The Philosophical Quarterly, Vol. , No. 
ISSN –
October 
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B J S
I. THE DIFFERENCES BETWEEN MATERIAL THINGS
The fundamental division of the world according to common sense, that is,
according to our theoretically and observationally unaided and natural
understanding, is between the living and the non-living. Among living
things, the next fundamental dichotomy is between plants and animals, and
within the latter the next is between humans and the rest of the animal
universe. These lines of separation are only imperfectly reflected in scientific
disciplines. There is biology, which deals with all kinds of living things, and
there is psychology, which deals with the special properties of humans,
namely, their mental properties, which distinguish them if not from all then
at least from most other animals. Subdisciplines such as botany and zoology
correspond to the division between plants and animals, but do not form institutionally different disciplines, as biology and psychology do. The division
between the living and the non-living is reflected by the existence of the
biological and non-biological sciences, although there is not one discipline
dealing with the whole of the non-living world but two, physics and
chemistry.
The absence of a distinction in our common-sense world picture corresponding to the distinction between mechanical and chemical properties of
non-living things can be explained by the fact that entities in our commonsense picture belong exclusively to one class. No living thing is at the same
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JÜRGEN SCHRÖDER
time a non-living thing, no animal a plant (mushrooms are counted as plants
in the common-sense picture); and although humans are mammals, they are
distinct from every other mammal. By contrast, chemical things are physical
things too. Chemical elements have physical properties such as mass,
density, velocity, etc., as well as chemical properties: they combine in such
and such proportions with such and such other elements. Physics, however,
is not restricted to a certain class of entities. Galileo’s and Newton’s laws are
supposed to be valid for atoms as well as for human beings, in short, for
(almost) every kind of individual material thing, whereas chemical laws
pertaining to chemical elements and molecules are not applicable to organelles, cells, organs or whole organisms. In this respect physics differs not only
from chemistry but from every other scientific discipline as well. This
difference is sometimes expressed by saying that physics (or a certain part
thereof, viz., mechanics) is the one general science, and the other disciplines
are special sciences. The special sciences are special in the sense that their
laws cannot be applied to all kinds of material objects but only to a special
sort of them.
C.D. Broad asked in the first paragraph of his chapter on ‘Mechanism
and its Alternatives’: are the apparently different kinds of material objects
irreducibly different?1 The point of this question is not whether the differences between animals and plants, or the differences between plants and,
e.g., water, can be explained away, or whether animals can be reduced to
plants. Nor is it whether the differences between species can be explained on
the assumption that all the species now living grew out of a smaller number
of species in the past. Broad raises his question in the context of the debate
between vitalists and mechanists about the fundamental distinction between
the living and the non-living.
Vitalists, or ‘substantial’ vitalists, as Broad (p. ) calls them, assumed that
the characteristic behaviour of living beings is dependent on the existence of
a special component in them, which was called an ‘entelechy’ by Hans
Driesch.2 It was supposed that entelechies explain the characteristic behaviour of the living (e.g., the healing of tissue after being wounded, the capacity
of an early-divided embryo to develop into two whole organisms) only when
the material complex in which they reside has a certain structure. Mechanists, on the other hand, claimed that all characteristic behaviour could be
explained without assuming such special components. The known material
substances, together with their arrangement and their action in living things,
should be enough to explain whatever had to be explained.
1
2
C.D. Broad, Mind and its Place in Nature (London: Routledge & Kegan Paul, ), p. .
H. Driesch, The Science and Philosophy of the Organism (London: A. & C. Black, ).
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Broad’s position in this debate, which he called ‘emergent vitalism’, was
to dispense with the special components of the vitalists and at the same time
to claim that the reductions that mechanism was looking for would not be
forthcoming. The concept of emergence or of emergent properties thus
played a crucial role with respect to the most fundamental division of the
world according to the common-sense picture.
A honey bee, as a living thing, has some properties (capacity for reproduction, locomotion, preservation of structure while components change)
which a rock has not, and these properties ground our impression of a big
difference between the rock and the bee. The difference is irreducible if the
properties which ground it are emergent. When we construe Broad’s
question in this way we would have a possible account of the big divisions in
our world picture. They exist because the properties which ground them are
emergent properties. We can therefore set down as a first function of emergent properties their capacity to explain the main divisions we make
between kinds of material objects in our common-sense picture of the world.
But the concept of emergence is supposed to serve other functions as well.
For example, Broad’s emergent vitalism was meant to break the apparently
exhaustive dichotomy between mechanism and vitalism. Emergence may be
equally well suited for an explanation of the existence of the special sciences
and of the belief that reality is hierarchically structured in levels. These
levels are thought to be generated by the fact that the basic constituents of
matter combine to form complexes of a certain form and stability, which
themselves combine to form higher-order complexes, and so on. But the
part–whole relation is not the only determinant of our view of levels.
