First things first. The technical terms used in this short andsimplified piece will be somewhat controversial. The main reason for this is that many philosophers tend to invent their own terms or use old terms in new ways. In addition, even a neologism can still be very closely related to previous words or concepts. Thus we have “sense”, “intension”, “reference”,“denotation”, etc. used in different ways by different philosophers. So it’s often merely — or at least partly — a case of (to use Derrida’s term) “sign-substitutions” in which the new “signs” hardly differ from the substituted ones — at least not in important ways. And then there’s always the (fairly) new kids on the block — such as David Chalmers’ “primary intension” and “secondary intension” (in his two-dimensional semantics).
That said, all these different terms do refer to different concepts or functions and there’s no claim in the following that they’re all (really) identical.
Intensions and Extensions
The German philosopher Rudolf Carnap (1891–1970) believed that an expression means what it means because it possesses both an extension and an intension(alternatively, a reference to a class and a sense). He believed that this approach would effectively bring the meanings of expressions down to earth and away from abstractaand/or “private” mental entities. Extensions are, after all, collections of concrete objects and intensions are the means to get at those extensions (or at the objects within those extensions). That means that concrete objects (or collections of such objects) are important when it comes to extracting the meanings from expressions.
In much of the traditional picture, it was either abstract objects or private mental entities which fundamentally determined the meanings of expressions. Of course Carnap still allowed for the existence and use of abstract intensions (that’s if if they are in fact abstract).
Expressions (or statements) have references for the parts and then for the whole; and also an intension for the parts and then for the whole. (In this manner, Carnap in many ways simply updated Gottlob Frege with new jargon or with his own sign-substitutions.)
So did Carnap see intensions as concepts, predicates or properties?
Carnap on Necessity
Carnap saw modality in terms of semantics. That effectively means that modal properties don’t belong to objects qua objects: they belong to statements qua statements. That is, statements have modal properties, not objects.
Take this statement:
“A is necessarily B."
Basically, traditionalists believed that this statement is about things; not about sentences or words. Thus Carnap — to paraphrase — offered this alternative:
“The expression ‘A is [necessarily] B’ states a semantic necessity.”
What’s necessary in the above isn’t the properties of things: it’s a property of the sentence itself(e.g., as is the case with analytic statements).
Thus the expression “A is B” implicitly (or elliptically) refers to prior expression which generate this further necessary statement. That means that necessity is either stipulated and/orconventional. In other words, it’s a property of the sentences and concepts we use (i.e., it isdedicto necessity): not the things we refer to with those sentences (i.e., de re necessity).
Thus, according to Carnap, modal logic is actually a branch of semantics. This means that necessity is generated by the meanings or concepts which are themselves properties of the sentences which are taken to assert certain necessities about things. So if necessity were really a property of things and their properties, then it would be ontology (rather than semantics) which deals with such properties.
Thus Carnap explained necessity by way of analyticity. That is, in
a = b
both a and b are “concepts of the same individual”; not variables which stand in for concrete objects. So perhaps we should write the above in this way instead:
[Ca] = [Cb]
Thus if a and b are concepts (C) of — or for - the same individual, then we can create an analytic statement.
For example, in the often-used example
“All bachelors are unmarried men.”
the words “bachelors” and “unmarried men” both refer to (or denote) different concepts of the same set of individuals (i.e., they have the same extension). Thus, according to Carnap, modal contexts were really “disguised quotational contexts”. In other words, the sentence
i) “Bachelors are unmarried men.”
is transformed into:
ii) “The sentence ‘Bachelors are unmarried men’ is analytic.”
Thus i) above is an example of de re necessity: it’s a statement about concrete objects: bachelors and unmarried men.) And ii) above is an example of de dicto necessity: it’s a statement about the words “bachelor” and “unmarried”.
