If I understand correctly, both presentism and eternalism accept that spacetime can be described by the same coordinate system and following the same rules of physics. Moreover, both of them affirm that a huge amount of matter-energy (or information) exists distributed throughout space. The difference is that presentism posits that time is special in the sense that matter-energy only exists in a slice of time, that encompasses all of space. Whereas eternalism posits that matter-energy exists at all points of this coordinate system.

Now, my question is: can Occam's razor be applied in favor of presentism? The argument would go that both perspectives explain exactly the same facts and are compatible with all known science, but eternalism demands much more matter and energy to achieve the same explanatory result. Whereas presentism needs just a very tiny part of all that matter and energy to explain everything we see. Therefore it would seem that presentism would be the simplest theory. In other words, why would we need to assume much vaster quantities of matter-energy if we gain no explanatory power?

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    No, not really. As you can see from SEP's entry on Simplicity, Occam's razor, as understood today, is about simplicity of explanation, not about quantities of matter involved in it. Eternalism fits better with modern relativistic physics, and so makes for a simpler overall explanation. Presentism has some serious problems with relativity, and requires unnatural extra constructs, such as the privileged frame, to be reconciled with it.
    – Conifold
    Oct 22, 2020 at 19:44
  • "It has often been commented by physicists that Einstein’s GTR {thus} reintroduced the relations of absolute simultaneity that his STR had denied". Craig W., Simth Q, 2007. Einstein, Relativity and Absolute Simultaneity,p.8 .
    – sand1
    Nov 23, 2020 at 10:40
  • Two events at the same place and time are simultaneous for all under gtr and str. From here, consider two events at the same time and place not as two events at all but as one: While it’s true that gtr allows for absolute simultaneity, there are still events which are simultaneous for some observers but not others under gtr (such events are not absolutely simultaneous). So absolute simultaneity doesnt carry the weight people often think. And observers dont generally agree about the time between two events, even two absolutely simultaneous events. There is no single timeline, still spacetime.
    – Al Brown
    Jul 21, 2021 at 6:48

3 Answers 3


I don't really see where you are going with this matter and energy story, but it seems quite clear that presentism mobilizes less unnecessary entities than eternalism, so unless there is something that it can't explain, Occam's razor should favor it.

Under presentism, only exists the present, an ensemble of stuff that moves and change. Because of the conservation of energy stuff keeps moving uniformly, unless it bumps into other stuff.

Time is only the perception of the uniform movement of stuff, for example how long it takes for a planet rotating at constant speed to complete one revolution is one day. Timespace becomes a necessary concept because when stuff moves uniformly into a curved space, the perception its movement from far away changes.

Future is just the consequence of change. It does not need to "be" anywhere, it just happens. Past is just how we remember things were before changing. It does not need to "go" anywhere. We can imagine the future or the past by applying our knowledge of the laws of change on the present situation (retroactively in the case of past, but the procedure is the same), yet it is merely extrapolated information from the present state of things.

Eternalism, on the other hand, requires a place for the past and future to be, a reason for time to pass, a reason for why we perceive only the present. It seems to raise many more questions that it answers, which is precisely what Occam's razor is designed to avoid.

