Motus Mentis

Bill Vallicella, the Maverick Philosopher, recently posted an item critiquing a defense of “presentism” by the Canadian philosopher John Bigelow.

Philosophical presentism is the view that only presently existing things have any sort of existence at all. It stands in contrast to “eternalism”, which maintains that all points in time — past, present, and future — are equally real. Both of these views present (pun intended) some difficulties.

I joined in the discussion over at Bill’s; I’ve always found strict presentism oddly narrow and exclusive, in particular because its radical denial of the reality of past events seems to preclude any kind of causal explanation for what exists now. (How can something that has no existence of any kind form any part of a causal chain?)

Eternalism, which is the view that sees all of time as a 4-dimensional “block”, raises questions of its own: most importantly why “now” should be privileged in any way at all, despite our experience of the “passage” of time through past and present into an unknowable future. (My own feeling is that this is perhaps the greatest of all mysteries, deeply connected to the mystery of consciousness itself.)

Bill’s essay was focused on presentism, though, and so I raised the following objection (slightly edited here):

In special relativity, there is is a concept of “spacetime separation”, which describes the four-dimensional relation between events.

A negative value means that light can travel from event A to event B; events so positioned relative to one another are said to have a “spacelike interval”, and it means that event A can have a causal influence on event B. (In such cases, event B is said to be within the “light cone” of event A.) This also means that A and B can be placed in an unambiguous temporal sequence — meaning that all observers, regardless of their state of motion and reference frame, will agree that A happens before B.

A zero value means that the events have a “lightlike interval”: only something traveling at the speed of light can move from event A to event B. (This means also that the time interval between them is zero!)

A positive value means the events have a “timelike interval”, and this is why it’s interesting here: because B is outside the light-cone of A, there is no objective “fact of the matter” about which comes first. Observers in different reference frames (i.e., in different states of motion relative to the two events) can legitimately disagree about which precedes which (and in some particular frame, they will be simultaneous).

This is known as the “relativity of simultaneity“. How does presentism account for this?

To clarify this: presentism makes a definite ontological claim about the non-existence of past events — but the relativity of simultaneity calls into question the nature of “past” and “present”, by demonstrating that the ordering of events is not always objective, but can (in the case of events separated by a “timelike” interval) be observer-dependent. How can presentism withstand such a challenge?

In response, Bill suggested that we read a paper, published in 2012 by the mathematician Christian Wüthrich (Erdös Number: 3), entitled The Fate of Presentism in Modern Physics.

Because the paper is dense and technical, Bill suggested we read it together, section by section. The first is called “Introduction: ersatzist presentism“. Here’s a summary (please follow along in the original if this topic interests you, which of course if very well might not!):

The gist of this introductory section is a look at how a presentist might even find a way to make a case against eternalism without acknowledging the ontological reality of the past. We read:

Defining ‘presentism’ in a way that saves it from being trivially false yet metaphysically substantively distinct from eternalism is no mean feat… The main issue in the triviality debate as I see it concerns the representation of events without an accompanying ontological commitment. If the presentist can find a way to represent non-present events without eo ipso [thereby] committing herself to their existence, then expressing her metaphysical disagreement with the eternalist seems rather straightforward. This naturally leads to an ersatzist position which introduces non-present events merely for representational purposes without imbuing them with physical existence.

So: “ersatz” events are those that comprise the vast majority of time — of which the “present” is a fleeting subset — but which don’t really exist, and are mentioned by the presentist only for the sake of discussion. (There is a considerable literature, apparently, about the nature of this abstract, “ersatz” time, with which I am completely unfamiliar, and about which Bill has said he will write a clarifying post.)

The author proceeds to clarify what is meant by presentism and eternalism, in the context of the four-dimensional manifold I referred to above as “block” time. (In this paper this manifold, this “block” containing all of time, is referred to by the symbol M.)

Eternalism and presentism are then taken to disagree as to overwhich points of M they quantify when quantifying over all spatiotemporal events where physically existing entities can be located. In this context, eternalism is understood as the position claiming physical existence for all events in M. In contrast, presentism partitions M into past, present, and future events. This partition results, e.g., from assuming an equivalence relation S (‘simultaneity’) to be defined on M such that the equivalence classes contain co-temporal events. Time, on this view, is the one-dimensional linearly ordered quotient set of these equivalence classes. One such equivalence class is privileged in that it contains the ‘present’ events, the set of equivalence classes to its past according to this ordering contains the ‘past’ events and the set of equivalence classes to its future the ‘future’ events.