In terms of necessity, it’s not necessary that the concrete objects unmarried men are bachelors. Yet in terms of stipulational/conventionalnecessity (which may sound like an oxymoron), it is necessary that the word “unmarried man” can be substituted (to use a term Quine used)salve veritate with the word “bachelor”.
Alternatively, we can used a conditional to get this point across:
(Uc ⊃ Bc)
However, that conditional above (in which ‘c’ means concept) doesn’t (really) capture the modal property necessity. Thus we can have a biconditional instead:
J.S. Mill believed that the
meanings of singular terms were their denotations. Denotations are
usually seen as objects of some description. Therefore any proposition which had a term whose meaning was its denotation would literally(?)
have an object within it.
Thus
Gottlob Frege argued that meanings can't be denotations. Instead of
objects being in propositions, Frege suggested that “different ways
of thinking” of an object are contained within propositions. Thus
if a belief (or desire) can't literally contain the object of that
belief (or desire), then it must contain “specific ways of thinking”
of that object. That is, we shouldn’t mix up the ontology of
objects with propositional structure and meaning. They belong to
different ontological worlds.
Similarly,
ways of thinking can't literally contain objects within them
and neither do minds generally. It followed, for Frege, that what he
called “sense” had to be distinguished from what many call
“reference” or “denotation”. (Fregean) senses therefore
determine references. They determine the ways in which the referents
are thought about. Thus senses are “modes of presentation” of
objects; not the objects themselves. Indeed we have no access to an
object other than via a mode of presentation or a sense.
Think
about what the contrary would entail.
It
would mean that the object itself would be in the proposition
or in its linguistic expression. That can't possibly be the case.
Propositions are abstract entities (according to Frege); whereas
objects are mainly concrete. An abstraction can't contain something
in concreto.
The Individuation of
Objects
In terms of the modes of presentation of an
object, it can also be said that a criterion of identity must come
along with a “principle of unity”. In addition, an object must
have some kind of temporal longevity if it's to be deemed an object
in the first place.
How
can an object have temporal longevity?
It
does so because it has a principle of unity. That principle
tells us that certain facts about that object unify it and they do so
because they tell us what things about that object must remain in
order for the object to remain as that very object. The unity of the
object is what makes it the thing it is over time.
Why
should an object have a single criterion of identity? Why not many
criteria?
It's
traditionally thought that an object’s “essence” will determine
what we take to be a criterion of identity for an object.
However, just as we had choices as to what could be criteria of
identity, so we may choices as to what constitutes the essence of a
single object.
This
is where we depart from Frege and from many other philosophers.
One
set of essential properties may work for one group of individuals (or
one set of situations) and another set may work for another group of
individuals (or set of situations). Why assume that there's one real
essence (which may contain a set of properties) of an object and no
more?
Perhaps
it all depends on the modes of presentation of that object.
Each
different mode of presentation may determine its own essence. For
example, under a mode of presentation that's supplied by physics, an object
may have an essence specified in terms of its molecular or atomic
structure. This would be a constitutional or inherent essence.
However, under the mode of presentation of people who relate to
- or use - the object under scrutiny, the essence may be specified in
terms of that object’s role or its relation to the scrutiniser.
Many
people will have different ways of individuating the very same
object. It will depend on how that object is seen - both literally
and metaphorically. It will depend on our particular relation to that
object. It will also depend on the cognitive baggage which we bring to
the object under scrutiny.
People
with different beliefs - or different agglomerations of knowledge - will
individuate the very same object in different ways. We could of
course have a God’s eye view of an object. Wouldn’t that view
involve an infinite conjunction of properties and relations which
belong to the scrutinised object? Alternatively, perhaps a God’s
eye view of an object would entail an infinite disjunction of properties instead.
That is, an infinite set of possible characterisations or
individuations of the object. Mortal individuaters can't of
course use infinite conjunctions or infinite disjunctions. Mere mortals can't
even comprehend them. Thus a God’s eye view of the object at hand
would only be of use to the person with God’s eye – viz., God
himself.
