5.1 An Attempt at Reconciliation

In the philosophy of time, one generally opposes two traditions of thought: there are the subjectivists who are mostly concerned with how time is commonly experienced (e.g., phenomenologists, philosophers of inner life), and the scientists who are mostly interested in time as science, especially physics, describes it (e.g., empiricists, naturalist metaphysicians). However, as it was argued (Sect. 1.1), this opposition rests on a caricatural picture of science, according to which scientific concepts are completely divorced from our actual experience. A paradigmatic example of this idealization of science is Ladyman and Ross (2007, 2013), who famously claim that “[a]ttaching epistemic significance to metaphysical intuitions is anti-naturalist […]” (2007: 10). A more realistic view recognizes that human experience and intuitions play an important role in science, at least in the sense of background assumptions influencing how scientific data are interpreted (cf. Morganti & Tahko, 2017). Taking this latter view seriously, it seems that a third way of doing philosophy of time can be envisaged: reconciling the time of human experience with that of contemporary science. This mainly requires developing a framework within which pre-theoretic and scientific data can be articulated non-paradoxically. The present book was written in accordance with this general philosophical scheme.

The main purpose of this book was to account for the intuitive asymmetry between the ‘open future’ and the ‘fixed past’. This asymmetry was mainly, but not exclusively, illustrated by examples drawn from our practical life: there are things we can do to affect how the future will unfold (e.g., finding a cure for cancer, acting in an ethically responsible manner), whereas the past is beyond our control (e.g., Napoleon definitely lost the battle of Waterloo). Against some form of skepticism brought by science (especially by contemporary physics), it was argued that this aspect of human experience reflects how time truly is: unlike space, time is of an asymmetric nature. To make that case, the book was divided into three main parts; each of them devoted to a separate methodological step: (i) the characterization of the asymmetry, (ii) the accommodation of the asymmetry through a model of the temporal structure of the world, and (iii) the reconciliation of this model (and hence the asymmetry) with our best physics. From a broader perspective, (i), (ii), and (iii) brought a framework within which human experience and science both contribute to a better understanding of our familiar, but paradoxically elusive, concept of time.

Specifically, in the second chapter, I reviewed various philosophical characterizations of the asymmetry between the ‘open future’ and the ‘fixed past’. I especially explored the confrontation between two types of characterizations: perspectival (e.g., semantic, epistemic, anthropocentric) and substantial (e.g., physical, modal, metaphysical, ontological). In the third chapter, I proposed a new way of introducing the A-theoretic models of the temporal structure of the world, based on some relevant geometric features, and I figured out which of these models is best designed to accommodate the asymmetry in openness between the future and the past. In this chapter, I answered questions such as ‘How the existence in the past should be conceived?’ and ‘Why the existence in the past differs sharply from the existence in the present?’. In the fourth chapter, I wondered whether fundamental physics, as many philosophers would have us believe, undermines any attempt to defend an open-future view. To address this question, I first introduced both (neo-)Newtonian and Einsteinian basics. Then, while (neo-)Newtonian mechanics appears to be a friendlier environment, I wondered whether A-theoretic models could be expressed in relativistic settings. Finally, I looked for some process of ‘growth’ (in which new events come into existence) in nascent approaches to quantum gravity, especially in the causal set theory (CST), in order to restore the manifest image. In the last section, I moved from science to science fiction to wonder whether GBT is in principle compatible with time-travel scenarios.

5.2 The Summary of the Results

In the second chapter, I argued that the intuitive asymmetry between the ‘open future’ and the ‘fixed past’ reflects a fundamental (rather than derivative) phenomenon that can only be observed in a non-deterministic world, where the future history is not nomologically necessitated by the current history. More specifically, I argued that this phenomenon should be characterized as a kind of worldly unsettledness that is to be expressed in terms of underdetermination (rather than overdetermination): there being facts of the matter about what happened, but not about what will happen. This characterization was contrasted with some form of skepticism, mainly supported by scientific arguments, which aim to show that no asymmetry is to be found within the ‘fundamental features’ of reality. After all, the ‘block universe’ view of time, favored by physicists, is isotropic (spacetime has no intrinsic direction), and the fundamental laws of physics are time-reversal invariant (they do not distinguish the future-direction from the past-direction). These scientific arguments lead to think that the asymmetry is merely perspectival (i.e. it arises from the peculiar way our minds interact with reality), and should therefore be characterized in semantic, epistemic or even anthropocentric terms.

