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I've always treated eternalism and the B theory of time as largely synonymous and thought they just refer to the perspective of viewing the universe as a single static entity with "slices" or "instants".

I read here https://philosophy.stackexchange.com/a/30362/27054 that eternalism is a metaphysical theory and the B theory of time is a semantic theory, but I'm not exactly sure what that means in this context. What would an eternalist but non-[B theory] viewpoint look like, for instance?

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    It is a very legalistic distinction in that A/B theories regulate which claims can be legitimately made about time, while presentism/eternalism are concerned with the nature of time itself. So technically, one could combine eternalism (future events already exist) with A theory by using some extra property of "moving present" to ground tense intutitions, but that is just confusing and almost never done.
    – Conifold
    May 30, 2017 at 23:42
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    Hi. I find the claim that the B theory is "semantic" rather than "metaphysical" strange, and probably wrong. McTaggart's essay, which originated the B theory notion, was about metaphysics, not semantics. One subtle point is that, at least for McTaggart himself, the B theory was not a theory of time at all. He believed that only the A theory was recognizable as a theory of time. And that if the A theory fails (as he also believed, and argued for) then time just does not exist. May 31, 2017 at 19:47
  • The central question in the philosophy of time is whether time is tensed or tenseless. If you see the formula of Tesla about Tension, then "T.L=MC^2". Then it would be a good question to Interprete , B-Theory of Time using Tesla formula!. B-Theory of Time claiming that Time is Tenseless which contradicts with the relation between Time, Light and Tension.
    – user47436
    Sep 28, 2020 at 11:26

2 Answers 2

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Special Relativity could be considered an eternalism that does not admit a B-Theory of time.

Spacetime is a single 4-dimensional object. So this is an eternalism. But simultaneity is not the same across different locations, so there is no B-series.

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The A-Theory/B-Theory distinction is about tense. Are fundamental truths about the universe tensed or not? To make it clearer, assume the statement "Sam is six years old" is true. An A-Theorist would say that the truth-value of this statement changes over time. It was false a year ago, it is true now, and it will be false again next year. A B-Theorist would say that "Sam is six years old" is really another statement in disguise: "In 2022, Sam is six years old." The truth value of this statement doesn't change over time.

A-Theorists think that there is a fact of a matter which moments are past, which one is present, and which ones are future. A certain time is "now," and that time moves ever forward. A B-Theorist would say that "now" is an indexical term. It just refers to whatever time a speaker is uttering it. One moment's past can be another moment's future. There is no "objective" past, future, and present. That is entirely perspectival. Rather, some moments are before other moments and after other moments. The relationship of "before" and "after" doesn't change (from a given reference frame).

This is different from the presentist/eternalist distinction. This distinction is about what exists. A presentist thinks that only the present exists. The past existed, but it doesn't anymore. The future will exist, but it doesn't right now. An eternalist says that the past, future, and present all exist. For a presentist, dinosaurs no longer exist. For an eternalist, dinosaurs exist in the past. Different times are just like different places. Paris exists, but its location is over there rather than here. Dinosaurs exist, but their location is back then rather than now.

A-Theory pairs nicely with presenting, and the B-Theory pairs well with eternalism. But you can mix and match them, to an extent. You can be an eternalist who believes that the past, present, and future all exist, but still be an A-Theorist. This is because you think that there is an objective present, rather than "present" simply being an indexical. One moment is present, and it moves forward over time. Other times exist, but they aren't present. This would make you a Moving Spotlighter.

So, you can be an Eternalist A-Theorist, an Eternalist B-Theorist, or a Presentist A-Theorist. You can't be a Presentist B-Theorist, though. That combination doesn't work. How can you think that only the present exists without thinking that which moment is present is objective rather than perspectival?

But there are other things you can be. You can be a "Growing Blockist." This means that, like the Eternalist, you think that the past and present both exist. However, like the Presentist, you deny that the future exists. It hasn't happened yet! The present moment is the three-dimensional slice at the end of an ever-growing block. The number of moments is increasing every second, and the newest one is the present.

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