It can be said (just to get it out of the way) that
there's no direct denial of the world (or of an “independent
reality”) by any/most theorists of embodied cognition. Neither is there
a stress on subjectivity. What theorists of embodied cognition do
claim is that the way the world is perceived (as well as how it's
conceptualised and categorised) is determined by how each animal (including human beings) is
embodied within the world.
It
can also be said that metaphysical realists have always known about the “limitations” of our senses and minds
when it comes to correctly understanding the world. They believed
that those limitations, however, must be transcended in order to
represent the world in the most accurate (or truthful) way possible. Of course many would argue that human beings can't
transcend their embodiment in that they can't escape from their
physiological and anatomical nature (which, in turn, determines our
sensorimotor experiences). Yet reason and cognition itself (even if
embodied) can surely transcend some of these limitations.
In a certain sense
(weak or strong) it can be said that some of those engaged in
embodied cognition are anti-realists or even idealists (of some
other appropriate metaphysical ist). Indeed many
uphold the self-description "constructivists".
In terms of
anti-realism, it's the case that theorists of embodiment don't uphold
the cognitivist/classicist (or, for that matter, realist)
position that the world is determinate and “cut at the joints”
into kinds/universals, facts or whatever. Or, rather, they happily
admit that this may well be so. Nonetheless, such a world isn't
passively mirrored or even represented by our cognitive system/s.
Instead, because human beings are built in very particular ways
(as well as being essentially “goal-directed”), this largely
accounts for how the world is represented or even given to us.
Thus even if the world is determinate and neatly cut at the joints,
that wouldn't matter because our sensorimotor systems aren't built to
perfectly replicate that cut-up world.
Whether
cognitivists/classicists are realists in the traditional metaphysical
sense is hard to say. They're certainly seen (by workers in embodied
cognition) as viewing human beings (as well as other organisms) as
passive re-presenters of a determinate and circumscribedworld.
Or, in Richard Rorty's phrase, (at least some) cognitivists see human
minds as “mirrors of nature”.
I've just compared
embodied cognitivists to anti-realists; though pragmatists is
also a good description of them – at least in a limited
sense.
Many evolutionary
theorists and pragmatists have
often said (in the last 100 years or so) that, from an
evolutionary point of view, there's often little point in mirroring
or even (precisely) representing nature. (That's if it's
metaphysically possible in the first place.) Like the theorists of
embodiment, they've argued that the precise mirroring of nature can
often be counter-productive from an evolutionary (or goal-directed)
point of view. More precisely, accurate representations (or even
truthful ones) may end up being time-consuming or irrelevant. What
matters is getting a job done. And that job is (usually) survival.
Other questions to
ask may be about how we human beings are embodied (along with our
sensorimotor machinery) and how that will determine how we represent
the world. Indeed how we represent/view the world is also determined
by how we deal with the world. In the words of the theorists
of embodiment, we human beings “construct” the world (or
construct “our worlds”).
Concepts
and Categories
Are the theorists of embodiment taking a line similar to that which the
British Empiricists took when they argue that the formation of mental
concepts and categories is determined – or made possible – by the
specific sensorimotor experiences (as well as their limitations)
animals have?
This is empiricist
in that our senses (or our sensorimotor
experiences) are seen to be the basis of later concepts
and categories. That parallels (at least to some extent) the
British empiricist theory that “sense
impressions” are the basis of later “ideas”.
Indeed John Locke famously said
that there's “nothing in the mind
that wasn't first in the senses” [1690]. Cognitivists of
embodiment, however, add the important detail that humans - as well as other animals - are limited to which concepts and categories they can
form by the nature of their sensorimotor experiences and therefore by
their physiological and anatomical nature.
So what about Kant?
Kant argued that
the our a priori and transcendental “categories” and
“concepts” determine (therefore also limit or constrain) all
experiences [1787]. In the case of embodied cognition, however, it's
not that
i) mental
categories and concepts determine experience.
it's that
ii)
experiences determine our categories and concepts.