Otherwise we would think that plants, animals and humans belong to the
same level. So there seem to be further criteria. But before I try to answer
the question of whether the concept of emergence is really suited for these
tasks, it is necessary to specify its various ingredients.
II. STRONG EMERGENCE AND NON-DEDUCIBILITY
First, we may ask what kinds of things can be or are supposed to be emergent. The inventory of emergents basically consists of three classes. There
are (or are supposed to be) emergent things, properties and laws. The class
of emergent properties seems to be more basic than the other two. A thing
can be emergent only if it has at least one emergent property, and a law is
said to be emergent if it connects emergent properties. The fact that a
property of a whole is said to be emergent if it occurs in a basic law that
connects this property with properties of the whole’s parts does not imply
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JÜRGEN SCHRÖDER
that emergent laws are more basic than emergent properties. By ‘emergent
law’ I understand a law connecting two emergent properties, not a law connecting non-emergent properties of parts with the property of a whole. The
latter kind of laws is called ‘transordinal’ by Broad (p. ). In what follows I
deal with emergent properties rather than emergent things or laws.
In almost every discussion of emergent phenomena there are two
constraints on the concept of emergence. First, emergent properties are
always the properties of complex systems. Elementary particles do not have
emergent properties. Second, a property of a complex thing, in order to
be emergent, must not be a property of a proper part of that thing.
Properties like mass, velocity and charge are thus excluded from emergents.
These two constraints on emergent properties yield a relatively uncontroversial concept of emergence, because they say nothing about the possible
relations between the properties of the parts of a thing and its emergent
properties except that they must not be the same. The concept is not
entirely uncontroversial, because one might think that if there are no more
ingredients it is rendered trivial.3 We may call this concept ‘weak emergence’, and distinguish it from a concept of ‘strong emergence’, which contains a further ingredient placing emergence in contrast with reduction. The
further relation between an emergent property of a whole and the property
of its parts is here one of non-explanatoriness. In order to be emergent, the
property of the whole must be not explainable, in a sense yet to be defined,
by the properties of the parts.
This is the concept of emergence we find exclusively in Mill and Broad,
and partly in Morgan and Alexander.4 In his treatment of the composition of causes Mill contrasts two kinds of cases: first, cases where the joint
effect of two or more causes is equal to the final effect if every cause had
acted separately (his example is the motion of a body subject to two forces
acting in different directions – the joint effect, that is, the real motion of the
body, is then computable by vector addition); second, cases where the result
of a combination of causes cannot be deduced by something akin to vector
addition. Mill (and Broad) thought that the combination of chemical
elements would be an example of this second kind.
It was because of the lack of a composition principle connecting certain
properties of chemical elements with the properties of their compounds that
Mill and Broad deemed the deduction of the compound properties unlikely.
The concept of emergence that takes non-deducibility to be the essential
3
See M. Polanyi, ‘Life’s Irreducible Structure’, Science,  (), pp. –, at p. .
S. Alexander, Space, Time, and Deity (London: Macmillan, ); J.S. Mill, A System of Logic,
th edn (London: Parker & Son, ); C.L. Morgan, Emergent Evolution (London: Williams &
Norgate, ).
4
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criterion is, therefore, based on the assumption that for emergent properties
no composition principles exist.
We can now say what the requirement of non-explainability amounts to.
A property of a whole is non-explainable if and only if it cannot be deduced
from properties of the parts in isolation (or in other wholes) and from a composition principle which would combine these properties. It is of the greatest
importance to understand what kinds of composition principles would be
acceptable, because some of them would trivialize the reduction.
III. THE GENERAL FORM OF COMPOSITION PRINCIPLES
If you want to explain why the combination of hydrogen and oxygen results
in a liquid which dissolves salt, and your composition law is ‘If oxygen is
combined with hydrogen the resulting compound dissolves salt’, this would
not be an acceptable composition principle because it is completely ad hoc. It
has been constructed only to cover the special case at hand, the combination
of oxygen and hydrogen. It might have been that such composition principles were the only ones we could ever get. Such was Broad’s conviction
when he wrote that ‘the law connecting the properties of silver chloride with
those of silver and of chlorine and with the structure of the compound is, so
far as we know, an unique and ultimate law’ (p. , his italics). Broad goes on
(ibid.) to characterize unique and ultimate laws by the following two criteria:
(a) that it is not a special case which arises through substituting certain determinate
values for determinable variables in a general law which connects the properties of any
chemical compound with those of its separate elements and with its structure. And (b)
that it is not a special case which arises by combining two more general laws, one of
which connects the properties of any silver-compound with those of elementary silver,
whilst the other connects the properties of any chlorine-compound with those of
elementary chlorine. So far as we know there are no such laws.
