Introduction

Abstract

In this introduction, my aim is threefold. First, I situate the present book in the vast landscape of the philosophy of time. Against powerful traditions of thought, I argue that a perspicuous philosophical account of time can only be obtained through the reconciliation of the manifest image of the world and contemporary science. Second, I introduce the book’s primary objective, namely to account for the intuitive asymmetry between the ‘open future’ and the ‘fixed past’, and contrast it with some current forms of skepticism brought by science. Third, I outline the three-phase structure of the book – characterization, modeling, reconciliation – and expose the main expected results.

1.1 Metaphysics of Time: The Science of Reconciliation

When we think of time, no accurate definition comes to mind. As Augustine famously puts it: “[w]hat then is time? If no one asks me, I know; if I want to explain it to a questioner, I do not know” (2006: 242). However, some features of time some are experienced quite vividly. First, time flows uniformly and universally. It indeed seems that time passes at the same speed everywhere, i.e. no matter where we are located. For example, if two friends separate – one starts living in the Swiss Alps, the other in Paris – and they meet up again years after, we think that time will not have elapsed more for one than for the other; one will not have lived longer than the other. Second, it seems that ‘our present’ extends throughout the whole universe. In particular, it makes perfect sense for us to call a friend who is visiting New York to ask him what he is doing now. Regardless of the jet lag issue, we assume that our friend and ourselves can have simultaneous activities. For example, while I am writing this introduction, he is visiting the Guggenheim Museum. Third, and perhaps most importantly, it seems that there is a difference between the future and the past: the former is open while the latter is fixed (or closed). For example, whereas we think that there are things we can do to affect how the future will unfold (e.g., making a significant donation to an NGO, acting in an environmentally responsible manner), we think that the past is beyond our control (‘what is done is done’). Although these features of time play roles of varying levels of importance in our epistemic life, they are all pieces of the ‘manifest image of the world’, where by ‘manifest’ one should understand ‘the world as it appears to us’ (cf. Sellars, 1962: 37–38). This is supported by various empirical works (cf., for instance, Latham et al., 2020a, b, 2021a, b).

The problem is that science, especially contemporary physics, contests these features of time, and hence contradicts our intuitions. First, Albert Einstein understood – a century before we had clocks precise enough to measure it – that time passes more slowly in some places, more rapidly in others.¹ To use the previous example, if two friends separate – one starts living in the Swiss Alps, the other in Paris – and they meet up again years after, the one who has stayed in Paris will have had a shorter life than the one who has stayed in the Alps. This is a consequence of General Relativity (GR), which posits that a large mass – such as the Earth – warps spacetime and thus slows down time in its vicinity. This so-called ‘time-dilation’ effect is more important in the plains than in the mountains, because the plains are closer to the center of the gravitational field.² Second, as Special Relativity (SR) has stated, there is no unique present, for there is no absolute relation of objective simultaneity: two spatially separated events may be simultaneous for one observer, and temporally distant for another observer. It therefore makes no sense to call a friend who is visiting New York to ask him what he is doing now (or while I am writing this introduction), for there is no privileged moment that constitutes the present. Third, since the ‘block universe’ view of time (which seems implied by SR) 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), it appears that any intuitive asymmetry in time is at best a non-fundamental phenomenon (that plausibly results from how matter is contingently distributed through spacetime), and at worst an illusion (due to the peculiar way our brains interact with the world). This leads us to think that our intuition that the future is open while the past is fixed is not physically grounded, at least at the fundamental level.

Given this, it seems that any philosopher who aims at providing a perspicuous account of time has to deal with at least two conflicting concepts of time: on the one hand, phenomenal time, of which we possess direct knowledge through mental awareness and, on the other hand, physical time, of which we have indirect knowledge. Whereas phenomenal time is embedded in the perspective of a particular encounter with the world, physical time is inferred from important results obtained in contemporary physics (especially SR and GR). Once the conflict between these two concepts of time has been properly identified, the issue that arises for philosophers is ‘How to manage this conflict?’. Roughly, in recent history of philosophy, two rival options have been privileged, though they both issue from a ‘conflict-avoidance’ strategy: subjectivism and scientism. Subjectivism consists in being exclusively concerned with intuitions to the detriment of science; scientism in abandoning intuitions for the benefit of science. Although these two options have taken on a variety of forms, for our purpose it will suffice to briefly describe each of their two most famous manifestations: phenomenology and the philosophy of inner life (or Bergsonism) on the one hand, and empiricism and naturalized metaphysics on the other.

