, they will be dead, and the assertion is true" - This is not true for Jesus. – Billy Rubina Jun 29 '12 at 2:22
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    yes it is true for Jesus - he was dead after having been killed. Being dead is necessery for resurrection. – artm Oct 27 '13 at 10:46
  • Perhaps in math you can reverse the arrow. This is not standard in philosophy. Your answer does not address what propositions are and you only take a practical approach which does not hold 100 percent. – Logikal Apr 26 '18 at 1:33
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Just to add to the answers given, I would like to give a slightly different way to arrive at the same conclusion. My background is in mathematics, and so my answer might seem amateurish from a philosopher's or logician's perspective.

Sufficiency

I'll start with sufficiency, since it is easier. This answer is the same as those given. If p is sufficient for q, then, p is enough to have q. That is, if we have p, we have q, or

p ⇒ q.

Necessity

The statement "p is necessary for q," means, we must have p to get q. In other words, if we don't have p, then we can't have q. In symbols,

~p ⇒ ~q,

where I use ~ to denote negation. The logical equivalent, called the contrapositive, of this is

q ⇒ p.

Necessity and Sufficiency

Thus, p is necessary and sufficient for q means

p ⇔ q.

Another way this is commonly written is

p if and only if q.

It is sometimes abbreviated as

p iff q.

Example of confusing a claim with its converse

Consider the claim "All dogs go to heaven." This means

you are a dog ⇒ you go to heaven.

That is, being a dog is sufficient for entrance to heaven. The necessity statement, its converse, is this:

you go to heaven ⇒ you are a dog.

It's possible that you might be a cat in heaven, and so this is not true. The only way this could be true is if there is a law that requires all dogs and only dogs to enter heaven. That is, being a dog is necessary to enter heaven.

This is also sometimes called confusing cause and effect. There is more explanation and examples on this website:

http://www.nizkor.org/features/fallacies/confusing-cause-and-effect.html

  • Your necessity is incorrect or misleading. The conditional q-->p does not express q is impossible without p. This is a may it is or maybe it isn't so I can't tell statement. You need a better way to write & express you can not have q without also having p. – Logikal Apr 27 '18 at 22:29
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    I'm not sure I understand. I read your answer, which I think says the same thing. I take p ⇒ q to be the same as ~p v q. I think this is the conventional meaning for p ⇒ q? In your answer, "a is necessary for b" is ~(b & ~a). "This is read as it is impossible to have b and also not have a." By De Morgan's laws, this is ~b v a, or b ⇒ a. – Anthony Pulido Apr 29 '18 at 21:26
  • The context in real human language does not express the same things. What you express is that in mathematical logic the truth tables are equivalent. Real language is not based on truth tables. I am not saying it is not possible to do so but this is not frequent. – Logikal Apr 29 '18 at 21:46
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Sufficient is an upper bound. Necessary is a lower bound.

Stopping the heart for X minutes (I don't know how long it needs to be to be legally dead) is necessary to make someone dead. (Whatever method you use to cause someone to be dead, it results in their heart stopping. If their heart does not stop as a result of the action the action has not made them dead.

Setting off a 100 megaton warhead next to someone is sufficient to make them dead. It does not require nearly this much to make someone dead, but if you do this they will certainly be dead.

There is room for error in both cases. you could decapitate someone but keep their heart beating through some artificial means, and you could theoretically devise some sort of clothing in some distant future that could protect against a nuclear blast but the situations in which these exceptions occur are bizarre and unlikely enough that we can effectively dismiss them. Otherwise we must accept that nothing is sufficient nor necessary to make someone dead.

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    It is, of course, debatable whether someone can be "alive" with or without a heartbeat. Many in the philosophic tradition would argue that consciousness is what makes us truly "alive". And with that definition, it might well be the brain that must be dead in order for the body to be pronounced deceased. But putting quibbles aside, the point made in your first sentence is absolutely correct. – Cody Gray Jun 9 '11 at 14:32
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Necessity excludes other methods or possibilities, sufficiency does not. To respond to your example, though I find your use of the phrase "make them dead" uncomfortable:

It is not necessary to kill someone to make them dead. You could incite someone else to kill them, and that would still be you starting a chain of events that "made" them dead. It is sufficient to kill someone to make them dead, as in killing them you've "made" them dead immediately.

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Water is necessary to bake bread. Without water, you can't make bread. But water alone is not sufficient to make bread. You want to have some powder, too. And an oven, or at least a campfire.