Accordingly, an acceptable composition principle needs either variables
which can be filled in with values of the elements that participate in a
reaction, or a combination of laws connecting the properties of any xcompound with the properties of x.
As things stand, however, Broad’s constraints on the form of composition
principles either are incomplete or fail to do their intended work. The first
constraint is that the composition principle has to have variables. But if it
were only of the form ‘If you combine an element with value x (e.g., the
number of free electrons in the outermost orbital) and an element with value
y, then the compound will have value z’ it would be of no help if the function that assigns pairs of numbers x and y to z is unknown. So in order to be
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JÜRGEN SCHRÖDER
a usable law it has to be combined with such a function. Moreover, what we
want to be able to deduce are certain properties of the compound, such as,
say, whether it is flammable, whether it is an acid or whether it dissolves
other substances. But what we have are only some numbers. There has to be
an assignment of these numbers to specific properties too, because otherwise
the compound properties would not be deducible. If, alternatively, we do
not take the values of the variables to be numerical values but more determinate properties, there would have to be a function taking pairs of such
properties to a third property. What would be important about such a
function, however, as in the case of numerical values, would be the existence
of an algorithm that yields the compound property for any arbitrary pair of
elementary properties. Otherwise we would just have a mapping between
two lists; and the generality of the law (which is due to the generality of the
algorithm) would be lacking.
In contrast to the incompleteness of the first constraint, the second seems
not to be workable at all. For if we took its formulation literally we would
have a huge set of properties, viz., the properties of all x-compounds, connected with the properties of x. The same goes for y and the y-compounds.
The next step would be the combination of these two ‘laws’. The simple
conjunction of them would yield a proposition in which the properties of the
elements would figure in the antecedent, and the properties of the wholes
would be mentioned in the consequent. But, of course, the x-y-compound
does not have all of these properties. Only a fraction of them will be true of
it. So this conjunction of two laws does not yield all and only the emergent
properties of the x-y-compound.
But perhaps this interpretation of the requisite laws does not do justice to
Broad’s understanding of them. Perhaps the intended properties of the xcompounds are not the union of properties of every specific x-compound but
the intersection of these properties. Suppose that there are three properties a, b
and c which are common to all x-compounds. And there are three other
properties d, e and f common to all y-compounds. The conjunction of the
two hypothesized laws would mention the properties a, b, c, d, e and f in its
consequent. If both laws were true the x-y-compound would have these properties. But would it also be the case that among these six properties were all
the ones originally (but as it is to turn out now, mistakenly) thought to be
emergent? Why should the operation of taking only the properties which are
common to all x- and to all y-compounds yield any properties that we
considered emergent? Why, for example, could it not be the case that the
common x-compound properties are the same as the common y-compound
properties, namely a, b, c, these properties not being emergent but resultant,
as they were called by the emergentists (see Morgan p. )? In view of the fact
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that resultant properties are general properties par excellence because they are
physical properties, physics being the only general science, it is clear that a
part of the common properties will consist of resultants. If, on the other
hand, an emergent property of an x-compound must not be a property of x,
it is very unlikely that it will characterize every x-compound, because there
seems to be no special reason, as there is in the case of resultants, for this
generality. But even if there were emergent properties which, by chance,
were common to all x-compounds, there will certainly be others, e.g., explosiveness, which will characterize only a few. Such properties would then be
excluded from the x → x-compound laws, and thus there would be emergent
properties which are not deducible by such laws. This is the first reason why
such laws cannot be the right composition principles.
The second reason is that, even if all the properties a, b, c, d, e and f were
thought to be emergent properties, and were the only ones, it is still unclear
how the conjunction of the x-compound and the y-compound laws would
differ from an ultimate and unique law, that is, how it would differ from just
those laws that are to be excluded from the deduction of emergent properties. For what such a conjunction does is nothing but connect the
properties of x and y with the properties of their compound. Such laws, if
they cannot be further explained, are just demonstrations of the emergence
of a, b, c, etc., rather than arguments against their being emergent.
In the case of chemical properties, then, a composition principle of the
first form, viz., a numerical function connecting the values of chemical variables with one another, and a list assigning these numerical values to properties of the compound, will be necessary for the deduction of compound
properties. The list which connects numbers with compound properties is
not needed in the computation of resultant properties, for the simple reason
that a resultant property is qualitatively the same for an element and for the
compound, e.g., weight or mass: only its quantity differs, and this quantity is
computed by an algorithm computing a certain function. For emergent properties, however, the list seems to be of paramount importance. If such a
connection is not available, the macro-property cannot be deduced.