First of all, many twentieth-century philosophers have been interested in the phenomenology of temporal awareness. Maurice Merleau-Ponty, for example, was highly critical of scientific conceptions of time. He famously put forward a view of time that agrees more with “[...] the descriptions given by literary men and artists than that given by physicists” (Mays, 1972 : 355). As Merleau-Ponty puts it: “[t]here is more truth in mythical personifications of time than in the notion of time considered, in the scientific manner, as a variable of nature itself, or, in the Kantian manner, as a form ideally separable from its matter” (1962: 422). Although “[...] his remarks apply specifically to the human cultural situation rather than to the scientific context [...]” (Mays, id) (“mythical personifications of time” would be regarded as a gross anthropomorphism by physicists), they still inform us about the role that philosophy is, in his view, meant to play. According to Merleau-Ponty, philosophy must concentrate on describing time as it is reflected in human experience, i.e. as the ultimate subjectivity, which “[…] reveals the subject and the object as two abstract moments of a unique structure, namely, presence” (1962: 474).

Of course, Merleau-Ponty’s view may, in some respects, appear to be caricatural. But the fact remains that phenomenology has, from its earliest beginnings, excluded physical time from philosophy’s primary concerns. Edmund Husserl, for example, with his watch-word ‘back to the things’ argues that philosophy must begin with the phenomena themselves: philosophers must concentrate on describing the authentically given – the contribution of empirical sciences is relegated to the background, even within transcendental phenomenology, which reproaches science for not questioning the foundations of the objectivation of the world. As a result, physical time, which is not adequate to the task of explaining how consciousness experiences temporal objects, is neglected in philosophical inquiry, understood as an epistemological analysis of temporal lived experience. For example, whereas Newtonian time (conceived as an empty container of discrete, atomistic nows) can explain the separation of moments in time (e.g., separated tones), it cannot explain the continuity of these moments (e.g., a melody). Such a quantitative view of time must therefore be supplanted by a phenomenological attempt to articulate how flowing objects are experienced. As Husserl (1964: 26) makes clear, a perspicuous account of time must explain how temporal objects, though composed of distinguishable moments (which can be measured by clocks), are apprehended as a unity (rather than as a convoluted patchwork).

Second, some philosophers have been interested in the notion of duration as being a qualitative multiplicity, i.e. a temporal heterogeneity in which “[…] several conscious states are organized into a whole, permeate one another, [and] gradually gain a richer content […]” (Bergson, 2010: 122). This approach is heir to Henri Bergson’s philosophy of inner life, which famously criticizes the scientific spatialization of time: scientists conceptualize time as an ordered arrangement of defined events, rather than as an unceasing flux impressed upon consciousness.³ Specifically, in the Bergsonian view, space is fundamentally unlike time: space is homogenous (in space, things exist separate from alongside each other), divisible, and infinite, whereas time is heterogenous (in time, things interpenetrate and are never completely independent), indivisible and finite. Space is a homogenous medium, whereas time is a process that emerges continuously in the absolute new.⁴ For example, while one often draws a parallel between the ticks of a clock and time itself, Bergson argues that there is ‘no point of contact’ (other than conventional)⁵ between these things: the clock’s hands move through space, not time. According to Bergson, duration (or ‘real time’) is ineffable and can only be grasped through a simple intuition of the imagination; attempts to intellectualize or measure duration are doomed to failure, at least if one seeks to understand reality in its deepest nature. The Bergsonian intuition is, however, not merely a pre-theoretic experience of the world (as described above), but a method that consists in entering into the things (rather than going around them from the outside) to get absolute knowledge. As a result, the common representation of time as a one-dimensional line (on which the present is a point) is, according to Bergson, a pure abstraction: time is mobile and incomplete, and the present consists in the consciousness of our body experiences. Moreover, a scientific concept such as ‘spacetime’, according to which time is just as much a dimension as any of the spatial ones, is quite alien to Bergson: “[b]asically space and time play opposite roles in his philosophy, which might even be regarded as a dualism of space and time” (Lacey, 1999: 17). Crucially, Bergson’s approach should not be confused with phenomenology: whereas phenomenology takes the multiplicity of phenomena to be always related to a unified consciousness, Bergsonism takes the immediate data of consciousness to be a multiplicity.

An immediate objection that could be raised against this first ‘subjectivist’ option of managing the conflict between intuitions and science is to observe that the world existed in time before human beings and human consciousness ever appeared on the Earth.⁶ For example, it appears that the emergence of the first living organisms temporally follows ocean formation, which implies that time was already passing, say 4 billion years ago. It therefore seems simply wrong to claim that time has no reality beyond our subjectivity. Moreover, philosophers (especially metaphysicians of time) may have more ambition than merely describing phenomena. They may want, for instance, to contribute (together with physicists) to the exploration of the fundamental structure of reality and, therefore, to the exploration of physical time. For example, philosophers may aim to account for the notion of ‘the passage of time’, not conceived as a subjective phenomenon, but rather as a central aspect of the scientific picture of the world. Tim Maudlin, for instance, argues that “[t]he passage of time is an intrinsic asymmetry in the temporal structure of the world, an asymmetry that has no spatial counterpart” (2007: 108-109). Of course, such a revisionary (as opposed to descriptive)⁷ way of undertaking metaphysics must be scientifically well informed. After all, assuming that philosophers and physicists can both investigate physical time, they cannot deliver contradictory truths about it. For these reasons, it seems that dismissing scientific resources from philosophical inquiry is methodologically doubtful.

