Keywords

9.1 Introduction

In the last years, some attention has been given to what has been called “New Mechanicism” or “New Mechanical Philosophy” (Craver & Tabery, 2019; Glennan, 2017). One of the primary motivations for New Mechanicism is that in opposition to some tradition in the philosophy of science, theories and universal laws are not the basis for explanations in science – for instance. The discussion occurs within a naturalised approach to metaphysics; however, as noticed by Kuhlmann and Glennan (2014), despite increasing interest among science philosophers, physics has never been the focus of more significant debates concerning the New Mechanicism. That may come as a surprise, not only because of the central role that physics always has taken in the philosophy of science but considering the long tradition that mechanisms and Mechanical Philosophy have in physics. The simplest explanation for this absentness is that fundamental physics not only may not be compatible with the mechanistic approach, but even more dramatically, fundamental physics may undermine the mechanistic program in science. According to Kuhlmann and Glennan (2014):

There is prima facie reason to be concerned that the two pictures do not fit well together. The neo-mechanists suppose that mechanisms are composed of objects with definite properties, where these objects are connected via local causal interactions. Quantum mechanics (QM) calls into question whether there are such things as objects with definite properties and whether causal relations can be understood in terms of local interactions between such objects. Moreover, mechanisms are hierarchical in the sense that the parts of mechanisms may themselves be complex objects composed of subparts which are components of lower-level mechanisms. It seems then that even complex macroscopic mechanisms must supervene on a set of “objects” that behave non-classically. This dependence upon a non-classical micro-level might seem to infect the ontological and even explanatory claims of the New Mechanists.

Thus, considering the above description, there are four suppositions are still made by neo-mechanists about mechanisms: (1) that there are objects with definite properties; (2) these objects are connected to each other through causal interactions; (3) relations can be understood in terms of local interactions between objects; (4) there is a hierarchical structure between mechanisms.

Taking these assumptions, the problem, the clash between neo-mechanists and fundamental physics would be the “fact” that fundamental physics challenges all the four suppositions stated above. If this is the case, then, as stated by Kuhlmann & Glennan, it seems that not only we can be sceptical about the existence of mechanisms in fundamental physics, but since macroscopic mechanisms must supervene on fundamental physics entities and processes, and if these do not fit into the mechanistic picture, then fundamental physics can even undermine mechanistic ontology and ambition. Kuhlmann clearly restates that:

The primary concern is not how the notion of mechanisms can be captured and how exactly mechanistic explanations work. Rather, the main question is to what extent physics, in particular fundamental physics, deals with mechanisms in the first place. A second question concerns whether the character of the physical processes that underlie all natural and social phenomena may even endanger the tenability of mechanistic reasoning in the special sciences. Kuhlmann (2018)

This raises at least the following questions: (1) are the problems that fundamental physics places on the New Mechanical Philosophy any new in the history of physics? (2) Even if it is the case that fundamental physics is incompatible with mechanisms, why does it place a problem with the suitability of the New Mechanistic approach on special sciences?

9.2 Traditional Mechanical Philosophy in Physics

9.2.1 Descartes

Mechanical Philosophy has a long tradition in Physics. It can even be said that his foundational philosophy, both in the ontological, epistemic, and methodological sense. Beginning with Descartes. As Garber notes:

“Descartes saw the physical world and its contents as a collection of machines. At the end of his Principia Philosophiae, Descartes tell the reader that “I have described this earth and indeed the whole visible universe as if it were a machine: I have considered only the various shapes and movements of its parts” (Pr IV, 188). Later in the Principia he writes:

“I do not recognize any difference between artifacts and natural bodies except that the operations of artifacts are for the most part performed by mechanisms which are large enough to be easily perceivable by the senses—as indeed must be the case if they are to be capable of being manufactured by human beings. The effects produced in nature, by contrast, almost always depend on structures which are so minute that they completely elude our senses. (Pr IV, 203)”

Similarly, Descartes suggests to an unknown correspondent, seeking to clarify his position that “all the causes of motion in material things are the same as in artificial machines.”” (Garber, 2013: 16)

According to Descartes, physical objects or bodies are Res Extensa. That is, physical objects are similar to three-dimensional shape-geometrical objects, with length, breadth, and depth. Therefore, there is a clear and radical cut between mental and physical properties, between subjects and objects.