IV. TWO EXAMPLES: SPATIAL STRUCTURE AND
SOLUBILITY OF MOLECULES
In order to bring an abstract discussion into contact with scientific reality, I
shall consider two examples of the explanation of chemical properties. As is
well known, the basic phenomena of chemistry such as the chemical bond in
general, or the covalent bond in particular, or the fact of saturation, that is,
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JÜRGEN SCHRÖDER
the fact that every atom has a maximum number of possible H-atoms that
can bond with it, can be explained by quantum mechanics.5 (A chemical
bond is covalent if the constituents are electrically neutral, as in the case of
H2 molecules; classical physics is unable to explain this bond, because there
are no classical forces that explain the attraction of the H2-molecules.) However, bonds do not seem to be emergent properties of compounds. It seems
rather that covalent bonds and saturation are properties (dispositions) of the
chemical elements. So, although there are some basic facts concerning
the existence of certain compounds, and the constraints which hold for compounds in general, which cannot be explained by classical mechanics or
electromagnetism, these facts would not (even in the absence of quantum
mechanics) argue for the existence of emergent properties.
There are, however, other properties satisfying the first two conditions on
emergence, viz., being a property of a complex which (a) is not a property of
its parts, and (b) cannot be deduced from physical theory. The spatial configuration of a molecule, for example, is surely an emergent property in the
weak sense, because it is a property of the whole molecule and of none of its
atoms. But it is not emergent in the strong sense, because it can be derived
from knowledge about the behaviour of the electrons in the atoms.
The spatial structure of a water molecule, for example, forms a triangle
with an angle of ° between the two hydrogen atoms. How does this
configuration come about? Oxygen has in its ground state (i.e., its state of
lowest energy) four electrons, in orbitals which are called p-orbitals because
they are at the second lowest energy level and because their electrons are
described by a so-called p-wave function. For any p-level there are three
orbitals (wave functions) px , py and pz which have preferred directions corresponding to the three spatial co-ordinates x, y and z. Since the four
electrons are distributed over three orbitals, one orbital, e.g., the pz -orbital,
contains two electrons. According to Pauli’s exclusion principle, there must
be not more than two electrons in the same orbital. The remaining two electrons can be in states described by px and py respectively. When the H-atom
comes from the z-direction its electron cannot form a common pair with the
electrons of the O-atom in the pz -orbital, since in this orbital there are already two electrons. When, on the other hand, the H-atom comes from the
x- or y-direction, its electron can form a common pair with one of the electrons in the px - or the py-orbital. We would therefore expect the two H-atoms
to form an angle of °. With the help of further details (forces between the
two protons) it can be explained why the angle is larger than °.6
5
6
See L. Pauling, The Nature of the Chemical Bond (Cornell UP, ).
See W. Heitler, Elementäre Wellenmechanik (Braunschweig: Viehweg, ), pp. –.
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Another example of an explanation of a compound property in terms of
the properties of the parts concerns the solubility of a compound substance.
Strictly speaking, solubility is not an emergent property, because parts of
substances, e.g., H-atoms, can equally be dissolved in water. The example,
however, has a certain feature, viz., the translation of a macro-property into
micro-property terms, which is why I have chosen it. The solubility of alcohol in water is due to the hydrogen bonds between the hydrogen atoms of
water and the OH-group of the alcohol. Such hydrogen bonds already exist
between the alcohol molecules. If the proportion of the OH group to the
size of the rest of the molecule is not far away from equality, as it is with
ethanol (CH3CH2OH), the solubility will be perfect. To the extent that the
number of CH chains increases, the solubility diminishes. An alcohol like
pentanol (CH3CH2CH2CH2CH2OH), for example, dissolves in water only
to a small degree in comparison with ethanol. A high degree of solubility
can also be attained if the substance has more than one OH group, as the
example of glucose with five OH groups shows.7
In general, there are cases of compound properties which would count as
emergent by the criterion for weak emergence but which would not count
as such by the criterion of non-deducibility. As we see in these examples, the
composition principles that help combine the properties of the parts are of
a different character. In the explanation of the spatial structure of water
Pauli’s exclusion principle had to be used. This principle is completely
general. It can be used to reconstruct Mendeleev’s periodic systems of elements, once the proper sequence of electron orbitals has been derived from
the three quantum numbers n, l and m characterizing the size, shape and
orientation of orbitals respectively.
Pauli’s exclusion principle conforms more or less to the outlines Broad
gave for composition principles as well as to our supplementation of these
outlines. It can be seen as an algorithm for the function which takes different
numbers of electrons with various spins as arguments and which yields the
binary values ‘allowed’ and ‘not allowed’. So what we required to supplement Broad’s proposal, viz., the existence of a computable function together
with an algorithm, is clearly there. Only the second requirement, namely
the existence of a function from numerical micro-properties to qualitative
macro-properties, is not satisfied in this example. The reason why such a
function is not necessary here is that the description of the situation in which
the electronic explanation sets in is already a spatial one. The three porbitals of the O-atom are orientated along the axes of three-dimensional
space. It is then asked what happens to the H-atom if it moves towards the
7
See T. Brown and H. Lemay, Chemistry (Englewood Cliffs: Prentice-Hall, ), p. .