The second option of managing the conflict between phenomenal and physical time is to ignore our intuitions and to concentrate solely on science. This scientistic option has taken on a variety of forms, but we will focus on two of them: empiricism and naturalized metaphysics.⁸ First, many philosophers of physics tend to think that it is solely empirical science that provides access to the fundamental structure of reality. For instance, Otto Neurath writes that “[t]here is no such thing as philosophy as a basic or universal science alongside or above the various fields of the one empirical science; there is no way to genuine knowledge other than the way of experience; there is no realm of ideas that stands over or beyond experience” (1973: 136). This thought is historically based on some ‘verificationist principle’, according to which every synthetic statement must be empirically verifiable.⁹ As a result, most metaphysical statements are meaningless. For example, empiricists typically believe that a true picture of time could only be arrived at by a study of such things as atomic clocks and physical processes. In this perspective, the way the world is pre-theoretically grasped has no relevance. Against the rationalist tradition, empiricists do not see intuitions as cognitive tools designed to produce guidance toward the truth, but rather as a misleading product of our immediate environment, which must in turn be studied by the precise methods of empirical science.¹⁰ According to empiricism, the role of philosophy is merely to react to scientific discoveries by reshaping traditional concepts, such as time and reality. But, while (at least some) philosophers make use of intuitions, their work is of no value when it comes to discerning the fundamental structure of reality. Of course, this is not to say that there are no difficulties in the scientific theories, but rather that training in philosophy or discussing these difficulties with subjectivist philosophers would not help to solve them. Their work is fundamentally misguided, after all.¹¹

Another, though less radical, way of dismissing intuitions is promoted by some metaphysicians of science, who assume that the main role of philosophy is to unify hypotheses and theories that are taken seriously by contemporary science. As James Ladyman and Don Ross famously claim: “[n]aturalized metaphysics must go beyond mere consistency with current science; it must be directly motivated by and in the service of science” (2013: 109). Here the starting point is clearly science; metaphysics comes second and builds upon scientific results. For example, when statistical mechanics was extended to the quantum realm, it was found that in order to obtain empirically adequate statistics, particles of the same kind (e.g., fermions) must be indistinguishable, though they can be counted. This induces a metaphysical response, namely that the Leibnizian Principle of Identity of Indiscernibles is false. Here again, although philosophy contributes to the inquiry into the nature of reality (e.g., in terms of theoretical unification), the use of intuitions is strictly limited. In particular, naturalized metaphysics explicitly excludes any reasoning based on intuitions and a priori considerations, for “[…] there is no reason to imagine that our habitual intuitions […] are well designed for science” (Ladyman & Ross, 2007: 2). Such a metaphysical stance should be contrasted with ‘Lewisian-style’ metaphysics. In short, whereas Lewis takes for granted that abandoning intuitions (e.g., the intuition that causes occur prior to their effects) should be regarded as a cost associated with accepting a metaphysical thesis, naturalists, such as Ladyman and Ross, are not concerned with preserving intuitions at all; they are merely concerned with serving science.

An first objection that could be raised against this scientistic option of managing the conflict is that it rests on a caricature of science. It is wrong to claim that scientists exclusively rely on empirical data and rational methods, at least in the context of discovery. As Jonathan Tallant (2013, 2014) has highlighted, the term ‘intuition’ is frequently applied by scientists to explanations, pictures, and results. This can be illustrated by an ethnographic study, carried out by Ference Marton and Peter Fensham (1993), which is based on data collected from 83 Nobel Prize-winners, drawn from physics, chemistry, and medicine. This study reveals that most of the Laureates (72) say they use scientific intuitions to guide the shape of their inquiries. For example, as Michael Brown¹² declares, “[…] we almost felt at times that there was almost a hand guiding us. Because we would go from one step to the next, and somehow we would know which was the right way to go. And I really can’t tell how we knew that, how we knew that it was necessary to move ahead” (cf. Tallant 2014: 295). Of course, one might reply that the use of intuitions by scientists is not the same as that of metaphysicians. For example, one might argue that scientists, unlike metaphysicians, never use intuitions to justify their claims. But, even if one grants this difference, it is does not follow that metaphysicians always use mere intuitions to this end, nor that they cannot but do so. In particular, although the term ‘intuition’ is notoriously vague, one should agree that both scientists and metaphysicians may use intuitions at least “[…] in the sense of background assumptions influencing how they interpret (empirical) data […]” (Morganti & Takho, 2017: 2570).