Since all physical objects are extended entities, there are no atoms; all physical objects must be divisible, no matter their size. However, for the same token, there cannot exist extension without physical objects; that is, there are no atoms nor empty space. Therefore, all space is completely “filled” by bodies and material objects; Space is a plenum. Space is the composition of physical objects of different sizes. Some are so tiny, that has an indefinite extension – corpuscula.

Consequently, every physical object is in direct contact with its surrounding bodies and, indirectly, with all other objects, via the imbricate of corpuscula that composes the plenum. Hence, all physical objects are deeply intertwined, and any motion of one physical object is necessarily communicated to the others. It is the image of the natural world in Descartes: a colossal clock with nothing more to consider than shape and motion. A machine that underlies and grounds all physical phenomena. A machine in which each part, or different sub-machines, works together in complete harmony and crosses all compositional levels. The natural world (physical, biological, etc.) is the arrangement of interrelated geometrical parts (like cogs in a clock). The scientific task is to analytically decompose any phenomenon or body into its components since all phenomena is fully explained by the causal and local interaction of physical objects, according to the laws of physics. All natural phenomena can and should be explained by the motion and collision of particles of matter and its composition alone. This is the epitome, the zenith, of the mechanistic philosophy in physics.

However, on the one hand, motion is a necessary condition to explain all phenomena, but why the universe, the “clock”, and the “machine” started to move in the first place is not explainable in mechanistic terms. Even in Descartes, Mechanical Philosophy is not self-sufficient.

9.2.2 Newton

The decisive challenge to the mechanistic approach in physics came with Newton. As put by Cohen (1999:57):

“Newton was still “a mechanical philosopher in some sense,” but not any longer in the strict sense in which that designation was usually understood. Whereas a strict mechanical philosopher sought the explanation of all phenomena in terms of what Boyle once called those “two grand and most catholic principles of bodies, matter and motion”, Newton came to believe that “the ultimate agent of nature would be . . . a force acting between particles rather than a moving particle itself”.

On the one hand, likewise traditional mechanical philosophers, such as Descartes or Boyle, Newton regarded the universe as a machine ruled by universally applicable axioms. Axioms that could be discovered by scientific analysis and that would ground the explanation of physical phenomena. In fact, that would be the fundamental role of science or experimental philosophy. Like traditional mechanical philosophers, the basic ontology is constituted by movable material bodies with extension (shape and size), hardness and impenetrability, as explained by Newton in Rule III of the Rules of Reasoning in Natural Philosophy of the Principia.

On the same path, in the introduction of the Principia, there is a moment where Newton seems to be hopeful concerning the possibilities of Mechanical Philosophy:

For in book 3, by means of propositions demonstrated mathematically in books 1 and 2, we derive from celestial phenomena the gravitational forces by which bodies tend toward the sun and toward the individual planets. Then the motions of the planets, the comets, the moon, and the sea are deduced from these forces by propositions that are also mathematical. If only we could derive the other phenomena of nature from mechanical principles by the same kind of reasoning! (Newton (1999): 382)

However, it is unclear how Newton’s characterisation of forces can be integrated into a mechanistic ontological view. As Janiak (2021) stated:

His second law indicates that a body moving rectilinearly will continue to do so unless a force is impressed on it. This is not equivalent to claiming that a body moving rectilinearly will continue to do so unless another body impacts upon it. A vis impressa—an impressed force—in Newton’s system is not the same as a body, nor even a quality of a body, as we have seen; but what is more, some impressed forces need not involve contact between bodies at all.

The main concern is gravity – of course. Gravity is both a kind of central force and an impressed force. Thus, a body moving in a straight line will be instantly deviated by a gravitational force that was originated by another material body placed far away without any intervention (collision) of another body(ies). Therefore, as Janiak (2014: XXVII) recalls:

These elements of the Principia make conceptual room for a causal interaction between two bodies separated by a vast distance, one enabled by Newton’s concept of an impressed force. Aspects of this idea became known in philosophical circles as the problem of action at a distance.

