The Shrinking Block (SB) theory of time holds that only present and future objects and events exist—our spatiotemporal reality is a four-dimensional block that ever shrinks. I have never seen anyone advocate SB. However, interestingly, SB has some advantages over either Presentism ("only the present is real") and Growing Block (GB; "only the present and the past are real"). For instance, it is puzzling on Presentism and GB why it is that we should worry more about future physical pains than about past ones, and why we shouldn't worry about past physical pains. After all, on those two theories, future pains don't exist. SB neatly explains this: future pains are real, but past ones aren't. Moreover, SB neatly explains why it is better to have a past of vice and a future of virtue rather than the other way around, which neither Presentism nor GB on their own explain. SB fits with such maxims as "Let bygones be bygones" and encourages us not to "live in the past."
Of course, SB has disadvantages. It fits better with a reformative view of punishment than with a retributive one, and I think retribution is going to be an ingredient of any good theory of punishment (retribution is at least needed to place limits on punishment). Moreover, SB has the same difficulties with propositions about the past that GB has with propositions about the future and that Presentism has with propositions about the past and future. However, if we take the pragmatic stance that what matters most is deliberation, then it is propositions about the future that are the most important anyway.
Now, as I said, I've never seen anyone advocate SB. Why are GB and Presentism more popular? One answer is that both GB and Presentism may seem to have the advantage of accommodating the intuition which some have that the future is open, in the sense that it is neither the case that there will be a sea battle nor that there will be no sea battle. But GB and Presentism do not by themselves yield an open future. Presentists hold that bivalence about the past should hold and that there are non-trivially true propositions about what happened in the past, and if they have the right to say that, then by the same token it is compatible with GB and Presentism that bivalence about the future should hold and that there are non-trivially true propositions about what will happen. In other words, to get any help with regard to an open future, GB and Presentism have to be supplemented with either the thesis
- Bivalence fails for propositions about the future, and it is neither true nor false that there will be a sea battle tomorrow
- "Not (will p)" does not entail "will (not p)" even when conjoined with the thesis that time will go on, and it is false that there will be a sea battle tomorrow and it is false that there will not be a sea battle tomorrow.
But why can't SB be supplemented at least with (1)? According to SB, there are future events, yes. But SB is compatible with saying that it is not true that there is a sea battle tomorrow, and it is also compatible with saying that it is not true that will not be a sea battle tomorrow. Moreover, unless bivalence is a correct rule of logic—which Type (1) Presentists and GBers deny—why couldn't SB be comaptible with saying that both it is not true that there is a sea battle tomorrow and that it is not true that there will not be a sea battle tomorrow?
Here is a tempting line of thought. If SB holds, future events make propositions about the future true. Now, it either is or is not true that tomorrow's sea battle exists. If it exists, then it's true that tomorrow there will be a sea battle. If it doesn't, then it's not true. But there are two objections to this line of thought. First, the line of thought assumes a truthmaker/falsemaker principle about the future? But why should the SBer accept it? After all, neither the Presentist nor the GBer do! Maybe the line of thought is: "If there are some future events, then they are apt truthmakers for some statements about the future, and so the SBer should accept an appropriate truthmaker principle about the future." But this seems a shaky line of thought: it moves from there being some truthmakers to a full truthmaker principle. Second, the line of thought assumes bivalence about what does or does not exist. But if one can deny bivalence about what will or will not exist, why can't one deny bivalence about what does or does not exist?
Moreover, the SBer can accept (2). She can simply say that as a matter of fact neither tomorrow's sea battle nor the non-occurrence of tomorrow's sea battle exist. Granted, if she says this, she is committed to a changing future—maybe in an hour, once the free choice is made, tomorrow's sea battle event will pop into existence. That's weird, but perhaps not much weirder than in an hour it coming to be true that there will be a sea battle tomorrow, which all open futurists think can happen.
In fact, the above shows that if one can be an open futurist, one can be an eternalist and an open futurist.
Now, if there is very little reason to prefer GB or Presentism to SB, and SB is absurd, we should say that GB and Presentism are little better than absurd. But SB is absurd. Ergo, etc.