Article
Reference
Priority Monism Beyond Spacetime
LE BIHAN, Baptiste
Abstract
I will defend two claims. First, Schaffer's priority monism is in tension with many research
programs in quantum gravity. Second, priority monism can be modified into a view more
amenable to this physics. The first claim is grounded in the fact that promising approaches to
quantum gravity such as loop quantum gravity or string theory deny the fundamental reality of
spacetime. Since fundamental spacetime plays an important role in Schaffer's priority monism
by being identified with the fundamental structure, namely the cosmos, the disappearance of
spacetime in these views might undermine classical priority monism. My second claim is that
priority monism can avoid this issue with two moves: first, in dropping one of its core
assumption, namely that the fundamental structure is spatio-temporal, second, by identifying
the connection between the non-spatio-temporal structure and the derivative spatio-temporal
structure with mereological composition.
LE BIHAN, Baptiste. Priority Monism Beyond Spacetime. Metaphysica, 2018, vol. 19, no. 1, p.
95-111
DOI : 10.1515/mp-2018-0005
Available at:
http://archive-ouverte.unige.ch/unige:110912
Disclaimer: layout of this document may differ from the published version.
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1
Priority Monism Beyond Spacetime
Baptiste Le Bihan, University of Geneva
Forthcoming in Metaphysica: International Journal For Ontology and Metaphysics
Abstract: I will defend two claims. First, Schaffer's priority monism is in tension with many
research programs in quantum gravity. Second, priority monism can be modified into a view more
amenable to this physics. The first claim is grounded in the fact that promising approaches to
quantum gravity such as loop quantum gravity or string theory deny the fundamental reality of
spacetime. Since fundamental spacetime plays an important role in Schaffer's priority monism by
being identified with the fundamental structure, namely the cosmos, the disappearance of spacetime
in these views might undermine classical priority monism. My second claim is that priority monism
can avoid this issue with two moves: first, in dropping one of its core assumption, namely that the
fundamental structure is spatio-temporal, second, by identifying the connection between the non-
spatio-temporal structure and the derivative spatio-temporal structure with mereological
composition.
Keywords: priority monism, quantum gravity, mereology, spacetime, fundamentality.
1. Introduction
According to “priority monism”, a view defended by Schaffer (2009, 2010), the whole
cosmos is more fundamental than its proper parts. The cosmos is identified with a substantivalist
spacetime
1
instantiating natural properties without the mediation of objects or substrates: properties
are directly pinned down on a spatio-temporal substance (a view called “monistic substantivalism”
or “super-substantivalism”
2
). In Schaffer’s account proper parts are derivative with respect to the
whole they are parts of, and so the cosmos admits of derivative proper parts (spacetime regions
instantiating properties). It means that there exists a relation of ontological priority connecting the
whole to the parts: when a collection of parts xs composes a whole y, y is ontologically prior with
respect to the xs.
3
Spacetime regions are identified with material objects, implying that material
objects possess derivative existence. Because any proper part of the cosmos is a material object, it
1 I will always refer to “spacetime” rather than “space and time”, or “space”, taking for granted the lessons of
2 The name originates in Sklar (1974).
3 Whether or not this relation of ontological priority requires a modal analysis is a delicate matter. Steinberg (2015)
has argued that priority monism is in conflict with the ontological dependence of any whole on any of its parts.
Matteo (2017) has claimed that this argument relies on the view that priority monism is necessarily true, and argued
that we should prefer the view that priority monism, if true all, is contingently true (a view also defended by Siegel
2016). My own view is that priority monism might be wholly disconnected from modal matters by being neither
necessarily nor contingently true, in any interesting sense of “necessarily” and “contingently”, contra a realist
interpretation of metaphysical modality (cf. Sidelle 1989, Le Bihan 2015a). Therefore, I will leave aside the delicate
question of the modal status of priority monism.
2
means that the category of material objects includes both ordinary objects like tables and chairs and
non-ordinary monstrous objects such as the sum made of the Eiffel Tower and the top of your nose.
Also, any proper part of the universe, seemingly empty, is a material object since any conceivable
volume of spacetime is a spacetime region and therefore a material object. In brief, the whole
cosmos (identified with spacetime) is more fundamental than its parts (identified with spacetime
regions/material objects).
Schaffer's view is interesting for many reasons described at length in Schaffer (2009). For
instance, priority monism recognises that many features of spacetime regions and material objects
are redundant. Indeed, parts of objects do follow the same mereological pattern that spacetime
regions they occupy: any part of an object occupies a region of spacetime which is itself a part of
the spacetime region occupied by the object. Then, why not just identify objects with spacetime
regions in line with super-substantivalism in order to account for this mereological harmony?
Priority monism also gives an explanation of why material objects are always located in
spacetime: material objects are nothing other than chunks of spacetime. Therefore, by nature, no
material object can escape spacetime: each object simply is (a hunk of) spacetime. Likewise, it
explains why there is always at most one material object for each spacetime region. Since a material
object is a region of spacetime, if two numerically distinct material objects were to be located at the
same region of spacetime, these two numerically distinct material objects would have to be
numerically identical, which is clearly a contradiction. Therefore, priority monism excludes the
possibility of co-located material objects.
This ontological picture is also taken to be nicely consistent with both general relativity and
quantum field theory, since the category of material object (including physical particles) does not
seem to be fundamental in quantum field theory
4
and, at the very least, is not necessarily required to
give an ontological interpretation of these physical theories. But, as we shall see, general relativity
and quantum field theory are not the end of the story, and by looking more generally to the whole of
physics, it will become clear that classical priority monism could well enter into conflict with the
future theory of quantum gravity.