However, as it was shown, perspectival characterizations of the asymmetry raise at least two issues, although they may be relevant to explain our having certain practices. First, they seem illegitimate: the fact that science fails to capture certain phenomena does not entail that these phenomena are merely perspectival. For example, there are dozens of natural phenomena (e.g., northern lights, will-o’-the-wisps) that, although science had for long regarded as subject-dependent, turned out to be objective features of reality. Second, assuming that the open future merely reflects some sort of human ignorance about what will happen, it has to be explained where this ignorance comes from, while no convincing explanation has been found yet. Most attempts involve the second law of thermodynamics. For example, Jenann Ismael claims that: “[w]hat explains our greater knowledge of the past than the future is that along that [thermodynamic] gradient [produced by the rise of entropy], inferences from the present, surveyable macroscopic state of the world to its past […] are much more powerful inferences than inferences from the present to the future” (2016: 143). But this kind of Reichenbach-inspired explanation at best postpones the problem: given that there is no directedness in fundamental physics, where does the thermodynamic asymmetry in time come from?

In the third chapter, I argued that the problem of the asymmetry in openness between the future and the past should partly be conceived as the problem of establishing a difference in some of the geometric properties of our spatiotemporal models. Accordingly, I proposed a reformulation of the A-theories of time, which are traditionally introduced as various answers to the ontological question ‘Do the future and the past exist?’, in geometric terms. In particular, I distinguished two kinds of A-theories of time, the symmetric and the asymmetric theories, which differ with respect to whether, when reflection symmetry is operated around ‘the present’ axis, the outcome is an unchanged or a transformed spatiotemporal structure. More specifically, a symmetric theory is a theory such that possibly always the structures it describes is reflection invariant; conversely, an asymmetric theory is a theory such that necessarily sometimes the structure it describes is not reflection invariant. In that respect, eternalism and presentism were called ‘symmetric’, whereas GBT and SBT were called ‘asymmetric’ A-theories of time. Then, in order to distinguish between the various forms symmetric and asymmetric A-theories may adopt, a new question was introduced: ‘Is temporal becoming (i.e. the creation of new things in the present) real?’. One consequence of this proposal is that GBT is no longer to be seen as an ill-conceived hybrid between two polar opposites (eternalism and presentism), but as a real alternative: it is the only asymmetric A-theory that accepts Temporal Becoming. This new characterization revealed GBT to be better positioned than its rivals to accommodate various past-future time asymmetries, including the asymmetry between the ‘open future’ and the ‘fixed past’. In particular, unlike presentism, GBT can avail itself of the ‘no fact of the matter’ account of the openness of the future, while keeping the past fixed. Moreover, assuming that physical determinism is false, GBT implies, through Temporal Becoming, that new things are created in the present, while (at least) some of them are not made inevitable by how things located in the present or the past of now are or were. GBT thus meets the two necessary and sufficient conditions for regarding the future and the past as open and fixed, respectively.

However, despite these attractive features, GBT is often criticized for not being a viable alternative to presentism and eternalism, because of (i) the epistemic objection, according to which GBT would lead to absolute skepticism about where we are temporally located, and (ii) the apparent contradiction between GBT and relativistic physics, especially with respect to absolute simultaneity. In the second half of the third chapter, I addressed the epistemic objection, by showing that it relies on a mistaken assumption, namely that the reality of the past entails that events are occurring in the past. Specifically, I argued that the past should be conceived as a spatiotemporal region where nothing occurs, since it is exclusively populated by ‘bare particulars’ that are responsible for the continuity of existence of both continuants (people, tables, planets, etc.) and occurrents (events, processes, etc.), through the passage of time. In that sense, becoming past involves alteration in things’ intrinsic properties to such an extent that they cease to belong to their natural kind (rejection of natural kind essentialism). But, this alteration does not cause the things to cease to exist (rejection of Annihilation), since they now correspond to bare particulars, which are at least freed from all the properties that made these things belong to the natural kind to which they belonged when present (e.g., the property of occurring, if the things in question were events). Finally, assuming that if one’s belief is occurring, then one introspectively knows it, I concluded that we (as constituents of conscious events) can be confident of being right when we think that the time at which we exist is the objective present; no such conscious event (or any other event) could occur in the past, after all.