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JÜRGEN SCHRÖDER
O-atom along the z-axis if in that p-orbital there are two electrons. Since the
other two p-orbitals are orientated along the x- and y-axes, it is clear that
there will be a bonding of H-atoms at these two sites. The spatial frame is
thus already set, and the question is only to know which of the possibilities of
this frame are real.
The second example, of water solubility, uses a more restricted
composition principle. In this case, it is hydrogen–oxygen bonds together
with the relative size of the OH groups which explain the good watersolubility of ethanol. The composition principle has nevertheless a general
aspect, in that it is valid for all sorts of compounds with an OH group. The
property to be explained is water-solubility, and not solubility in general (if
that makes sense at all), so the explanation operating with hydrogen bonds
could not be more general. The algorithmic character of the principle
consists in the part concerning the relative size of the OH groups. Different
degrees of solubility can be computed on account of this relative size. Here,
moreover, if we want to see the example in this way, we have something like
an assignment of quantitative values to a quality. The proportion of the
number of the OH groups to the rest of the molecule is a numerical value
which has to be assigned to the property in question, i.e., the degree of solubility. To be sure, it is not only a numerical value but a value of the variable
‘proportion of parts of a molecule’, but the important point is the assignment of certain values of this variable to the compound property of
solubility. This example shows that, in order to explain the macro-property
physically, we have to ask first what water solubility is in terms of the parts
of a substance. When we have the general idea of molecules and of bonds
between some of the parts of these molecules, we may then ask what kind of
parts are involved in these bonds. Without the ‘translation’ of solubility into
some kind of bonding between molecules, the macro-property could not be
explained!
The reductive explanation of a weakly emergent property does not
presuppose the reduction of whole theories or of whole disciplines. Nevertheless there are always cases of properties which cannot be deduced by the
theory about the parts of a system, composition principles and identities or
realizations. The tetravalence of carbon is an example. In order to deduce
this property, a principle of the hybridization of orbitals had to be postulated. This principle does not belong to quantum mechanics, but constrains
it in such a way that the tetravalence of carbon can be deduced from it and
this principle. Since this principle concerns the parts of an atom, viz., its
orbitals, it can be considered as an extension of the physical theory,
Although this extension has been driven by a chemical fact, the explanation
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of tetravalence is reductive because the extension concerns the physical
aspects of a C-atom.8
In general, then, explanations of properties are reductive when they are
based on the theory of the parts of a system, further principles governing the
behaviour of these parts in wholes, composition principles and identity or
realization relations.
V. IDENTITY, MULTIPLE REALIZATION AND THE
STATUS OF NON-DEDUCIBILITY
For any emergent property which is not already ‘sketched’ by certain properties of the parts, as in my first example, there has to be an answer to the
question ‘What does this property amount to in physical or chemical or
physiological terms?’, depending on the kind of property involved. You can
explain the solubility of a substance by the ratio of its OH groups to the rest
of the molecule only if solubility is the bonding of parts of some molecules
with parts of other molecules. Accordingly, an explanation of arbitrary
emergent properties requires an answer to the question ‘What does property
E amount to, physically (etc.) speaking?’. A satisfactory answer can take one
of two forms. First, it can be an identity, so that in every situation where the
emergent property is instantiated the same micro-physical property is
instantiated. Second, for more abstract emergent properties, it may be that
in different contexts, that is, in organisms in the first place, the property is
realized by different micro-physical properties. If that is the case, and if the
number of such realizations is finite, the explanation of the macro-property
eventually has to specify the various micro-properties that realize it. When
this exhaustive specification can be given, the macro-property is explained
just as it is in the first case. Multiple realization is, then, not incompatible
with reduction.9 That there is no incompatibility can also be seen when we
ask if the macro-property is something ‘over and above’ the complex of
micro-properties. Since it cannot exist without this complex, and since its
causal role in any given instance is exhausted by the causal role of this
8
See M. Lévy, ‘Les rélations entre chimie et physique et le problème de la réduction’, Epistemologia,  (), pp. –, at p. ; and M. Bunge, ‘Is Chemistry a Branch of Physics?’,
Zeitschrift für allgemeine Wissenschaftstheorie,  (), pp. –, at pp. , . Both authors
speak of ‘chemical assumptions’. But these assumptions are not chemical because they contain
specifically chemical terms but because they are based on chemical facts.