What is
transcendental (if in a weak or metaphorical sense) is the
psychological and anatomical embodiment of the animal concerned. It's
that embodiment which will determine the types of sensorimotor
experiences animals have. Thus those embodied experiences also
determine the nature of the resultant concepts and categories formed
by human beings and perhaps also by other species of animal.
The
Construction of the Physical/Social World
Instead of
emphasising how "conceptual schemes", languages, "language games", "paradigms", epistemes, etc. somehow “construct” the world (as
relativists, anti-realists, etc. have done), theorists of embodiment
emphasise how bodies, sensorimotor experience, engagement and the
environment play parts in constructing the world.
In the case of
relativists, anti-realists and older types of constructivist, the
arrow pointed from mind to world. However, in the case of embodied
cognition, the arrow seems to point from world to mind. Thus:
world
→ mind (metaphysical realists, etc.)
world
← mind (anti-realists, relativists, idealists, etc.)
world
→ mind/body (theorists of embodiment)
On close
inspection, however, it may be better to say that for the theorists
of embodiment the arrow points in both directions: from world to
mind/body and from mind/body to world. That's partly because the
world hasn't completely dropped out of the picture. The environment
(along with the goal-directed behaviours and actions aimed at that
environment) clearly has a causal impact on minds and bodies.
However, the sensory and embodied nature of human beings helps us
“construct” (to some extent) that world at the very same time.
Thus:
world
↔ mind/body (theorists of embodiment)
Nonetheless, some
philosophers have moved beyond the simple physiological and embodied
construction of the physical world into areas which are a
little bit more controversial.
George
Lakoff and Mark Johnson [2003], for example, argue that such
embodied constructions involve the projection of “schemas” which
help us to understand the world. The important point here is that this
understanding is deemed (by Lakoff and Johnson) to be “metaphorical”
in nature.
Constructivist
positions can also take on an explicitly political hue. In
constructivist epistemology, for example, construction is seen (at
least by people like Ernst von
Glaserfeld [1989]) as being a “radical, creative, revisionist
process”; not one of mirroring nature or even one of representing
(as such) the world. In this process, human beings (or persons)
construct a tailor-made “knowledge system” that represents not
nature and society; but a subject's experiences of nature and society. Not only that:
the knowledge-system is intended to (politically) rectify the
existing representations/constructions of the world which the subject
is unhappy with.
These
pieces are primarily commentaries on the 'Ontic Structural Realism
and the Philosophy of Physics' chapter of James Ladyman and Don
Ross's book Every Thing Must Go. There are also a handful of
references to – and quotes from – other parts of that book.
Ladyman
and Ross (L & R) offer a list of four statements which they
believe summarise the position of “standard metaphysics”.
Take
(i).
(i)
“There are individuals in spacetime whose existence is independent
of each other. Facts about the identity and diversity of these
individuals are determined independently of their relations to each
other.”
The
problem is how to take the word “independent” in the above. One
can accept individuals and also believe they that they aren't
independent of other individuals. That is, the existence of
individuated objects and their lack of independence aren't
mutually exclusive. What's more, one can accept the “identity and
diversity of these individuals” and also deny that such
“individuals are determined independently of their relations to
each other”. In other words, I don't see why a commitment to
individuals necessarily means that one also accepts their complete
independence from all other individuals (or from events, processes,
conditions, states, etc.).
In
addition, it's simply false that metaphysicians have accepted all
that's claimed in (i). Randomly, take the various monists in history and
philosophers like Bradley and A.N. Whitehead. They certainly didn't
believe that individuals are “independent of each other”.
What
about L & R's second statement? They say that “standard
metaphysicians assume” the following:
(ii)
“Each has some properties that are intrinsic to it.”