In the fourth chapter, I treated the objection based on the apparent incompatibility between GBT and relativistic physics. I first explained what this incompatibility is about, by contrasting Newtonian mechanics with the Special theory of relativity (SR). Then, I showed that, since SR does not privilege any way of slicing four-dimensional Minkowski spacetime into three-dimensional hyperplanes, it challenges the two components – geometric and dynamic – at work in the definition of GBT. Specifically, SR does not allow one to pick out a hyperplane as being (i) the unique ‘present’ axis around which reflection symmetry can be operated, and (ii) a universe-wide border where new events come into existence. Faced with this objection, I classified the possible reactions into two families of options: the compatibilist and the incompatibilist options. Whereas the compatibilist options take the metaphysical relevance of SR for granted, the incompatibilist options contest it. I then argued that the incompatibilist options are more promising, since compatibilist options conflict with both our common intuitions (e.g., the intuition that there is single universe-wide border between the past and the future) and some considerations from quantum mechanics (e.g., experimental results connected with John Bell’s theorem).

In the second half of the fourth chapter, I argued that the General theory of relativity is of no help to the compatibilist strategy, since (i) the possibility of ‘closed time-like curves’ renders GBT even less plausible, and (ii) relativistic causality puts GR in no better position than SR when accommodating quantum considerations (e.g., the violations of John Bell’s inequality, the collapse of the wave-packet). Taking that for granted, I suggested that a naturalistic basis for GBT (if any) should rather to be found in the nascent theories of quantum gravity. In that respect, I considered the causal set theory (CST), which rests on the idea that continuum spacetime disappears on sufficiently small scales, and is superseded by an ordered discrete structure: the causal set. This structure seems sufficient to reproduce the geometry of four-dimensional spacetime. Although CST is still in its infancy, it promises to underpin a growing block model of time, which was so far regarded as purely speculative. For, CST, when augmented with the ‘classical sequential dynamics’ (CSG), which takes the form of a discrete stochastic process of ‘growth’, provides an objective correlate of the intuitive notion of temporal becoming. Since this dynamics is said to unfold in a generally covariant manner, it renews the hope of reconciliation between GBT and relativity. Whereas temporal becoming within CST is often regarded as a local phenomenon, Earman (2008), and Wüthrich and Callender (2016) showed that CST permits global forms of temporal becoming that allow one to preserve the intuitive picture of a single physical world that grows and changes. CST might therefore be the missing link between the manifest image and science. Assuming indeterminism, it preserves the possibility of being both a scientific realist and a defender of the view that the future is open. Finally, I showed that GBT is in principle compatible with time-travel scenarios, provided that one accepts, for instance, a revised conception of perdurantism.

5.3 Future Directions

The question of the nature of time has intrigued philosophers for centuries, but much work still remains to be done, especially with regard to the reconciliation between the manifest and the scientific images. Whereas the present book was mainly concerned with the intuitive asymmetry between the ‘open future’ and the ‘fixed past’, many other aspects of human experience of time need to be reconciled with contemporary physics. For example, one can think of time passage, the spatially-extended present, and enduring objects, which all a priori seem to conflict with what relativistic physics tells us of the world. It would therefore be interesting to apply the three-step methodology – characterization, modeling, reconciliation – to these aspects too, in order to get a more comprehensive picture of the temporal structure of the world. Second, it would be desirable to give further consideration to the conception of the past – ‘the bare past’ – developed in the third chapter. One could, for instance, wonder whether this conception is compatible with time-travel, or whether bare particulars are sufficient to underpin the complexity of the past (what about past properties, or past instants?). Third, whereas the second law of thermodynamics has been shown insufficient to explain intuitive temporal asymmetries, one still needs an account that articulates GBT with the increase in entropy. Perhaps a starting point could be to insist, as Maudlin (2007) does, that a one-way temporal evolution is required to express the increase of entropy away from a low-entropy. Then, an idea would be to show that this temporal evolution is packed into Temporal Becoming, which is constitutive of GBT.