This more realistic picture of science seems to match with how Arthur Eddington and John Wheeler introduce physical knowledge. Contrary to Albert Einstein (1936), who claims that “[w]e can view only as miraculous that our sense-experience can be unified by our freely created concepts”,¹³ Eddington and Wheeler do not believe in miracles. In particular, they do not believe that scientific concepts are freely created by pure thought alone divorced from our actual intuitions. They both emphasize the role of inferences from ‘data’, ‘pointer readings’ or ‘information’ to phenomena and knowledge of physical reality. As Eddington points out, all physical knowledge is ‘hypothetico-observational knowledge’, which means that science is an inference from good observations (cf. 1939: 49).¹⁴ This view can even be pushed further if we consider, as Alfred North Whitehead does, that some important scientific concepts – such as velocity, acceleration, momentum, and kinetic energy – are unintelligible when disconnected from temporal experience. For example, it seems impossible to define ‘velocity’ without some reference to the past and the future, which makes essential the importation of this distinction into the physical picture of time. As Whitehead puts it, “[t]his conclusion is destructive of the fundamental assumption that the ultimate facts for science are to be found at durationless instants of time” (1919: 2).

A second objection against the scientistic option is that if our intuitions are indeed a misleading product of our immediate environment, then it should be explained why it is so. There is a need to explain why people across the countries and the centuries are systematically deceived by their intuitions when apprehending time. Such a ‘massive error hypothesis’ cannot be put forward without providing any proportionate explanation. For example, given the ‘block universe’ view of time (which seems implied by SR), the spacetime geometry must have been fixed and settled all at once in some single act of creation. Accordingly, you and I did not come into existence later than dinosaurs or the Big Bang. Rather, if the universe were created at all, then “[…] you and I and all its parts, and all the associated events in which these parts are involved, were created together” (Brogaard, 2000: 345). But if everything is in existence (in some sense of ‘is’ that is both tenseless and timeless), then the advocate of such a model is faced with the problem of accounting for the common fact that “[…] new perceptions enter and exit our minds in successive fashion at what appear to be always later and later times” (Brogaard id.). How does one explain that our cognitive abilities are limited in such a way that we have no access to the future, if there is no difference between the present, the past and the future? And why is it that the times at which we perceive the world appear to be ordered successively? It is not clear that the advocate of the ‘block universe’ view can account for these facts. Of course, one could reply that potential solutions to these issues are to be found outside the philosophical field (e.g., in psychology). But these kinds of buck-passing answers are clearly unsatisfying: one cannot skip the question as to why everybody is mistaken in apprehending time, under the pretense that this could hypothetically be answered by other scientists. Worse still, as Mauro Dorato argues: “[…] the ontology posited by a physical theory should in principle be capable of establishing connections with the world of our experience, for the latter world is the source of the empirical tests of the theory” (2008: 54).

A last objection is that, although it should be acknowledged that many things that have seemed obviously true have turned out, upon inspection, to be false (e.g., that the sun rotates around the Earth), it seems that appealing to rational intuitions is epistemically justified. Two arguments can be invoked in favor of this claim.

First, what we have learnt from early modern attempts to find absolute certainties is that, unless we are willing to become extreme skeptics, we must allow that “[…] it is reasonable to believe things that seem obviously true, in the absence of special reasons to doubt them; and we must allow this even if the beliefs are admittedly not certainties, and cannot be ‘proven’ in any interesting sense of the word” (Zimmerman, 2008: 222). This leads to a methodological principle, sometimes referred to as ‘the principle of credulity’ (cf. Broad, 1939; Swinburne, 1979), that seems reasonable to accept: intuitions must be preserved as long as they are not proven wrong. In that sense, intuitions are ‘innocent until proven guilty’ and their relinquishing should only be envisaged as a last resort, when our best science leaves no hope of preserving the manifest image of the world.