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    Unfortunately presentism omits many necessary entities along with the unnecessary ones. The physical fact of the relativity of simultaneity would be impossible in a presentist universe where the past does not exist. Oct 23, 2020 at 22:32
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    I dont see why. Take the common exemple of the train hit by thunder at both ends while observed simultaneously by an observer in the train and another standing near the railway: it does not require any concept of abstract past, just photons moving at constant speed hitting observers in motion relative to each other.
    – armand
    Oct 23, 2020 at 23:38
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    Subjective failure to comprehend the issue doesn’t mean the issue doesn’t exist, lol. It’s not about when the photons hit the observers. A lightning detector in the observer’s initial frame would record the lightning strikes as simultaneous; lightning detectors aboard the train (like something attached to the carriage) would record different times for the strikes. The “presents” experienced in both frames are physically, not just perceptually, different. Oct 24, 2020 at 0:17
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    Each lightning is perceived by both observers in a different sequence, but it is only because the observers are moving relative to each other. The photons move exactly in the same way, and this is why there is no privileged frame of reference. Because no information goes faster than light, both observers can't know any better than what they see and come to different conclusions about the sequence of events. Pry explain why it requires the existence of some "past" time to store information? Please remember, when people are not convinced, it MIGHT be because you're not making sense ^^
    – armand
    Oct 24, 2020 at 3:58
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    If basic relativity “isn’t making sense” to you, is that Einstein’s fault, or your fault? 🤔 Your continuing conflation of differences in “appearance simultaneity” with relativistic failure of absolute simultaneity (see pitt.edu/~jdnorton/teaching/HPS_0410/chapters/…) suggests that you really have failed to understand the issue. The lack of absolute simultaneity is physically real, and fatal to presentist theories of space-time. Oct 24, 2020 at 9:27

In its original form, the razor went something like, "Do not multiply entities unnecessarily." If theory T1 requires n unobservables and theory T2 requires m unobservables, and if n < m, then T1 is preferable (modulo the razor).

Now with presentism, we don't have all times given. But with eternalism, is it correct to speak of "times" anymore, or just the one all-encompassing eternal time? One could, then, formulate presentism and eternalism such that there is only one actually existent temporal object (so to speak) required by either theory, in which case adjudicating between the theories can't default to applying the razor to them.

(Or so it seems, on the face of it, to me. However, one might object, "The temporal object required by eternalism is potentially infinitely larger than the one required by presentism; it has more parts than the 'moving spotlight.'" I have no reply to that objection at the time being, except to say that even the "moving spotlight" might be otherwise internally divisible into infinitesimal parts, of which there would therefore be infinitely many even if the "spotlight" were a single integral unit of time.)


Occam’s razor would seem to (slightly) favor eternalism over presentism, if anything. As Conifold points out in the comments, the relativity of simultaneity as theorized by Einstein and verified by experiment would seem to be a fatal blow to any strict presentist theory. Yet eternalism would seem to conflict with both quantum physics (Bell’s theorem against hidden variables) and our everyday experience of free will. One potential reconciliation is that of the “growing block universe”. A presentist might claim a version of the growing block universe where parts of the block “get deleted” or cease to exist gives the necessary correction to presentist theory, although it’s not clear why or how they would get deleted.

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    Are you saying eternalism would require hidden variables, so that the lack of them conflicts with eternalism? If so, why?
    – Hypnosifl
    Oct 23, 2020 at 5:51
  • @hypnosufi Quantum mechanics makes probabilistic predictions about the future which are incompatible with deterministic explanations, one of which is Bell’s theorem. Oct 23, 2020 at 19:44
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    Eternalism is not the same as determinism, eternalism just says that past, present, and future all have the same ontological status (past and future things 'exist' in exactly the same sense as present ones), not that later states of the world can be perfectly predicted from earlier ones. Consider a row of numbers on a piece of paper sitting in front of you--all the printed symbols representing the numbers are equally real, even if the numbers don't follow a rule such that later ones can be predicted from earlier ones.
    – Hypnosifl
    Oct 23, 2020 at 20:04
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    Also, Bell's theorem only shows that quantum mechanics is incompatible with local hidden variable theories where every measurement has a single definite outcome, it doesn't rule out Bohmian mechanics or the many-worlds interpretation, which are both deterministic.
    – Hypnosifl
    Oct 23, 2020 at 20:05
  • @hypnosifI To say that the future exists and is real and fixed is to say that the future is determined from the beginning. A deterministic universe need not be predictable-in-practice, or even predictable-in-principle, from inside of itself in order to be deterministic. To go with your sequence analogy, there may not be a way of telling what the next symbol is from the previous ones; but someone wrote down all the symbols before, and if you could have watched them do that, you would be able to predict the sequence perfectly. Oct 23, 2020 at 20:48

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