Hence, one of the significant successes of the Principia – gravitational force – is not explained by any underlying mechanism. Of course, the same could be said about hardness, for instance—or inertial force. However, the gravitational force is more striking since, not only because it is the only force in the Principia with a specific law but because it is a force that acts at a distance. That is, a mass A instantly changes the state (of motion) of a mass B, and simultaneously, its state is changed by a mass B, without direct contact (collision, for instance) or by the transmission of force (throughout other masses or a medium) between them.

Why is this pose a problem to Mechanical Philosophy, and what is its relationship with contemporary fundamental physics?

The term mechanical in the context of Mechanical Philosophy meant (see, for instance, McGuire (1972)) many different things throughout the history of philosophy. Nevertheless, contact action was, in general, broadly accepted as a necessary (although not sufficient) condition for a mechanical explanation. Thus, if the only thing that is clear for most mechanists is that contact action is necessary, then since gravitational force in Newton is an “action at a distance” and therefore is not compatible with the mechanistic philosophy, then, as Leibniz would put it “Principia renders gravitation a “perpetual miracle” because it does not specify the physical mechanism underlying it” Janiak (2021). Alternatively, as also Andrew Janiak (2008: 53) puts “If Newton contends that gravity exists, he must admit that material bodies act on one another at a distance, thereby violating a crucial norm of the mechanical philosophy (in all its guises).”

So, with Newton’s axioms of movement and with the description of the world displayed in books I and II of the Principia, on the one hand, we find a realisation of the mechanical philosophers’ ideal: all phenomena seem to be explainable uniquely by the knowledge of the position and the momenta of material bodies, and its laws of momentum’ determination, transmission, and conservation (The Three Axioms of movement). All phenomena seem to be explainable by the application of this simple ontology. However, on the other hand, Newton’s account of gravitational force encompasses the instant action of a force on a distant matter without any intermediation, something that Mechanical Philosophy cannot follow.

9.3 Contemporary Fundamental Physics and (New) Mechanical Philosophy

9.3.1 Entanglement

One of the most challenging QM’s features to traditional Mechanical Philosophy is entanglement. The term “entanglement” was originally coined by Schrödinger (1935) and referred to a special case “where two (or more) particles exist in an eigenstate of a certain observable, such as angular momentum, but neither particle is in an individual eigenstate of that observable” (Huang, 2007: 62).

In the literature, it is possible to find several examples of quantum entanglement. Probably, the most well-known is the one from Bohm (1951: 611–622). Giving a very simplified version of that example (Cordovil, 2015), consider a system in a spin-zero state that decays into a pair of two particles, namely, two electrons—electron A and electron B—that head off in directed opposite directions. Since they are electrons, they have a half-value of spin. Using the standard convention, the spin state is either “up” (+1/2) or “down” (−1/2).

In this case—and only considering the spin factor—there are two independent spin wave functions α e β, representing respectively the state “up” and the state “down”. In this state the total spin is definite—singlet state—but the individual spins (of the electrons 1 and 2) are not defined. All we can know, by the conservation of angular momentum, is that if one is in the spin-state “up”, the other will be in the spin-state “down”, and vice-versa. More specifically, if electron A acquires (or shows) the value spin “up”, electron B will acquire (or show), through measurement, the value spin “down” immediately and no matter the distance (and vice versa). That is, the spin state of electron A is dependent on the spin state of electron B, and vice-versa. This is the reason why it is said that particle A and particle B are entangled. The mystery is: how can one account for something that was at one point indefinite regarding its spin (or whatever the property under investigation) and that suddenly becomes definite even though no physical interaction (direct or indirectly) with the other subsystem occurred? How, instantly, can the interaction via measurement with one particle immediately alter, at a distance, the state of the other particle?