Here
again what was said about claim (i) partly goes for claim (ii) as
well. Throughout the history of Western metaphysics there have been
metaphysicians who would now be classed as anti-essentialists. Indeed
we could go back to Heraclitus
to find anti-essentialists (or at least to find proto
anti-essentialists).In addition, we had the medievalnominalists.
Come the 20th
century, there have been many anti-essentialist metaphysicians and
philosophers.
What
do essentialists believe? That an individual “has some properties
that are intrinsic to it”. In addition, some of the ontologists
who've (broadly speaking) accepted the bundle theory of individuals
could also be classed as anti-essentialists. (In that they would have
denied the statement that each individual must have at least some
intrinsic properties.)
Discernibility
and Individuality
One
method for distinguishing two objects is basically Quine's reworking
of Leibniz. Quine called it
“absolute discernibility”. L & R express his position
this way:
“Quine
called two objects... absolutely discernible if there exists a
formula in one variable which is true of one object and not the
other.”
This
is a reworking of Leibniz(with
the addition of references to “formulas”) thus:
(x)
(y) (F) (x = y ⊃. F (x) ≡ F (y).)
One
way a and b can be deemed to be “absolutely
discernible” is if they “occupy different positions in space and
time”.
Now
for “relatively discernible”.
According
to L & R,
“[m]oments
in time are relatively discernible since any two always satisfy the
‘earlier than’ relation in one order only”.
This
clearly makes a moment in time relational in nature. Or at
least its relativelydiscernible nature is accounted
for by its relational nature (i.e., ‘earlier than’, 'later than',
etc.).
What's
just been said about time is similar to what's also said about space
(as well as the “mathematical objects” which measure it). L &
R write:
“An
example of mathematical objects which are not absolutely discernible
but are relatively discernible include the points of a
one-dimensional space with an ordering relation...”
More
precisely,
“...
for any such pair of points x and y, if they are not
the same point then either x > y or x < y but not
both.”
Here
there's a fusion of points in space with moments in time. Thus x
and y are absolutely discernible because x is before
(or “earlier than”) y or x is after (or “later
than”) than y. In other words, x can't be both
earlier than and later thany (as well as vice
versa) at one and the same time.
Now
let's take L & R's definitions of discernibility and
individuality. They write:
“The
former epistemic notion concerns what enables us to tell that one
thing is different from another. The latter metaphysical notion
concerns whatever it is in virtue of that two things are different
from one another, adding the restriction that one thing is identical
with itself and not with anything else.”
At
first glance these definitions come across as two different ways of
saying the same thing. Clearly the second definition (“whatever it
is in virtue of that two things are different from one another”) is
ontological in character and the former (“what enables us to tell
that one thing is different from another”) is, as L & R say,
epistemic. However, don't the two definitions fuse? That is, in order
to know “whatever it is in virtue of that two things are different
from one another” (an ontological fact) one would need to employ
the epistemic tools which “enable us to tell that one thing is
different from another”. Thus the ontological question merges with
the epistemological question (or vice versa).
Black's Spheres, Substantivalism & Relationalism
We
can say that because of the spatial differences between Max
Black's two spheres (in his well-known thought experiment),
sphere a and sphere b would only be classed as “weakly
discernible” on L & R's picture.
L
& R make Max Black's example more concrete (as well as
scientific) by talking about fermions instead of spheres (which are a
mile apart). According to L & R:
“Clearly,
fermions in entangled states like the singlet state violate both
absolute and relative discernibility...”
Fermions
“in entangled states like the singlet state” aren't absolutely
discernible because there are neither spatial nor temporal means
to disentangle each fermion from other fermions. (Hence the technical
term “entanglement”.) However, Max Black's two spheres are also
spatially indiscernible in that they're in constant movement
around a figure of eight. Thus sphere a would be continuously
occupying a spacial point which had only just been occupied by sphere
b – as well as vice versa.
(The only way out of this would be to either literally or
imaginatively freeze the movements of both spheres – though surely
that's unacceptable.)