Second, intuitions have demonstrated success in the past. In particular, there are many examples in which a philosophical theory was found flawed because it contradicted an intuition (especially in specific thought experiments). For example, in epistemology, the ‘classical’ theory of knowledge was abandoned because, in Gettier cases (1963), we share the intuition that one can have the justified true belief that p without knowing that p. Moreover, in the philosophy of language, the descriptivist theory of proper names was rejected because, in Kripke cases (1980), we share the modal intuition that, although someone other than the U.S. President in 1970 might have been the U.S. President (e.g., Humphrey might have), no one other than Nixon might have been Nixon. Finally, in applied ethics, the naive utilitarian theory was rejected because, in Thomson cases (1976), we share the intuition that it is not morally permissible to take five organs of a healthy person in order to prevent five people from imminent death, though this action would maximize well-being. These three examples – but there are many more – show that intuitions decisively contribute to philosophical progresses in a great variety of fields and, without further arguments, it is hard to see why it would not be the case in the metaphysics of time. Of course, one might emphasize that these intuitions crucially differ from the intuitions we have about the nature of time, as they are not in tension with important scientific results. But, this does not answer the question as to why, whereas intuitions seem reliable in such various fields as epistemology, philosophy of language, and applied ethics, they would not be so in the metaphysics of time.

A further criticism could be that the three former cases (Gettier, Kripke, Thomson) are not relevant, as they do not involve any reasoning about the metaphysical structure of the world from our intuitions about the world. For example, the knowledge case differs from metaphysical theorizing in that it crucially rests on some concept that we commonly use: knowledge. Assuming that we are competent with that concept, we can elucidate its content by asking about the conditions under which we would employ it. In brief, we ask people if they would employ it in Gettier case, and we find they would not. So, that provides a reason to suppose that our concept of knowledge (as we commonly use it) is not just justified true belief. Yet, this does not seem like a very robust sense of an appeal to intuition in settling how things are with the world: we merely investigate how we are disposed to use some concept (viz. knowledge), to determine what we mean by it. It therefore seems that there is no analogy with metaphysical theorizing. In reply, two things can be said. First, the three former cases were not intended to provide an analogy, but merely to illustrate the fact that intuitions are widely and successfully used in various domains of philosophy. Second, one can easily think of cases where intuitions have played a decisive role in metaphysical theorizing. Consider, for example, Kit Fine’s influential rejection of modal conceptions of essence, according to which “[…] an object [has] a property essentially just in case it is necessary that the object has the property” (1994: 3). The rejection is based on five properties (e.g., ‘being a member of singleton Socrates’, ‘being distinct form the Eiffel Tower’) that necessarily belong to Socrates, but that are intuitively not essential to Socrates. The five properties thus function as five counterexamples to the classical, modal conception of essence, which is thus explicitly rejected by Fine on the sole basis of intuitions. The above arguments against scientism provide as many reasons to think that our intuitions must not only be explained but also vindicated.

Now, while none of the two ‘conflict-avoidance’ strategies (subjectivism vs scientism) is fully satisfying, one may wonder whether all hope of solving the conflict between intuitions and science is gone. In that respect, the situation is not altogether desperate, as a third way – perhaps more demanding, but also more interesting – of managing this conflict can be imagined: the way of reconciliation. As Craig Callender puts it: “[w]e seem to have, to echo another debate, an ‘explanatory gap’ between time as we find it in experience and as we find it in science. Reconciling these two images of the world is the principal goal of philosophy of time” (2008: 339). The main idea behind ‘reconciliation’ is to bridge the gap between the time of human experience and that of science, by critically analyzing and conceptually improving the way non-physicists think about the nature of time. This requires working in close contact with both pre-theoretic and scientific data and, thereby, to develop a framework in which these data can be articulated non-paradoxically. In order to do so, it is worth following three methodological steps. First, one has to provide a rigorous philosophical characterization of our intuitions, by surveying, for example, the different senses in which ‘time passes’, ‘our present extends throughout the whole universe’ and ‘the future is open while the past is fixed’. Second, one has to determine which model of the temporal structure of the world is most appropriate to accommodate these intuitions. Third, one has to describe this model in such a way that it meets the main imperatives of our most salient scientific theories. The present book fits into this general philosophical scheme.

Although this scheme places a lot of weight on the vindication of intuitions, this should not overshadow the fact that other factors (e.g., simplicity, parsimony) also play an important role when evaluating a model. In that sense, our main objective is not to arrive, whatever the cost, at the model that best fits a set of specific intuitions – such a model might turn out to be complex, unparsimonious, or in tension with our best science, after all. And, clearly, even if intuitions play a crucial role in theorizing, they certainly do not trump all these other factors. Rather, our main objective is to find an equilibration between how well a model coheres with best science, the picture it provides of the world, how parsimonious, simple, and so on the model is, and how well it accords with our intuitions. This methodological precision is intended to avoid the risk of settling on models that are pretty baroque.

