- Presentism, the view that only present entities exist, is a theory in the ascendant, and this book, a defence of presentism, will contribute to its rise. It is a clear and well argued monograph which systematically develops a metaphysical theory of presentism, and ably defends it against common objections. Most significantly, Bourne defends his theory against the criticism that presentism is false because it is inconsistent with the special theory of relativity (STR), a well confirmed scientific theory. This is probably the biggest hurdle for any proponent of presentism, since if a metaphysical theory is inconsistent with any of our best current scientific theories, the smart money will be on the scientific theory, and the best the metaphysician can hope for is to have developed a consistent but false theory. Furthermore, presentism is, prima facie, inconsistent with (STR). Nevertheless, this is a hurdle which some presentists are reluctant even to attempt to jump. Not so Craig Bourne, who presents the physics in a clear and manageable way for philosophers who may be unfamiliar with it, and argues that both (STR) and his version of presentism can be maintained. Whether his arguments ultimately succeed is another matter.
- Philosophers engaging in debate in the philosophy of time have come up with countless ways of drawing the battle lines between various opposing theories. Some distinguish between static and dynamic theories, others between A- and B-theories, or tensed and tenseless theories. But there is disagreement over whether the descriptors 'tensed' and 'tenseless' should be reserved for theories about temporal language, or whether they can properly describe metaphysical theories. For example, is a tenseless theory one which holds that tensed sentences are reducible to tenseless sentences, or is it the more robust metaphysical view that there are no real tenses, no ontological distinction between past, present and future? Furthermore, it is not the case that each of these ways of dividing up the metaphysical terrain maps neatly onto the others. In the face of this lack of consensus over how the debate is to be set out, Bourne offers a simple and compelling way of doing so. According to him, 'if one's theory postulates an ontologically significant notion of the present, then that theory is a tensed theory' (p. 10). This allows him to maintain the view that the debate is robustly metaphysical, and not merely linguistic, and it also allows him to separate all the contending theories into two groups solely on the basis of their response to one simple question.
- Bourne begins the development of his positive thesis by posing a problem he calls the 'present problem', and which he claims can only be adequately met by the tenseless theory and presentism (the material in this chapter has been previously published in his 'When am I?', Australasian Journal of Philosophy, 80 (2002), pp. 359-71). He thus eliminates all opponents save the tenseless theory in one fell swoop. His 'present problem' is the challenge of guaranteeing our knowledge that we are present, and not past or future. The tenseless theory meets this challenge by effectively deflating it: we know that we are present because this is merely the indexical knowledge that we are located at the time at which we are located. But this option is not available to tensed theories, which postulate an ontologically significant notion of the present. According to them, our knowledge that we are present must be knowledge that we are located in this ontologically privileged moment. So we can distinguish between knowing that we are 'present' in the indexical sense, and knowing that we are *present* in the referential sense which refers to the ontologically privileged present moment. Bourne then argues that any tensed theory which is pluralist about times, that is, any which holds that there is an ontologically significant present moment, and recognizes the existence of more than one time, will not be able to meet this challenge. Such a theory could guarantee our knowledge that we are 'present', but this will not do, for we could have all the same beliefs and experiences as we currently have if we were in fact *past*, or even *future*. The only tensed theory which can adequately meet this challenge, according to Bourne, is presentism, because it does not postulate the existence of any times other than the present. Given that we exist, we must be *present*.
- Bourne sets up his version of presentism via the truthmaker problem for presentism. This is the problem of finding truthmakers for truths about non-present times. We would all want to agree, says Bourne, that 'Socrates taught Plato' is true, but what makes it true? According to the tenseless theorist it is the tenseless existence of the past fact that Socrates taught Plato. But this option is not available to the presentist who denies that there are any past facts. Bourne then gives us three desiderata which any satisfactory solution to this problem must deliver. It must preserve our views about which statements are true and which are false; what the truthmakers are for those statements must be transparent; and the solution must accommodate the truth-value links between various times. Looking to these desiderata, Bourne then considers and rejects a number of attempts to solve the truthmaker problem before finally offering his own.