Within the paper, I shall defend two claims. First, in spite of its many theoretic virtues,
Schaffer's priority monism is in tension with a genuine possibility described by many research
programs in quantum gravity. Interestingly, it means that priority monism, in its original version,
could turn out to be empirically refuted. Second, priority monism can be substantially amended in
order to be turned into a view more amenable to this physics. The first claim is grounded in the fact
4 Although, it is still debated whether quantum field theory is consistent with an ontology of particles (see for
instance Halvorson and Clifton 2002). At the very least, it seems that if real at all, physical particles will have to be
very different from classical entities persisting through time, in such a way that it would more natural to stick to an
ontology of fields by being eliminativist about physical particles (Le Bihan, 2015a).
3
that many approaches to quantum gravity such as loop quantum gravity or string theory are often
interpreted as denying the fundamental reality of spacetime. Since fundamental spacetime plays an
important role in Schaffer's priority monism by being identified with the fundamental structure,
namely the cosmos, the disappearance of spacetime would undermine classical priority monism. My
second claim is that priority monism can appeal to a two-step strategy in order to address this issue:
First, in dropping one of its core assumptions, namely that the fundamental structure is spatio-
temporal, second, in using the notion of composition to understand the connection between the non-
spatio-temporal fundamental structure and the spatio-temporal derivative structure. Therefore, this
article aims at using some preliminary results in physics and philosophy of physics in order to make
a point in the field of contemporary metaphysics.
As I have already argued (Le Bihan, 2016), substantivalism about spacetime is not a
necessary component of priority monism. Spacetime may be construed along a relationalist picture
as a collection of spatiotemporal relations obtaining between matter points or natural properties.
Actually, although Schaffer endorses monistic substantivalism (2009), he examines at length
priority monism without considering super-substantivalism (2010). Leaving aside the exact nature
of Schaffer's views about the connection between priority monism and super-substantivalism, I shall
argue that priority monists should prepare themselves to let go of both relationism and
substantivalism–at least, when these views are understood as describing the fundamental structure.
(There is room for interpretations in which the fundamental structure and the derivative structure do
not share the same basic ontology–relational or substantial. But let me leave aside this question in
order to focus on the basic ontology of the fundamental realm.) As we shall see, contemporary
physics may well establish that there is no fundamental-substantial or relational-spacetime.
2. Quantum Gravity
Quantum gravity is the name of the collection of research programs aimed at bridging the
gap between general relativity and quantum field theory, two theories known to be mutually
exclusive (for a general introduction aimed at philosophers, cf. Matsubara 2017). Indeed, gravity is
construed as a geometrical feature of spacetime in general relativity, whereas it is not described at
all by quantum field theory (although the framework of QFT uses spacetime, it does not give us a
theory of spacetime). In this section, I will briefly mention two popular research programs in
quantum gravity in order to show why they imply that spacetime emerges from a non-spatio-
temporal ontology.
According to loop quantum gravity (LQG hereafter), the natural world is fundamentally
4
made of a collection of superposed non-spatio-temporal structures called “spin foams”
5
and
displaying strange features like discreteness and fuzziness (see Rovelli 2004 and Rovelli and
Vidotto 2014, for a summary aimed at philosophers cf. Huggett and Wüthrich 2013). These
structures can be thought of as being made of “spacetime atoms” with particular properties, and
must give rise to a structure similar to general relativity in order to explain its predictive success in
the domains where general relativity has been well-confirmed. Each of these structures does have
its own system of locality, and they differ in general from the system of locality we observe at our
macroscopic scale. As in quantum physics, it is quite difficult to understand the ontological status of
quantum superpositions. But LQG triggers an additional issue, namely that the system of locality
we find in our ordinary phenomenal space, or as described by general relativity, is in general not the
same as the one we find in the fundamental structure (cf. Wüthrich 2017). Indeed, neither our
familiar macroscopic space and time nor the spacetime of general relativity can in general be easily
mapped onto these structures: the fundamental and the emergent structures are sometimes
geometrically deviant. Huggett and Wüthrich describe the situation as follows:
[Not] only does the quantum superposition frustrate the applicability of locality criteria,
but there is a sense in which even a spin network corresponding to a single term in the
superposition is not amenable to the kind of localization that may be required to ensure
empirical coherence. The problem is that any natural notion of locality in LQG—one
explicated in terms of the adjacency relations encoded in the fundamental structure—is
at odds with locality in the emerging spacetime. In general, two fundamentally adjacent
nodes will not map to the same neighbourhood of the emerging spacetime [...]. Hence
the empirically relevant kind of locality cannot be had directly from the fundamental
level. (2013, 279)
Therefore, if something like LQG is true, neither our familiar space and time nor the
spacetime of general relativity can be fundamental. Indeed, the fundamental structures described by
LQG differ in too numerous ways from them: in particular the fundamental structure is discrete and
has a different organization.
6
We may distinguish two conceptual issues with the disappearance of spacetime in LQG.
First, how are we going to explain the predictive success of general relativity? We need to account
for its predictive success by relating the new theory of quantum gravity, for instance LQG, to
general relativity, perhaps as an approximation under particular circumstances, or perhaps as a
genuinely emergent structure. In the absence of such a story, knowing why a spatio-temporal theory
5 Spin foams are the equivalent of classical GR spacetime, and spin networks are the equivalent of space. Spin works
are only a first step towards the recovering of GR spacetime: an explanation of the relation obtaining between GR
spacetime and spin foams.
6 The status to be played by time in LQG is still ill understood, but it is very likely that the strangeness affecting
space will also impact time. It will certainly impact the features of time that are common to space (they are both
dimensions made of relations) since, after all, we will need to recover GR spacetime. The recovering of specific
features of time (like flow, or direction) will also be a difficult task and is called “the problem of time”.