From a more general perspective, it may be worth contrasting the methodology promoted in this book with other philosophical attempts to reconcile the manifest image with the scientific image. In that respect, the Canberra Plan, initiated by David Lewis and Frank Jackson (cf. Jackson, 1994, 1998; Lewis, 1970, 1972), seems particularly relevant. This program of philosophical methodology and analysis (which brings together many people who were associated with the Australian National University in Canberra during the 1990s) primarily aims at reconciling a certain account of conceptual analysis with philosophical naturalism. Specifically, the approach can be broken in two steps (cf. Braddon-Mitchell & Nola, 2008: 7). First, we collect together the ‘platitudes’ concerning the X to be analyzed (e.g., colors) – these platitudes may simply be the large number of what we (or experts) can agree are the truths about X (e.g., snow is white and normal perceivers in normal conditions have experiences of white caused in them). These agreed-upon platitudes about X are expressed in two kinds of terms: the ‘outsider’ O-terms, i.e. the terms that get their meaning from outside the platitudes (e.g., ‘perceivers’, ‘conditions’), and the ‘insider’ T-terms, i.e. the terms that play a theoretical role specified in the platitudes (e.g., the terms for the colors of objects and experiences). Second, we discover what in our best theory of the world, if anything, plays the theoretical roles spelled out by the T-terms; or, to put it like Braddon-Mitchell and Nola, “[…] what our current best theories tell us there is in the world to serve as realizers of the theoretical roles specified in the platitudes” (2008: 7). Sometimes, nothing in the current sciences can serve as a realizer of the specified role. When that happens, most proponents of the Canberra Plan, who are usually physicalists, conclude that those realizers fail to exist (as it has been the case, for example, with the phlogiston).

Another current approach is more focused on our assertions and practices. First, we examine the conditions under which we use certain terms, and the conditions under which we have certain practices as part of the manifest image. Second, we find what is in the scientific image that explains those assertions and practices. For instance, if we go about talking about colors, and engaging in colors practices, we look at the scientific image and see what explains this: perhaps the fact that there is electromagnetic radiation of a certain range of wavelengths visible to the human eye. We then conclude that this is what it is for there to be colors. As before, there may be cases in which what we learn about what explains our assertions and practices leads us to say that, in fact, we are mistaken: there are no such things. So, here again, perhaps if what explains our use of ‘free will’ assertions and practices turns out to be very different from what we expected, we come to conclude that there is no free will. Alternatively, perhaps we revise our notion of what free will is, to come in line with what it is that explains our assertions and practices.

These two popular approaches share an important feature: they seek to ‘locate’ the manifest image inside the scientific image, allowing that what we find might both explain our having those intuitions as well as vindicate the relevant intuitions, or might explain our having those intuitions without vindicating them. In general, any program that aims at reconciling the two images should meet the following minimum requirement: it has to explain, by appeal to the scientific image, why things seem as they do according to the manifest image. In that respect, the methodology promoted in this book deserves the label of ‘reconciliation program’, although it diverges from the two previous approaches in some crucial aspects. First, my methodology is more speculative and exploratory than those described above. As it will become clear in §4, I do not only examine well-established scientific theories to explain and vindicate relevant intuitions, but I also explore nascent theories (e.g., the causal set theory) that, although they do not (yet) strictly belong to our best science, provide a valuable insight into what our best science might look like in the near future. Second, as a consequence of the previous point, the conclusion that my methodology allows me to reach is necessarily more modest than one might expect: all that can be shown is that science (including nascent theories) does not a priori rule out the possibility that certain relevant intuitions adequately reflect the structure of the world. Although the modest nature of the conclusion may be disappointing, it should be contrasted with the fact that any conclusion based on well-established scientific theories, at least in fundamental physics, is at best temporary. As it will be detailed in Sect. 4.6, General Relativity and Quantum Mechanics are incomplete; they fail to capture phenomena that combine high energy densities and strong gravitational fields (e.g., the very early universe, and the dynamics of black holes). These theories will therefore 1 day plausibly be replaced by a theory of Quantum Gravity (such as the causal set theory) with superior predictive and explanatory power.

1.2 A Temporal Asymmetry: The Open Future and the Fixed Past

The present book is mainly related to the metaphysics of time and the philosophy of science. It offers a detailed study and a systematic defense of a key intuition we typically have, as human beings, with respect to the nature of time: the intuition that the future is open, whereas the past is fixed. As will be shown in the second chapter, there are many ways in which this intuition may manifest itself. But, as a first approach, it will suffice to think of some future and past events. For example, whereas it seems unsettled whether there will be a fourth world war, it is settled that there was a first world war. In that sense, whereas nothing (in the present or the past) a priori predetermines that a large-scale armed conflict will blow up in the future (it is ours to prevent such a disaster!), there is nothing we can do to prevent WWI from having taken place in the past. Likewise, whereas it seems that behaving in an environmentally responsible manner may prevent some animal species from extinction, there is nothing we can currently do to bring back the dodo birds. It is however worth noting that, although the question of human abilities (what we can or cannot do) may inform us on the asymmetry in openness between the future and the past, it would be a mistake to reduce this asymmetry to purely anthropocentric considerations. In this book, one will also consider senses in which the future and the past may respectively be said ‘open’ and ‘fixed’ in a world without humans (or before humanity emerged).¹⁵