One
problem which can be raised about objects (as well as about L &
R's position on objects) can be expressed by stating two positions:
i)
Objects have their intrinsic naturesindependentlyof the rest of the world.
ii)
Objects can exist independently of the rest of the world.
This
problem specifically arises in the context of “points of spacetime”
rather than objects. (Although it may be said that they amount to the
same thing.) In this case, L & R use the word “exist” (as in
ii) above). This is also a product of two different positions:
substantivalism and
relationalism. Thus:
i)
According to substantivalism the “points of the spacetime manifold
exist independently of the material contents of the universe”.
ii)
According to relationalism “spatio-temporal facts are about the
relations between various elements of the material contents of
spacetime”.
The
idea that “points of the spacetime manifold exist independently of
the material contents of the universe” sounds a little like David
Lewis's take (1982) on intrinsic properties. Lewis wrote:
“A
thing has its intrinsic properties in virtue of the way that thing
itself, and nothing else, is.”
Lewis's
position (if not the substantivalist position) can be taken to its
most extreme in the following statement:
Object
a would still have intrinsic property P if, after the
world around it disappeared, a would still have P.
In
L & R's rendition of substantivalism, it's said that an object or
point in spacetime could “exist” regardless of everything else.
Could there ever be “the way that a thing itself is” regardless
of everything else? That is, can object a be the way that
it is regardless of its relations to other
properties/objects/events, its place in time and space and so on?
James Ladyman and Don Ross argue that particles can’t be individuated without reference to external and relational factors. (All particles are parts of package deals.) So, to them, this means that particles simply aren’t “things” at all.
The rather pompous term “ontic structural realism” basically refers to a realism about structures, not a realism about what the authors call “things” (i.e., things such as particles… or anything else other than structures).
*********************************
The philosophers James Ladymanand Don Ross’s philosophy of physics is primarily motivated by the findings and theories of quantum mechanics. Elements of their position can be traced back to — among others — the German philosopher Ernst Cassirer (who died in 1945).
Ladyman and Ross have much to say about Cassirer. For example, they wrote the following:
“OSR [ontic structural realism] agrees with Cassirer that the field is nothing but structure. We can’t describe its nature without recourse to the mathematical structure of field theory.”
Some readers may wonder what the words “nothing but structure” actually mean.
The usual assumption is that there can only be a structure if there are things (of some kind) that make up (or are part of) that structure. That said, this may be one of those everyday “intuitions” which the authors warn us against. Hopefully, Ladyman and Ross’s position on the nature of structure willbecome clear in the following. In addition, the phrase “[w]e can’t describe [a field’s] nature without recourse to the mathematical structure” is fairly commonplace — at least within physics — and has been since the late 19th century.
What Ladyman and Ross say about Ernst Cassirer’s position on objects (or “things”) is almost exactly the same as their own position on objects. Indeed it was also quantum mechanics which provided Cassirer himself with the motivation to reject (what Ladyman and Ross call) “individual objects”. Ladyman and Ross write:
“Ernst Cassirer rejected the Aristotelian idea of individual substances on the basis of physics, and argued that the metaphysical view of the ‘material point’ as an individual object cannot be sustained in the context of field theory. He offers a structuralist conception of the field.”
It may seem odd that, at least within the context of 20th-century physics, an “Aristotelian idea” is being discussed at all. Not only that: we also have the technical philosophical term individual substance too. That said, these are the words of two philosophers of physics, not the words of two physicists.
One can firstly ask whether or not a commitment to the existence of objects (or things) is also automatically a commitment to “individual substances”; as well as to intrinsic (or essential) properties. After all, the bundle theory (among other theories) rejects the notion of substance; though not that of an individual.