In a way, the challenges posed to Mechanical Philosophy by the Newtonian description of the force of gravity, until the advent of General Relativity, seem to be not much different to those posed by quantum entanglement. In both cases there is an instantly interaction between two spatially separated physical objects, without the mediation of other objects or entities. Furthermore, in both cases the change of the state of one object instantly changes the state of the other object, no matter the distance. Thus, if one of the critical elements of Mechanical Philosophy is that the world is composed of local causal interactions, then, since Newton, that does not seem to be compatible with fundamental physics. Consequently, maybe apart from the brief period between the appearance of general relativity and the formulation of quantum mechanics, Mechanical Philosophy always had a problematic relationship with fundamental physics. Then, the reason why physics has not been part of the contemporary discussions on New Mechanicism should not be because there is a clash between fundamental physics and New Mechanicism, as was defended by Kuhlmann & Glennan. First, because that clash has always been present in the history of physics. Secondly, because general relativity is also fundamental physics and the abovementioned clash between physics and New Mechanicism may not exist. Such analysis was not made.

It could be argued that the conflict between fundamental physics and Mechanicism is more acute in QM than with Newtonian gravitation. Although the force of gravity has been a mystery for the Mechanical Philosophy advocates, there was always the expectation that gravity would eventually be explained mechanistically. For instance, Leibniz, Boshokovic or even Kant strongly reacted against the non-mechanical character of gravitational force and tried to offer alternatives. Due to this well-known discomfort, one could defend that the non-local feature of Newtonian has not had the same status that entanglement does since the physicists and philosophers of physics alike accept the latter. However, that is not the case. For instance, some interpretations or reformulations of quantum mechanics try to incorporate entanglement into a mechanistic framework. This could happen, for example, by defending a local interpretation of quantum mechanics.

The de Broglie-Bohm theory, also known as the pilot-wave theory, proposes that particles have definite positions and trajectories, where the wave function serves as a guide or “pilot wave” that determines how particles move. The theory is fully deterministic and does not require any non-local effects. Another example would be the Many-Worlds Interpretation, where the wave function describes a “multiverse” in which every possible measurement outcome occurs in a separate parallel universe. Alternatively, any other interpretation that incorporates the idea of hidden variables, where the wave function is not a complete description of the system but instead reflects our lack of knowledge (for instance Araújo et al., 2009; Lopez, 2016). Or a relational kind of interpretation of QM along the lines of Esfeld’s Thin-Objects Moderated Ontic Structural Realism (Esfeld et al. (2015)).

So, in fact, as in the case of gravitational force, even though our standard fundamental physics may not be compatible with (New) Mechanical Philosophy, some physicists and philosophers are still committed to providing a mechanistic explanation (or understanding) of QM.

9.3.2 Still, the QM’s Challenges

According to Kuhlmann (2018) there are other non-classical features, besides entanglement, where QM seems to clash with the ontological commitments of the New Mechanism, namely the Indeterminacy of properties and Non-localizability of quantum objects. However, that is also a matter that falls on the ongoing discussion in the context of QM’s interpretations debate. For instance, according to Allori (2015), a particle position can be taken as a primitive variable, and therefore there is at least one property with always a value well-defined: position. Again, we could say that what drives those interpretations is a reaction against QM’s “weirdness” in the line of the ontological model based on mechanical philosophy. In that case, the situation in physics is not so different from what it was at the beginning of the eighteenth century.

On the other hand, Kuhlman argues that some of these features can be, in part, addressed by decoherence: at the “quantum level” that clash will still exist, but at the “classic level”, due to decoherence, it could be almost unnoticeable. Nonetheless – of course – at the ontological level, this does not solve the incompatibility.

9.4 Against the Universality Thesis of QM

Even if we accept that QM is incompatible with (New) Mechanical Philosophy, why would it place a problem to New Mechanism in special sciences?

Kuhlmann provides one main reason: the universality of QM. That is, QM applies to all physical domains, and all physical phenomena should be grounded and explained ideally upon QM. Let us call it the universality thesis of QM. That is the reason why, even though decoherence can give an approximative explanation of why the “classic level” seems to be different from the “quantum level”, it does not solve the problem of how to bridge the two realms since, in the end, everything is part of QM’s domain. The assumption that the properties of the upper compositional level are supervenient, reducible or identical to the properties of the fundamental level (those that are the object of fundamental physical theories) are very present in von Neumann-Dirac axiomatisation and in most of the QM’s debates. It is an atomistic heritage to assume that fundamental physical theories apply to all physical domains, or that, in principle, all physical reality is describable by fundamental physics.

