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I am confused about how "P is necessary for Q" means "Q$\implies$P" (source: Kenneth Rosen DMGT).

Intuitively, I interpret "P is necessary for Q" as "for Q to happen, P must happen", which I basically feel is equivalent to "if P happens then Q must occur", i.e, "P $\implies$ Q". Could someone correct my understanding?

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    $\begingroup$ "Having money is necessary to buy milk" means "If I bought milk it implies I must have had money". It does not necessarily mean "If I have money I will go to buy milk", maybe not. $\endgroup$ Oct 6, 2023 at 7:26
  • $\begingroup$ See this post $\endgroup$ Oct 6, 2023 at 7:26
  • $\begingroup$ "P is necessary for Q" means that we cannot have Q without P, i.e. the circumstance that Q holds and P does not is ruled out. This fir with the line marked False in the truth table of $Q \to P$. $\endgroup$ Oct 6, 2023 at 7:28
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    $\begingroup$ Another example: "In order to get a driver's license, it is necessary to pass a vision test." $\endgroup$ Oct 6, 2023 at 7:44
  • $\begingroup$ "for Q to happen, P must happen" --- Doesn't this mean "when Q happens, then P must happen", which is Q $\implies$ P? $\endgroup$ Oct 6, 2023 at 15:33

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I think this is an intuitive example:

Suppose that it is necessary to have a ticket ($P$) in order to board a certain train ($Q$). That is, if you board the train ($Q$), then you have a ticket ($P$).

(It is not true that if you have a ticket ($P$) then you boarded the train ($Q$)! You might have a ticket and fail to board, or you might have a ticket and be refused boarding because you would not extinguish your cigarette.)

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You need to understand that the material conditional is not saying anything about temporal or causal relationships ... it merely says that 'if this is true, then that is true'

Consider:

"In order to take Calculus II ($II$), it is necessary that you take Calculus I ($I$). "

Temporally you might think $I \to II$, but that is not right: I can take Calculus I without taking Calculus II. But what we do have is $II \to I$: if it is true that I take (or have taken) Calculus II, then we can infer that I must have taken Calculus I.

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I interpret P is necessary for Q as for Q tohappen, P must happen, which I basically feel is equivalent to if P happens then Q mustoccur, i.e, P ⟹ Q.

  1. P is necessary for Q means P must be the case for Q to be the case, which can be rewritten as for Q to be the case, P must be the case, which means if Q is true, then P is true, i.e., Q ⟹ P.

  2. You were likely thrown off by erroneously framing P and Q as events that “happen/occur” and by the phrasing “for Q to happen / come true / be the case”, which sounds like “to cause Q to happen”, which in turn sounds like “implies Q”.

    However, P and Q are of course just (true/false) statements, and there is no inherent sense of time or causation when discussing these truths/falsities.

  3. Thus, for example,

    • persistence is necessary for success (i.e., success ⟹ persistence)

    should be understood not as

    • ❌ persistence results in success,

    or even as

    • ❌ success results in persistence,

    but as

    • ✔️ success is evidence of persistence.
  4. To put it another way,

    • persistence is necessary for success (i.e., success ⟹ persistence)

    most assuredly means

    • if got no persistence, then surely got no success,

    which is logically equivalent to

    • if got success, then got persistence.
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  • $\begingroup$ This is amazing! Thanks. I understood my mistake of looking at these statements as timed events, rather than just simple english statements. $\endgroup$
    – Ved K
    Oct 7, 2023 at 7:46
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To get the intuitive idea you can think of it in simple logic, and avoid symbols:

$ Q \Rightarrow P $ means that at every single case that $Q$ holds, then $P$ will certainly hold as well. So there is not even a single case in which $Q$ holds but $P$ doesn't. So $P$ is necessary for $Q$, as it always holds given the fact that $Q$ holds. Or in other words, $Q$ cannot hold, if $P$ doesn't hold. That's what makes $P$ necessary for $Q$.

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    $\begingroup$ What's with the downvotes? Wonderful community, this is. Someone tries to be helpful (without committing any errors, as far as I can tell) and it's voted down. $\endgroup$
    – Simon SMN
    Oct 6, 2023 at 17:15
  • $\begingroup$ @SimonSMN Thanks for noticing. I don't get it as well. I've figured that the community can become way too toxic at times. $\endgroup$
    – notpron
    Oct 6, 2023 at 17:57
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    $\begingroup$ @SimonSMN unexplained downvotes are something you just have to get used to on StackExchange sites as the accepted ethos doesn't require a down-voter to give any kind of reason (not like when you vote to close a post). If you are being plagued by down-votes on multiple questions or answers, it is worth flagging the apparent hate campaign. If it's just the odd one or two, then that's just life. $\endgroup$
    – Rob Arthan
    Oct 6, 2023 at 22:46
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Consider that $(A\implies B)\Longleftrightarrow (\neg B\implies \neg A)$, so that it is necessary for $B$ to be true in order that $A$ might be true. For example, let $A$ denote that $x$ is a natural number and $B$ denote that $x$ is a real number. Clearly $A\implies B$ because every natural number is a real number. But if $x$ is not a real number, then it is certainly not a natural number: it is necessary that $x$ be real for it to possibly be natural, and so $B$ is necessary for $A$.

On the other hand, if $A\implies B$, then $A$ is sufficient for $B$: it is sufficient that $x$ be a natural number for it to be a real number.

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