We can also ask whether or not these positions are equally applicable to objects in the “classical” (or macro) world. Let’s put it this way. Cassirer’s, Ladyman’s and Ross’s positions are far more acceptable when applied the the quantum world than when applied to the classical world. More precisely, all this is far easier to swallow in the (to use Ladyman and Ross’s words) “context of field theory” than it is in relation to human persons, cups or trees.
Essential and Intrinsic Properties?
It does seem strange (if only intuitively strange) that all “quantum elementary particles of a given type” are deemed to “have the same mass, size, and shape (if any), charge, and so on”. As it is, mass, size and charge are (essentially) seen as intrinsicproperties. In fact it’s hard to imagine elementary particles having other (i.e., inessential) properties. After all, we’re not talking about classical objects here. So one can also easily imagine a natural kind having essential properties; though, when it comes to a particular member of that kind, having contingent properties too.
Can we also imagine an elementary particle having contingent properties?
If such particles don’t have such contingent properties (due to the nature of the micro-world), then it may literally be the case that the set {mass, size, charge} is all there is to them. Thus essentialism of some kind may be an easier option to uphold for particles than it is for classical objects such as lions or even a given sample of water.
That said, Ladyman and Ross do cite examples of what can be taken as a particle’s accidental properties. They cite “velocity or position at a particular time”. Indeed, in commonsense terms, it can be doubted that anyone would take the position or velocity of a particle at a particular time as being anything other than an “accidental” property. (Could it be that physicists could take a particular velocity — or at least mean velocity — of a type of particle to be essential to it?)
From what’s just be said, it’s therefore no surprise that different particles of the same type can be seen as what Ladyman and Ross call “indistinguishable”. However, particles may still have properties which aren’t intrinsic. That is, they may have properties which are relational or extrinsic.
Ladyman and Ross cite “spatio-temporal or other state-dependent properties” as examples.
In terms of spatio-temporal properties.
Does that mean that particles are even more likelier candidates for being four-dimensional objects than classical objects (see ‘Four-dimensionalism’)?
As for state-dependent properties .
That must mean that the nature of a particle must necessarily depend on the parallel (or corresponding) nature of the “state” (or system) to which it belongs.
Individuals?
Ladyman and Ross give a very concrete example of the physics which underlies the problematic nature of seeing elementary particles as single entities.
“[C]lassical physics assumed a principle of impenetrability, according to which no two particles could occupy the same spatio-temporal location. Hence, classical particles were thought to be distinguishable in virtue of each one having a trajectory in spacetime distinct from every other one.”
Clearly, in quantum mechanics, many — or all — the assumptions in the classical picture above are rejected. (At the least, on someinterpretations of quantum mechanics all these assumptions are rejected.)
Firstly, the “principle of impenetrability” is either questioned or rejected.
On the classical picture, if particles are impenetrable, then that means that “no two particles could occupy the same spatio-temporal location”. On the other hand, if particles are penetrable (or if the notion of penetrability does make sense), then one can conclude that two particles “could occupy the same spatio-temporal location”.
Thus one can immediately ask the following question:
If two particles occupy the same spatiotemporal location, then is it correct to talk about two particles in the first place?
In consequence, the second part of the classical picture is either rejected or questioned. That second part (which follows from the first) is that
“classical particles were thought to be distinguishable in virtue of each one having a trajectory in spacetime distinct from every other one”.
Clearly, if the penetrability argument is true and two particles may occupy the same location, then each particle can’t be seen to have its own trajectory in spacetime. In other words, it will share that trajectory with other particles.
This has the result that the Leibnizian picture breaks down in the case of quantum mechanics. Or as Ladyman and Ross put it:
“Thus for everyday objects and for classical particles, the principle of the Identity of Indiscernibles is true [].”
It can now be shown that the notion of a field plays an important part in Ladyman and Ross’s philosophy.
Particles as Package Deals
The central argument is that fields and particles are intimately connected. Indeed they’re so strongly connected that a distinction between the two hardly seems warranted.