Another direction would be to explore the practical significance of GBT. It indeed seems that GBT may help clarify our thinking about matters of moral and emotional concern. For example, consider our fear of dying. Death is traditionally defined as the “[…] unequivocal and permanent end of our existence” (Nagel, 1979: 61). As L. W. Sumner puts it: “[t]he death of a person is the end of that person; before death he is and after death he is not. To die is therefore to cease to exist” (1976: 153). This definition highlights what frightens us the most about death: our own annihilation (cf. Luper-Foy 1987).Footnote 1 Of course, death will always be a matter of fear, but GBT could persuade us that things are less bad than they seem. After all, GBT removes from death its existential significance: even if we become something intrinsically different after death (perhaps a bare particular!), we definitely remain something. In that sense, death does not bring absolute annihilation: lives do not cease to exist, they simply have a beginning and an end with, hopefully, creation of durable good in between. Accordingly, the loss of being is not something that should be feared, since the permanentness of being is guaranteed by GBT. The most we have to lose, so to speak, is our experience of life (or something closely related), which undoubtedly sounds less tragic. In short, one could say that the past is existence without afterlife. By contrast, the birth of a child (which is literally to be understood as the coming into the world of a new human being) should always be a matter of great rejoicing.

Closely related, GBT might provide a partial explanation as to why we care more about future experiences than past ones (especially when the experiences in question are of the painful and pleasure variety). This emotional asymmetryFootnote 2 can be illustrated by the fact that, in general, we would prefer that bad things (e.g., a headache, an awkward meeting) be in the past rather than in the future (cf. Hare, 2013: 507). Of course, there have been valiant attempts to account for this asymmetry. Arguably the most influential one is due to David Lewis (1979b), who argues that wanting bad things to be in the past involves wanting of myself (construed as a person-stage) that I be to-the-future of bad things. But, this account leads to arbitrary considerations. As Caspar Hare puts it: “[w]hy should I want my pains to be in the past if wanting pain to be in the past just amounts to wanting myself (construed as a person-stage) that I be to the future of pain? Wouldn’t that be just like standing in a row of soldiers […], knowing that one of them has a toothache, and wanting of myself that I be to the south of the pain?” (2013: 514). It indeed seems that Lewis’ account fails to explain why a desire that I (construed as a person-stage) be to the future-of-pain is less arbitrary than a desire that I be to the south of pain.

Interestingly, GBT might offer a partial solution to this problem. Let me sketch the general idea. The most plausible reason why we care more about future experiences than past ones is that we are typically both intrigued and scared by the unknown. As H. P. Lovecraft puts it at the very beginning of his classical essay on fear and the supernatural: “[t]he oldest and strongest emotion of mankind is fear, and the oldest and strongest kind of fear is the fear of the unknown” (1974: 13). In the same vein, Elias Canetti writes that “[t]here is nothing that man fears more than the touch of the unknown” (1962: 15). My wanting my pain to be past could therefore involves my preferring to have a detailed rather than a poor knowledge about what happens to me. Taking this idea seriously, it seems that GBT, at least when enriched with the ‘no fact of the matter’ account of openness (cf. Sect. 2.9), is well positioned to account for this emotional asymmetry. Indeed, since this theory provides an immediate explanation as to why future experiences remain largely unknown (we cannot know more about the future than what is settled about it), it also helps explain why future experiences are a matter of great concern (in comparison to past experiences).

At this point, it could be objected that it is simply wrong to assume that we always attach more importance to future events than to past ones. For example, it might seem that a person condemned to death could, just before his execution, care less about his future than about his past (since his future is limited to a few minutes). Arguably, this person would have no more plans or hopes, whereas he would remember the pivotal moments of his life, and perhaps feel some regrets about his past crimes. Yet, it seems that GBT cannot account for this case, and hence neglects the variety of our emotional concerns. In response, two claims can be made. First, in the above case, it is not clear that the person cares more about the past than the future. In such a situation, one can rather be inclined to think that he would be obsessed by his approaching death and other future events. He might ask questions such as ‘Will I suffer?’, ‘What is there after death?’, ‘Will my children have a happy life?’ or ‘Will my wife get remarried?’. Second, even assuming that the person sentenced to death mostly cares about past events, this can perfectly be accommodated by GBT. As a reminder, GBT (alone) does not entail that the future is open, nor does it entail that the future is a matter of great ignorance. For example, in a fully deterministic context, what will happen is fixed (no matter whether the future exists or not) and, therefore, everything is predictable (cf. Laplace [1814] 1951). So, if a person sentenced to death does not attach great importance to the future, this could easily be explained by the fact that he believes that his future is fully determinate (there is no way for him to avoid his execution – no escape plan, no legal recourse, etc.). He perfectly knows what is going to happen, and therefore his future contains no more mystery he might fear. But, here again, these are just naïve thoughts; a perspicuous account would require further investigations, especially within epistemology and the philosophy of emotions.

Of course, these future directions are merely intended as suggestions: one may choose to change them to match our own philosophical purposes; the future is open, after all.