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Maybe this help: [Definition: A necessary condition for some state of affairs S is a condition that must be satisfied in order for S to obtain.] [Definition: A sufficient condition for some state of affairs S is a condition that, if satisfied, guarantees that S obtains.] From here. It has got examples and a tests. ☺ It is not **sufficient** many persons who you do not like to be called *Donald*. It is **necessary** that a certain *Donald* becomes President of USA for you to consider to change the name

  • I don't see how the image fit with the reference. The reference was good by the way. – Frank Hubeny Apr 25 '18 at 15:36
  • Donald = Necessary Donald Trump = Sufficient However, the Donald (Walt Disney) seems to find "Donald" Sufficient. ☺ – KwanzaKymi Apr 27 '18 at 7:09
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Necessary means that something is required to achieve a goal. Sufficient means that is all you need to reach your objective.

In your case:

Killing someone is sufficient to make them dead.

Killing is sufficient to make someone dead.

Killing someone is necessary to make them dead.

Killing is not sufficient, you need killing plus something else. Your example is not a good one because, killing, by definition means that someone is going to be dead. You should use some other verb.

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The conditional proposition can be other things besides necessary & sufficient for the record. Do not commit a fallacy of bifurcation thinking all conditionals must be one or the other.

Sufficient expresses that some element is able to satisfy the result. This means there are also OTHER means to also meet the same result. For instance a grade of 65 on an exam is sufficient to PASS the exam. Surely any score greater than 65 also passes.

Necessary expresses that the result would be impossible to occur in the absence of a essential element. A triangle necessarily MUST have three sides. Any more sides or less sides, then you absolutely do not have a triangle. Many people associate this conditional statement with cause & effect. As in science oxygen is necessary for combustion to occur or your standard chemistry balancing equation problems use the arrow to express this context.

To put conditionals another way we can use symbolic language to express these propositions in a clearer sense instead of the ambiguous arrow notation. To say a is sufficient for b is to say it is possible that you can obtain b without a. That is ~a V b. The tilde (~) represents a negation.
To say that you must have a to get b is to say it is impossible to get b without a. That is ~(b & ~a). This is read as it is impossible to have b and also not have a.

Some examples: if you are saved, then you have salvation. This expresses there is only ONE way to salvation. There are no others. This is necessary. You would use the latter notation above.

If you are a citizen then you have unemployment benefits. This expresses there are people who are eligible for unemployment benefits but all citizens do not automatically get benefits. Perhaps the law allows people on visas to collect unemployment benefits as well. It is a possibility that there are other conditions to grant the benefits. We are given that one of the many accepted conditions is being a citizen. This is not mandatory.

If I shoot you with my gun, you will die. This clearly is one way to die. So you can also die by drowning, by poison, getting a main blood vessel cut and bleeding out, shark attack, thrown out of a sky scraper, etc. This is not mandatory or necessary but sufficient to cause death.

If someone is a woman, then she is a human being. This is necessary by definition because it is impossible to have the traits of womanhood and not have the traits to be a human being. The fact that men are humans do not affect the truth if the conditional. As you know all humans are not women. This is impossible to be otherwise or else you are using the wrong term.

  • How can a conditional express neither a sufficient nor a necessary condition? Can you give an example of what you have in mind? – Patrick R May 5 '18 at 1:31
  • In mathematics you may need to have a necessary and/or sufficient variable but in ordinary English this is NOT so. For example, if the moon is made of cheese, then I am a mortal. The truth value of my example will be true. Surely I don't mean true in reality. The antecedent has no bearing on the consequent. Without the antecedent I can be a mortal. Even if the antecedent was true for 5 seconds I would still be a mortal for that time. True or false i get the consequent which math guys call the conclusion that is the words after THEN form the consequent not a conclusion. – Logikal May 5 '18 at 1:49
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    I'm not sure I understand what you're getting at. Are you saying the example does not express anything informative about the two terms? – Patrick R May 5 '18 at 2:36
  • I am illustrating that math context does not apply to ordinary language as I can have a true antecedent with a false consequent. The proposition will not be false in the real world. This process is a no-no in mathematics. The terms have to have a special relationship! If the terms are independent as the moon & me being mortal then mathematical logic idea of a conditional will not hold in reality. In rhetoric as well the mathematical notion will not hold to Reality. For instance, if the Giants draft a quarterback I will eat my hat. The idea expresses i doubt the Giants will draft a quarterback. – Logikal May 5 '18 at 2:47

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