However, why should we assume that all physical domains of reality are reducible to the most basic set of properties, relations and laws of the putative ultimate physical domain? Why don’t we consider the possibility that classic objects have (some) different properties from quantum objects and endorse a pluralism ontology against the QM universality thesis? That is, to defend that there are ontologically emergent classical properties distinct and autonomous from quantum object’s properties - for instance, the property of position. Therefore, we could accept that quantum objects do not have nor position or trajectory, but that would be unproblematic to any ontological account of all non-quantic domain.

It can be argued that QM does not contain any precise criterion for identifying the frontier between micro and macro or between quantum and classic. That there is nothing in QM fixing such a border. Nevertheless, on the one hand, since classic objects do not share the same set of properties as quantum objects, then is unsurprising that QM does not fix such a frontier. On the other hand, that border is not settled on a specific spatial scale since the claim is that (some) classical properties ontologically differ from quantum properties. That is, if classical properties ontologically emerge from quantum properties, the distinction between quantum and classic (that is, ontological) is not identical to the difference between macro and micro (that is phenomenological). It, therefore, does not make sense to ask for a scale frontier. Also, it is essential to recall that at the macro-scale there are some quantum phenomena.

How to make sense of this emergentist hypothesis?

If we move towards a relational ontology according to which there are changeable structures of relations, then individuals are relational entities that can have qualitative transformations. The central assumption of this ontology is that every physical object is a relational entity occupying a relational location in a structural complex, and new levels of organisation or structure can emerge – against the one-level micro-physicalist picture of the world. Adopting this view, one is in a position to argue that a given structure can instantiate a new type of property that is not manifested at the level of the structure’s components, i.e. the relations and their relata. Furthermore, one can take a step forward and try to explain the emergence of a structure’s new property and its micro-irreducibility. This can be done by virtue of a qualitative change at the level of those objects as relata of the relations that compose the structure. That is, emergent structural attributes are attributes of specific macro-structured networks of transformative and interdependent relations between integrated system’s parts. This can and must be explained via a relational-transformative account of interlevel emergence (Santos, 2015, 2021) as specific modes of the composition of their parts’ transformative and structurally interdependent relations.

In this view, new emergent higher-level structural properties and laws may be generated from the transformative relations between lower-level sub-structures. Relations and structures could still be the actual star performers of science and reality, but a structuralist view of the world would not be equal to an essentially flat or static view of it. In particular, this view could be made possible if structures were seen as primarily ‘concrete structures’, taken as relations between first-order properties, in contrast to mere ‘abstract structures’, taken as higher-order, formal (logical or mathematical) properties of relations (Cordovil et al., 2022).

Within this ontological working hypothesis in a measurement there is a qualitative change of the quantum object/system being measured in virtue of the new relational network that integrates (quantum object/system – measurement object/system). Namely, since the measurement object/system is characterised by having the property of position, then every other object/system can only be a relatum of the measurement relation if it undergoes a qualitative change from which emerges the property of position. This qualitative change is not reducible to its microstructure. That is, we can think that, in essence, a measurement is an act of transformation of physical reality.

On the other hand, this ontological proposal would understand the role of decoherence as the interaction between the quantum object’s structure and the environment.

Moreover, this view seems to go precisely in the direction of the mechanistic claim that levels are not monolithic stratification of the universe, nor are they fundamentally a matter of size or causal interactions within a level.

Besides, if it is vital to New Mechanical Philosophy to reject the idea of universal laws that grounds, axiomatically, all explanations in science, then the rejection of the universality thesis of QM is the more natural move to make.

9.5 Conclusion

In conclusion, as in the late seventeenth century, contemporary fundamental physics (or at least, QM) is in contradiction with mechanistic ontology. However, the reaction to the mystery placed by the gravitational force in Newton, like the reaction to some mysterious’ quantum features, seems to have in common the (spontaneous) defence of some kind of mechanical ontology. So, in fact, the situation does not seem to be new and if Physics has not been playing a central role in the literature devoted to New Mechanical Philosophy that may not be simply a consequence of the hypothetical incompatibility between QM and Mechanicism ontology, but due to adhesion to a specific QM’s interpretation and the micro-physicalist assumption of the universal character of QM.