Professor Wüthrich immediately addresses the issue I mentioned above: in “block” time, why should any slice of it be privileged as “the present”? But he mentions it only to make clear that it isn’t relevant to the discussion at hand (I have bolded the key passage here):

An obvious worry arising from this manner of characterizing the position is that presentism does not just amount to the assertion that only present events or entities exist, but also that the present undergoes a dynamical ‘updating’, or exhibits a quality as of a fleeting whoosh, and that this additional dynamical aspect is what threatens the substance of the debate between the presentist and her eternalist opponent. In order to capture this dynamical quality, the thought goes, the presentist must quantify not just over the events contained in one equivalence class corresponding to the present present, but also over all events in all the other equivalence classes containing the past and future presents. Once this point is granted, it seems as though presentism deflates into admitting all the events of M as existing. But this clearly misses the presentist’s point: the presentist’s sum total of existence remains a proper subset of the eternalist’s, fleeting whoosh or not. I am not pretending as if to characterize in satisfactory detail what exactly constitutes this dynamical quality is without difficulty. But for present purposes, presentism should be understood as a merely ontological hypothesis making an assertion as to what exists, and not an ideological statement about the qualities—dynamical or otherwise—of that which exists. Perhaps this is a mistake. But if it is, at least not without precedent.

“Fleeting whoosh!” I like that very much. (That “fleeting whoosh” is, of course, the cardinal mystery of time.)

Now the author describes the heart of the problem that the relativity of simultaneity creates for presentism:

The remainder of this essay shall assume, however fallibly, that presentism is a metaphysically substantive thesis markedly different from eternalism. It contends that physical existence is restricted to a spatially extended manifold of events simultaneous with the here-now. This view comes under severe pressure from modern physics, most notably from special relativity (SR), as shall be explicated in Section 2. The source of the tension is found in the fact that in SR, andhence in modern physics, space and time are intertwined in a way such that whether two given spacetime events exemplify the relation of simultaneity is no longer an absolute and global matter. But if simultaneity cannot serve as on absolute and global basis for determining whether or not a spatially distant event is present (in the temporal sense), then we seem to lack an objective basison which matters of physical existence could turn for a presentist metaphysic.

This looks bad for presentism! But perhaps all is not lost:

Naturally, presentists have responded to the challenge. The problem, of course, should not be misconstrued as dealing with an in principle insurmountable inconsistency between presentism and physics; rather, the challenge amounts to grounding the necessary distinctions (past, present, and future) in a way that is responsive to modern physics while remaining faithful to presentist intuitions.

In other words: perhaps there is a way that we can define “the present” that addresses the apparent relativism of simultaneity in such a way as to leave presentism standing as a defensible position. The author offers four angles:

The presentist responses to this challenge, both actually stated and hitherto unarticulated, shall be chronicled in the remainder of this essay, together with an assessment of the prospects of success and the price tag for each response. In an attempt to bring order into the variegated multitude of presentist strategies to counter the challenge from modern physics, a systematization is offered in Section 3. The basic distinction of presentist responses is into compatibilist and incompatibilist strategies, with the former arguing that presentism is compatible with the truth of SR despite initial appearances and the latter accepting their incompatibility while rejecting that this entails the denial of presentism. It turns out to be useful to introduce a distinction orthogonal to the one between compatibilism and incompatibilism: presentism can not only be compatible or incompatible with respect to SR, but to fundamental physics—contemporary or prospective. This distinction derives its utility from the fact that there are a number of presentist retorts readily admitting that their view is inconsistent with SR but insisting on its compatibility with fundamental physics. These two distinctions span a matrix of four types of strategies. The two boxes of strategies accepting an incompatibility with contemporary, and possibly future, fundamental physics will be examined in Section 3. Responses purporting a compatibility with either contemporary or at least future fundamental physics will be dealt with in Sections 4, if they also allege a compatibility with SR, and 5, if they accept that presentism is inconsistent with SR. Section 6 will take stock and dare a rather negative comprehensive appraisal of the prospects of presentism to survive the pressure from modern physics in any form that permits retaining its appeal.

So: the stage is set. To hold on to presentism, what has to go? Special relativity? Modern physics (such as quantum mechanics)? Neither? Both?

We will resume in a later post.