9
See J. Kim, ‘Multiple Realization and the Metaphysics of Reduction’, Philosophy and
Phenomenological Research,  (), pp. –; A. Beckermann, ‘Property Physicalism, Reduction
and Realization’, in M. Carrier and P. Machamer (eds), Mindscapes: Philosophy, Science, and the
Mind (Univ. of Pittsburgh Press, ), pp. –.
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complex, we can conclude that it is not something ‘over and above’ its
realizer. So identities or multiple realizations have to be acknowledged if we
want to succeed with a reductive explanation. Simple bridge laws, as in
Nagel’s conception of reduction,10 would not be enough, because the existence of such laws is possible without either identity or multiple realization.
Reduction in the full sense requires an explanation of the existence of bridge
laws. Identity or realization are such explanations.
We may pause here and see what the effect of the criterion of nondeducibility is in the case of emergent properties of chemical compounds. If
we hold on to non-deducibility as a necessary conceptual component
of emergence, the spatial configuration of a water molecule will not be an
emergent property of water. If we wanted the concept of emergence to give
us an explanation of the different levels of complexity of our world, then the
feature of non-deducibility vitiates that goal, because it will not count
the difference between chemical elements and chemical compounds as a big
difference.11 Of course, we could take the part–whole relation in order to
generate the various levels, but then we would have to count a colony of
unicellular organisms such as Volvox as exemplifying a level in its own right.
The same would be true of mixtures of elements which constitute wholes but
are not unified in the same way as compounds are. The part–whole relation
together with principles of unification could do the job, but although these
principles are different from emergent properties as such, they are correlative to these properties. As Morgan said (p. ), when a new kind of
relatedness between already existent elements supervenes, this relatedness
will give expression to emergent properties.
In the worst case, we would have no differences between levels at all if
every property of every whole could be deduced by the properties of the
parts, composition principles and identity or realization relations. Nondeducibility seems therefore not to be apt for one of the functions the concept of emergence was meant to serve. If it cannot serve this function, and if,
for example, there is no particular problem in deducing the characteristic
properties of life, viz., growth, reproduction and development, from chemical properties of macro-molecules, e.g., DNA, then, it seems, this concept of
emergence cannot serve the other two functions either: it will not account
for the existence of different scientific disciplines, and it will not show that
the most basic apparent difference in our common-sense picture of the
world is a real difference.
10
See E. Nagel, The Structure of Science (New York: Harcourt, Brace & World, ).
For a reductionist treatment of more complex molecules see J.R. Platt, ‘Properties of
Large Molecules that Go Beyond the Properties of their Chemical Sub-groups’, Journal of
Theoretical Biology,  (), pp. –.
11
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A second shortcoming of non-deducibility, which is a consequence of the
first, is that it makes emergence an epistemic notion. A property is emergent
as long as either we do not have the right theories about the properties of the
parts or we have the right theories but not the computational power to deal
with the increasing complexity as we move from H-atoms to economies. In
both cases properties cease to be emergent when the right tools (either
theoretical or technical) are developed which convert the required explanations from the realm of possibility to actuality.
If we think that emergence, the supervenience of new properties along
with the development of ever more complex systems, is something objective,
something that essentially characterizes the natural world (and not only the
natural world, as TV sets and computers witness) we would be well advised
to abandon the criterion of non-deducibility and replace it with something
else. The emergent evolutionists like Morgan and Alexander would not have
thought of emergence as an epistemic concept. Otherwise they would not
have insisted on the ‘natural piety’ with which these properties were supposed to be accepted. Moreover, Broad’s question does not make sense if it
is interpreted epistemically. Of course, it makes sense to say that the
apparent differences between kinds of material objects are only apparent.
However, it ceases to make sense if emergence is what is to vindicate the
apparent differences and if emergence is at the same time itself only apparent. Emergence, in the context of this question, has to be something
objective in order to play its role consistently.
Since non-deducibility does not allow us to make the distinctions we
would like to make, we should abandon this criterion and see if the other
criterion which has been proposed by some emergentists (e.g., by Morgan
and by Sperry12 ), viz., downwards causation, fares any better.
VI. WHICH PROPERTIES ARE RESPONSIBLE FOR
DOWNWARDS CAUSATION?
Since downwards causation is sometimes thought to be somewhat mysterious, because it seems to be incompatible with bottom-up determination, we
first have to define the notion in such a way that the appearance of incompatibility will go away. For this purpose, we need to distinguish between
diachronic and synchronic explanations.
12
R. Sperry, ‘A Modified Concept of Consciousness’, Psychological Review,  (),
pp. –.