Firstly, there’s the problem of distinguishing a particle from the state it (as it were) belongs to. Thus, in an example given by Ladyman and Ross, we can interpret a given field/particle situation in two ways:
1) A two-particle state.
2) A single state in which two “two particles [are] interchanged”.
Since it’s difficult to decipher whether it’s a two-particle state or a single state in which two particles are interchanged, Ladyman and Ross adopt the “alternative metaphysical picture” which “abandons the idea that quantum particles are individuals”. Thus all we have are states. This means that the “positing individuals plus states that are forever inaccessible to them” is deemed (by Ladyman and Ross) to be “ontologically profligate”.
Ladyman and Ross back up the idea that states are more important than individuals (what’s more, that there are no individuals) by referring to DavidBohm’s theory. In that theory we have the following:
“The dynamics of the theory are such that the properties, like mass, charge, and so on, normally associated with particles are in fact inherent in the quantum field and not in the particles.”
All this means that mass, charge, etc. are properties of states, not of individual particles. However, doesn’t this position (or reality) have the consequence that a field takes over the role of an individual (or of a collection of individuals) in any metaphysics of the quantum world? Indeed doesn’t that also mean that everything that was said about particles can now be said about fields?
On Bohm’s picture ( if not Ladyman and Ross’s), “[i] t seems that the particles only have position”. Yes; surely it must be a particle (not a field) which has a position. Indeed particles also havetrajectorieswhich account for their different positions at different times.
To Bohm, “trajectories are enough to individuate particles”. It’s prima facie strange how trajectories can individuate. Unless that means that each type of particle has a specific type of trajectory. Thus the type of trajectory will tell us which type of particle is involved in that trajectory.
Ladyman and Ross spot a problem with Bohm’s position. That problem is summed up in this way:
If all we have is trajectory (as with structure), then why not dispense with particles (as individuals at least) altogether?
This is how Ladyman and Ross explain their stance on Bohm’s theory:
“We may be happy that trajectories are enough to individuate particles in Bohm theory, but what will distinguish an ‘empty’ trajectory from an ‘occupied’ one?”
Here again Ladyman and Ross are basically saying that if all we’ve got are trajectories (which is part of the structure?), then let’s stick with trajectories and eliminate particles (as individuals) altogether.
Ladyman and Ross go into more detail on this by arguing that
“[s]ince none of the physical properties ascribed to the particle will actually inhere in points of the trajectory, giving content to the claim that there is actually a ‘particle’ there would seem to require some notion of the raw stuff of the particle; in other words haecceities seem to be needed for the individuality of particles of Bohm theory too”.
Of course most physicists would have no time for a term like “haecceities”, but if Ladyman and Ross’s physics is correct, then what they say makes sense. Positing particles seems to run free of Occam’s razor. In other words, Bohm was (to mix two metaphors) filling the universe’s already-existing ontological slums with yet more superfluous entities .
One way of interpreting this position is by citing two different positions. Thus:
1) The positing of particles as individuals which exist in and of themselves.
2) The positing of particles as part of package deals which include fields, states, trajectories, etc.
Then there’s Ladyman and Ross’s position.
3) If there are never particles in splendid isolation (apart from fields, etc.), then why see particles as individuals in the first place?
Ladyman and Ross are a little more precise as to why they endorse 3) above. They make the metaphysical point that “haecceities seem to be needed for the individuality of particles of Bohm’s theory too” (see ‘Haecceity’). In other words, in order for particles to exist as individuals (as well as to be taken as existing as individuals), such particles will require what Ladyman and Ross call “individual essences” in order to be individuated. However, if the nature of a particle necessarily involves fields, states, other particles, trajectories, etc., then it’s very hard (or impossible) to make sense of the idea that it could have an individual essence.
To sum up. A specific particle is always part of a package deal. Indeed all particles simply can’t be individuated without reference to external, extrinsic or relational factors. Thus particles simply aren’t individuals (or things) at all.