Generally speaking, it seems very hard to discard the intuition that the future is open while the past is fixed. For, this intuition, which is to be understood as a pre-theoretic representation (or concept) of the temporal structure of the world, is reflected everywhere in our relationship to the world. We have certain practices that are time asymmetric: we plan for the future not the past; we deliberate about the future not the past; we take some past apparently fixed facts as inputs into our deliberation; we act as though we can causally influence the future but not the past; etc. We also have certain emotions and attitudes that are time asymmetric: we feel regrets about the past, not the future; we anticipate the future, not the past; we generally place greater value on future events over past ones, etc. Finally, we have observations, which, around here, are of asymmetric: we have records of the past, but not the future; we observe that entropy increases towards the future and away from the past, etc. All these things are taken as manifestations of a basic intuition that deeply structures our relationship to reality, and that must not only be explained but also vindicated for the reasons set out in Sect. 1.1.¹⁶

However, the main models of the temporal structure of the world do not reflect any asymmetry between the future and the past. According to presentism and eternalism, the future and the past are ontologically on a par. Eternalists hold that both the future and the past exist, while presentists hold that neither the future nor the past exists. In other words, the two main competing models of the temporal structure of the world do not ontologically distinguish the future from the past (either both of them exist or none of them exists). Therefore, neither eternalism nor presentism seems able to account for our basic intuition regarding the nature of time. Of course, one might claim that the asymmetry between the open future and the fixed past does not need to be grounded in the temporal structure of the world, but merely in some natural processes (e.g., the increase of entropy). Nonetheless, one will provide reasons to think that these natural processes are not as important as some philosophers would have us believe for temporal asymmetries. For example, the increase of entropy – which arises from the collective behavior of many microscopic entities – at best postpones the problem: if there is no directedness in fundamental physics, where does the thermodynamic asymmetry in time come from?

This project might be criticized for taking too seriously the intuitive asymmetry between the open future and the fixed past. After all, some arguments taken from science, especially from contemporary physics, have been put forward to show that the asymmetry is at best a non-fundamental phenomenon, at worst an illusion. For example, the ‘block universe’ view of time, which is inferred from the results of SR, does not reflect any asymmetry. It regards reality as a block-like four-dimensional ensemble, lacking a moving present, wherein all times and events are equally real. In other words, the spatiotemporal model favored by contemporary physics does not reflect any difference between space and time that somehow accounts for the fact that whereas there is no here-there space-asymmetry, there should be a past-future time asymmetry. Likewise, the fundamental laws of physics, which are time-reversal invariant, do not underpin any asymmetry regarding the nature of time: for every physically allowable sequence of events, the inverse sequence of time-reversed events is also physically allowable. For example, if one watches a movie that shows a ball rolling, the fundamental laws of physics cannot tell whether the movie is being projected correctly or in reverse. Therefore, accounting for the asymmetry between the open future and the fixed past as a fundamental phenomenon seems to require developing an alternative model of the temporal structure of the world to that favored by contemporary physics, namely a spatiotemporal model that has the intrinsic resources to ground the asymmetry. This model will turn out to be a specific version of C. D. Broad’s growing block theory of time (GBT).¹⁷

The final step of the book will be the reconciliation of this alternative model (GBT) with contemporary physics. Although physics cannot settle the debate about the nature of time,¹⁸ it crucially informs and frames the debate. As Yuval Dolev puts it: “[physics] has shaped the manner in which we set out to study our world, and our understanding of almost every aspect of it” (2006: 188–189). It therefore seems that no metaphysical contribution to the question of the nature of time can be provided without observing the main imperatives of our best physical theories. In that respect, considerable efforts have to be devoted to the understanding of the main postulates and consequences of both the Special and General theory of relativity, but also to the nascent theories of quantum gravity (which aim to unify GR with the principles of quantum mechanics). The main purpose of this final step is to show that GBT is expressible in a relativistic spacetime setting and can, thereby, offer a naturalistic basis to the intuitive asymmetry between the open future and the fixed past. In this perspective, the causal set approach to quantum gravity (CST) will be a matter of great interest, since it allows for the ‘coming-into-existence’ of events through a discrete stochastic process and might, therefore, underwrite a specific version of GBT. At the end of the day, the time of human experience might turn out to be a more faithful reflection of the time of science than what most philosophers believe.