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Diachronic explanations explain how something came about in the
course of time and how it developed out of something else. Typical
diachronic explanations can be found in evolutionary biology, but also
in embryology, developmental psychology and, for the whole universe, in
cosmology. Diachronic explanations generally have three components. First,
there are the properties of things out of which more complex and integrated
things develop. Second, there are the conditions under which there is a high
probability that the more complex things will come into being. Third, there
are the laws governing the behaviour of the constituents which will form the
complex whole.
Diachronic explanations are not to be confused with explanations that
rely essentially on so-called ‘transition’ theories.13 Transition theories are
used to predict a later state of a system from a former state, for example, the
position of the planets in our solar system. An explanation of these positions at a certain time only needs to make reference to the positions of the
planets at some former time and to apply the laws governing the movement
of the planets around the sun. What is to be explained is a property of the
system that has changed while the complexity of the system has remained
the same. In contrast, diachronic explanations are directed at the coming
into being of more complex systems.
Synchronic explanations, on the other hand, explain how a capacity of a
system is realized in subcapacities, and ultimately how it depends on
the system’s structure and on the properties of its parts. The explanation of
water-solubility in terms of hydrogen bonds is a synchronic explanation.
Now ‘bottom-up determination’ can mean two things. First, it can mean
that the properties of a whole are determined by the properties of the parts
and by their being related in such and such a way. Micro-determination is
then correlative with synchronic explanation. Second, it can mean that in
order for a complex thing to come about there have to be such and such
conditions under which less complex things go together and form more
complex things.
A synchronic explanation has to take for granted the relatedness of the
parts. This relatedness is not itself the target of the explanation. It may
become, however, the target of a diachronic explanation. If we understand,
then, by ‘bottom-up determination’ of a system property, first, its determination by properties of the parts and by this relatedness, and second, the
determination of the relatedness of the parts by diachronic conditions, we
see how downwards causation fits into this picture. The place for downwards causation is the relatedness of the parts, which is itself explainable but
13
See R. Cummins, The Nature of Psychological Explanation (MIT Press, ).
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which, apart from its role in the explanation of a capacity of a system, also
has a role in the explanation of the behaviour of its parts.
Suppose a system has different candidate emergent properties which are
all ‘expressions’ of the same relatedness of the parts, for example, thermal
and electrical conductivity and optical opacity in metals. Each emergent
property would not then have a specific effect on the behaviour of the parts,
the electrons, say. Instead, the behaviour of the electrons would be constrained by the lattice structure of the metal atoms. What is responsible for
downwards causation, then, is the ‘new kind of relatedness’ of the parts and
not its ‘expression’, that is, what we take to be the emergent property.
If, on the other hand, there are various sorts of relatedness with various
correspondent emergent properties, it would still be the relatedness which is
responsible for the constrained behaviour of the parts, and not its expression, the emergent property. Nevertheless, downwards causation can be
used as a criterion for emergent properties. Only those properties are truly
emergent which are weakly emergent and whose bases, i.e., the relatedness
of which they are the expression, are involved in downwards causation.
We are now able to define the notion of downwards causation. Downwards causation is the influence the relatedness of the parts of a system has
on the behaviour of the parts. It is not the influence of a macro-property
itself, but of that which gives rise to the macro-property, viz., the new
relatedness of the parts.
VII. WHAT DOES DOWNWARDS CAUSATION EXPLAIN?
Since this definition does not specify what aspects of the behaviour are those
which are influenced, and since the non-specification of these aspects might
be a source of misunderstandings, some remarks are necessary on the question of what kinds of differences downwards causation is supposed to explain. Morgan, for whom downwards causation seemed to be on a par with
non-deducibility, writes (p. ):
But when some new kind of relatedness is supervenient (say at the level of life), the
way in which the physical events which are involved run their course is different in
virtue of its presence – different from what it would have been if life had been absent.
What is caused to be different in a living organism is the course of events, as
compared with the course of events in a dead one. It is not that the physical
or chemical laws which govern the behaviour of, say, the molecules in the
cells, are different in living and dead organisms. These laws say only that if a
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JÜRGEN SCHRÖDER
molecule has properties a, b and c it will transform into a molecule or molecules with properties d, e and f when it combines with a molecule with
properties g, h and i. Or that if a concentration c of molecules is diffused in a
cell, the diffusion velocity will be such and such. As laws are conditional
statements connecting the properties of things and situations with one
another, they do not contain any information about the actual course of
events. They only say what will happen when their antecedents are instantiated or what would happen if they were instantiated. What does determine
the course of events are certain conditions which determine how often and in
which order the chemical and physical laws are to be applied.
