Can something be logically necessary now but not in the future? I probably always assumed it couldn't, that it followed from the laws of logic alone, and that these are immutable etc..

I don't think necessity in general is limited to one tense only. Temporal logicians ask if the past is necessary and the future contingent (not necessary). But do they include logical necessity in that, and if so what are some examples of logical necessity that isn't always the case?

  • A possible example would be, "The time is now," which seems true whenever it is uttered (since it is "now" when it is uttered), but which on another level seems to change in truth-value. This is still a temporal example, but an example from temporal logic, and such as satisfies some of the topic-neutral/permutation-invariant parameters for "pure logicality." Perhaps a more promising case would be the theory of contingently abstract objects. Commented Jul 17, 2023 at 4:22
  • I would have thought that either it does not change (it is always now) or it is not logically necessary @KristianBerry
    – user66760
    Commented Jul 17, 2023 at 4:49
  • It seems like a possible counterexample to, "Logical truths are absolutely unchanging." Granted, the relation under which it would count as changing would be as a sentence-token: the sentence-type is always true (or is beyond truth, being more like a variable by itself), but each sentence-token becomes in some sense false as time passes. I mean, I'm not really sure about all that; but so again, it just seems like it might be an example of what you're asking about. Commented Jul 17, 2023 at 5:40
  • OTOH, if you mean to ask whether the law of noncontradiction, or of the excluded middle, or those kinds of things, can change, the answer might be that the logical laws that could never change would be those used to define the concept of "change" itself, and if we could pry apart some logical laws from that definitional process, we would find changeable laws. Commented Jul 17, 2023 at 5:42
  • Necessity is not contingency, but even more: it is the opposite.
    – RodolfoAP
    Commented Jul 17, 2023 at 6:08

2 Answers 2


As far as necessity goes, forms of necessity are essentially defined with reference to all possible examples of some kind. A temporal necessity is true for all possible times; a physical necessity is true for all possible physical dynamics (or: for types of causation, a physical necessity is true of all possible tokens per type); a logical necessity is true for all possible logical systems—and so when logic is self-applied while standing both above and within all other kinds of systems, we end up with a picture of logical necessity as the most absolute necessity there could be.

Now, one might deny that there is such a necessity in the first place, although if this were so, then it would seem to be necessary that nothing was necessary: for if it were logically possible for something to be logically necessary, then in the end it would seem as though the mere possibility of something's absolute necessity would mean the actual necessity of that thing. As Kant put it: to say that something is necessary is to say that it is actual just by being possible; or in contemporary modal logic, "possibly necessary" collapses to "necessary."

However, one famous thinker who sought to imagine that eternal truths could be contingent was Descartes:

Another reason that Descartes’ view is difficult to conceive is that Descartes takes eternal truths to be necessary. That is, it is difficult to conceive how eternal truths could be necessary if they were created by a free act of God. Descartes is clear that the eternal truths are necessary: he says that “the necessity of these truths does not surpass our knowledge” (“To Mersenne, 6 May 1630,” AT 1:150, CSMK 25). If eternal truths are necessary, however, it should not be the case that they could have been otherwise. Yet Descartes’ commitment to divine omnipotence appears to commit him to this view...

So it is that some analysts don't accept that iterated modalities always collapse to whichever directly prefixes the modified terms. With respect to Descartes' viewpoint, they say:

An alternative view (developed and defended by Edwin Curley) is that Descartes holds that eternal truths are necessary, but that they are not necessarily so. On this reading, Descartes’ view involves iterated modalities: a number of truths are possibly necessary, but God chooses only some of these possibilities to be the actual necessary truths.

For a broader, recent discourse on iterated modalities, see Gregory[11].

  • “Possibly Necessary” only collapses to necessity in S5, Kristian. If accessibility is just a preorder like in S4, and not a symmetric, reflexive and transitive relation like S5, then in one accessible world w1 a proposition p might be true and necessary, and w2 also accessible has p false, such that w2 is not accessible to w1.
    – Paul Ross
    Commented Jul 17, 2023 at 6:12
  • idt i understood how your answer answers the question
    – user66760
    Commented Jul 17, 2023 at 6:16
  • While a fairly reasonable proposal that “logical necessity” should behave like S5, I’m not sure it’s absolutely obviously true. Consider - what if some mathematical objects exist in some possible worlds but not others, that have scope on the limiting behaviour of logical inferences?
    – Paul Ross
    Commented Jul 17, 2023 at 6:19
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    @doot_s for the most part, my answer is, "No: if something is necessary, it is unchanging, especially if it's logically necessary." Commented Jul 17, 2023 at 6:58
  • @PaulRoss as far as I know, S5 is the most widely accepted system nowadays, but at any rate, since I went on to discuss no-collapse iteration theories, I didn't mean to declare that the collapse thesis is certainly true (I myself don't even believe that the possibility and necessity operators are the "real" basic ones, here). Commented Jul 17, 2023 at 7:01

"This sentence exists" may be a tautology, but it hasn't always existed. So perhaps not all tautologies are always the case.

  • 2
    I'm not sure, "This sentence exists," counts as a tautology (admittedly, the concept of something being a tautology is not obviously absolute, though). Commented Jul 17, 2023 at 7:10
  • 1
    I guess that depends on whether you are a realist or not. Meaning whether sentences exist before someone has formulated them. Discovery vs. creation.
    – kutschkem
    Commented Jul 17, 2023 at 7:46

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