1.3 Three Main Desiderata

The present book aims to satisfy three main desiderata: (i) to provide a coherent, non-metaphorical, and metaphysically illuminating characterization of the intuitive asymmetry between the ‘open future’ and the ‘fixed past’; (ii) to determine which model of the temporal structure of the world is most appropriate to accommodate the asymmetry; (iii) to reconcile this model (and hence the asymmetry) with our best physics. Each of these desiderata, which echo the three methodological steps introduced in Sect. 1.1, will feature as a separate chapter within the book. The second chapter deals with characterization, the third chapter with temporal models, and the fourth chapter with our best physics. From a broader perspective, satisfying desiderata (i), (ii), and (iii) should allow one to obtain a framework within which pre-theoretic and scientific data can be articulated non-paradoxically. This seems required for bridging the gap between the time of human experience and that of science. Let’s have a closer look at these three desiderata.

First, although the intuition that the future is open and the past fixed is widely shared, it is not a straightforward matter to determine the nature of the asymmetry it reflects. So, in the second chapter, I review the various philosophical ways of characterizing the asymmetry in order to account for our intuition. In particular, I wonder whether the asymmetry should be characterized in a perspectival way (it merely reflects how humans interact with the world) or in a substantial way (it reflects how the world truly is). It is worth noting that perspectival and substantial characterizations do not exactly serve the same function: the latter aim both at explaining and vindicating our intuitions, whereas the former are more about vindicating our having certain practices. My conclusion is that substantial characterizations are more promising, as they are the only ones that offer a fundamental explanation (i.e. not causal or thermodynamic) of the asymmetry reflected by our intuition. Specifically, assuming physical indeterminism (i.e. the doctrine that the future history of the world is not nomologically necessitated by its current history), I argue that the asymmetry between the open future and the fixed past is a kind of worldly unsettledness that should be characterized in ontological terms: there being facts of the matter about what happened, but not about what will happen. This characterization, which stems from the ‘no fact of the matter’ account of openness, will be shown to be superior to the alternatives in explanatory power, intelligibility, and in how it coheres with interesting senses of openness.

Second, while the above characterization requires the asymmetry to be reflected in the temporal structure of the world, neither of the two main competing models – eternalism and presentism – ontologically distinguishes the future from the past. Eternalists hold that both the future and the past exist, whereas presentists hold that neither the future nor the past exists. Thus, neither eternalism nor presentism can avail itself for the ‘no fact of the matter’ account of openness, while keeping the past fixed.¹⁹ For instance, although the ‘no fact of the matter’ account of the open future is available to presentists, they cannot endorse it without acknowledging that the past is open in the very same sense. In the third chapter, I therefore propose to reject both of these models in favor of a ‘growing block theory’ of time (GBT), which is naturally seen as better designed to accommodate the asymmetry. Against powerful traditions, I do not introduce GBT as a hybrid between eternalism and presentism, but as real alternative: GBT is the only asymmetric theory of time (i.e. necessarily sometimes the spatiotemporal structure it describes is not reflection invariant) that accepts Temporal Becoming (i.e. the creation of new things in the present). Finally, I address the so-called ‘epistemic objection’, which purports to show that GBT leads to absolute skepticism about where we are temporally located, by appealing to the continued existence of bare particulars and to introspective knowledge. I take this occasion to explain (i) how existence in the past should be conceived and (ii) why it differs sharply from existence in the present.

Third, GBT is often criticized for conflicting with important results of contemporary physics. For example, Hilary Putnam (1967) and Wim Rietdijk (1966) have both argued that the view according to which the future is unreal requires an objective notion of absolute simultaneity, while such a notion is rejected by the theories of relativity. Yet, since one aims to end up with a scientifically coherent account of time, it has to be shown that GBT is expressible in a relativistic spacetime setting. In the fourth chapter, I reply to the Putnam-Rietdijk argument by putting forward a certain approach to quantum gravity, which is formulated in terms that naturally underwrite GBT. In particular, I argue that the causal set approach to quantum gravity (CST), when enriched with the ‘classical sequential growth’ dynamics (CSG), depicts reality more as a ‘growing being’ than as a ‘static thing’. This result apparently disproves the widespread claim that fundamental physics undermines any attempt to defend an open-future view. In short, it is possible to be both a scientific realist and a defender of the view that the future is genuinely open. Finally, I move from science to science fiction and show that GBT is, in principle, compatible with some scenarios such as time-travel.

As I hope to have indicated above, despite all that has already been accomplished, much work still remains to be done in reconciling the manifest image of the world and contemporary science. In what follows, I intend to contribute to this undertaking by offering my own thoughts on the challenges faced by philosophers who aim to bridge the gap between the time of human experience and that of science.