- Bourne calls his view 'ersatzer presentism', as it bears some similarities to ersatz modal realism, criticized by David Lewis in "Lewis (David) - On the Plurality of Worlds". According to Bourne's ersatzer presentism, there are non-present times: they are maximally consistent sets of unembedded present-tense propositions - specifically, those propositions which we would say are true at the time in question. So Bourne's version of presentism helps itself to the existence of as many times as does the tenseless theory, but tells a different story about what sorts of entities make up those times. For the tenseless theory, those times are populated by just the same sorts of concrete entity as populate the present time; other times are qualitatively just like the present time. This is similar to Lewis' account of other worlds, which are qualitatively just like the actual world. For ersatzer presentism, other times are sorts of entity radically different from the present time which we inhabit: they are constructed out of propositions. But that is not quite right. The present time too is constructed out of propositions, but it differs from all the rest in being the only one which is concretely realized, or perhaps, whose constituent propositions are all true. Similarly, ersatz modal realism has it that all possible worlds, including the actual world, are abstract entities constructed out of propositions, pictures, sets of sentences, or whatever; but only one of those worlds, the actual world, is concretely realized.
- Bourne insists that the propositions which make up a time are unembedded: that is, none of the propositions constitutive of a time contains past- or future-tense operators. Presumably this is so that the set of propositions which make up a time does not include any of the problematic cases of propositions about non-present times. But it raises some potential problems. The set of propositions which constitute a time is supposed to give a complete, maximally specific description of what is the case at that time (p. 54). But a complete maximally specific description of what is the case now will surely include some past- and future-tense truths, or at least some truths which cannot be fully accounted for without making reference to past or future events. For example, it is true now that my daughter's sixth birthday is approaching. This is a proposition which makes reference to a future event, and it is true in virtue of the past fact that she was born almost six years ago. Other propositions too will be omitted from the set of propositions which constitute a time because they make ineliminable reference to past or future times or events, for example, those involving causal processes; and this will render the description of what is the case at a time incomplete.
- But there is a further worry with this strategy of constructing times out of propositions. Unembedded present-tense propositions are made true by the concrete facts they represent. Past-tense propositions, however, are made true by ordered pairs of unembedded propositions and dates (p. 57). But what makes these latter propositions true, i.e., the ones which, together with a date, constitute the truthmaker for a past tense proposition? They appear to be ungrounded, which is a problem Bourne thinks afflicts other versions of presentism, but not his own.
- A similar worry arises with respect to Bourne's discussion of temporal relations. Bourne defines simultaneity in terms of a conjunction of two unembedded propositions, each stating that a given event occurs. So two events are simultaneous because the relevant conjunction is true. But the dependence relation here should go the other way: the conjunction of the two propositions is true because the two events are simultaneous. This strategy carries over into his discussion of absolute simultaneity in the section on special relativity. He defines absolute simultaneity thus: 'e is absolutely simultaneous with e* =dfP(p & q) v (p & q) v F(p & q)' (p. 173). Here p is the true present-tense proposition that event e occurs, q is the true present-tense proposition that e* occurs, P is the past-tense operator 'It was the case that' and F is the future-tense operator 'It will be the case that'. Again this definition seems to be getting things the wrong way round. Surely the conjunction of p and q either was, is, or will be true because e and e* are simultaneous; they are not simultaneous because the conjunction is true at some time. A conjunction, or any other well-formed string of symbols, is a representational entity which, if true, represents how things are in the world. So it is how things are in the world that grounds the truth of the conjunction. Bourne opts for a wholesale rejection of this way of thinking, assigning primary ontological significance to the representational entities. In doing so he nobly bites the bullet: 'For I say that it isn't that the conjunctions are true because the events are simultaneous, but rather that they are simultaneous because the conjunctions are true. And why not?' (pp. 173-4). But his definition of absolute simultaneity has wider ramifications than he seems to think. How are physicists supposed to make use of this definition of absolute simultaneity? A logical definition is quite useless for their purposes. They think themselves to be in the business of studying physical relations, but if Bourne is right, they should turn their attention to logical relations instead.
- My criticisms should not be taken as criticisms of the quality of this book. They are, rather, an attempt to engage with it, as philosophers of time from across the spectrum will have to do. It is a well written, well-argued and thorough defence of presentism which, in my view, gives that theory its best chance to date of being true.
Review of "Bourne (Craig) - A Future for Presentism".
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