In living cells, for example, there are certain reaction cycles, such as the
citric acid cycle, transforming various kinds of molecules into other molecules while gaining energy from these transformations and producing
molecules such as CO2 which leave the organism through breathing. For
such cycles to keep going, it is necessary for the molecules to be kept
together in a certain space, for the right molecules to be available for every
stage of the cycle, and for the right catalysts to be present. There would be
no cycle if the molecules diffused in huge amounts of water, or if they were
brought together with rare gases which are chemically inactive, or if there
were no catalysts.
The conditions which determine how often and in which order the antecedents of chemical laws are instantiated in the living cell consist in part of
the information the DNA molecules contain and in the regulatory mechanisms which exploit that information. For it is thanks to these molecules and
mechanisms that the right amount and the right kinds of chemicals are
synthesized in the cells so that the cyclic reactions keep going and the cell
stays alive. That it is the frequency and order of transitions governed by
physical and chemical laws, which differ owing to the presence or absence of
a certain relatedness of components, is, as compared with the transitions
themselves, not of minor importance. The difference between an animal
and a pond as an environment of chemical reactions is due to differences in
the frequency and the spatial order as well as in the temporal order of
transitions. We might say that the set of transitions contains all possible
forms of beings, but that the relatedness of parts on every level helps some of
these forms to actuality.
With this notion of downwards causation in hand, we can see what is
wrong with a critique of emergentism14 that castigates it for assuming ‘configurational forces’ which depend on the existence of material complexes,
14
Cf. B. McLaughlin, ‘The Rise and Fall of British Emergentism’, in A. Beckermann, H.
Flohr and J. Kim (eds), Emergence or Reduction? (Berlin: de Gruyter, ), pp. –.
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that is, forces which do not exist when the corresponding complexes do not
exist. Emergentists who adopt downwards causation as a criterion for
emergent properties need assume no such force. Just as they have no need to
assume special physical or chemical laws which only apply in the case of
complex systems, they are equally not committed to any special sort of force
which is only displayed by a complex of particles but not by the particles
themselves. In order to produce live and mindful beings, what is needed is
not special laws but special structures that constrain the sequence of possible
events in special ways.
VIII. THE ALLEGED MERITS OF DOWNWARDS CAUSATION
Now that we have seen that downwards causation is compatible with
bottom-up determination, and that it is the frequency and the order of
events which is the target of downwards causation, we are in a position to
ask whether this criterion for the emergence of properties is better suited
to its task than the criterion of non-deducibility. Downwards causation
already has an advantage over non-deducibility if it can account for even
one of the distinctions either in our common-sense picture or between the
sciences or between the levels of reality. In this respect the criterion vindicates the distinctions between the living and the non-living and between
biology and the non-biological disciplines. The living cell satisfies both the
conditions for weak emergence and for downwards causation. The criterion
does not succeed in grounding the distinction between plants and animals,
but neither does this distinction feature in the landscape of scientific
disciplines.
The distinction between animals and humans, or between biology and
(human) psychology, raises two different problems. For one thing, just as
plants are not literally parts of animals, so animals are not literally parts of
humans. If the part–whole constraint on adjacent levels tolerates no exceptions, organisms with a mind do not belong to a different level from organisms without one. So either we insist on that constraint, and then count
plants, animals and humans as all belonging to the same level (which would
be unsurprising by biological criteria), or we allow for exceptions in certain
cases. These cases should be such that new principles govern the behaviour
of a being. The capacity of reasoning would be such a principle in humans.
This move would, however, require us to count plants and animals as
belonging to different levels as well, for the existence of muscles, sense
organs and a nervous system, and the purposeful locomotion that is made
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JÜRGEN SCHRÖDER
possible by these organs, may equally count as a new principle. In any case,
however, it is not downwards causation that accounts for the distinction of
levels but the new principles.
The second problem is that the existence of mind is not an all or nothing
affair. The capacities to learn and to perceive, for example, are common to
many species. Even the capacities of problem-solving and of using a rudimentary language are not restricted to humans but are characteristic of
chimpanzees as well.15 So mind as such, if not further specified, does not
yield the requisite distinction. One mental capacity which clearly distinguishes humans from other animals is the use of a recursively structured
language which can be used for processes of very abstract and very rigorous
thinking. But again, if we take the advent of language to be an emergent
property, it is not downwards causation which distinguishes between
humans and other animals.
We can conclude that downwards causation, since it presupposes parts of
a system which are influenced by the way they are related, cannot account
for the distinctions inside the living world between plants, animals and
humans, for the existence of psychology as a distinct discipline, or for the
conviction that these three kinds of living things belong to three different
levels of reality. Although it is true that the capacity for reasoning has an
effect on behaviour, it is not an effect on parts of the reasoning organism, for
the behaviour is of the whole organism and not only of some of its parts. To
speak of downwards causation in this case would then be a metaphorical
extension of the term, for the causation would not be downwards. It would
be interesting to know, however, what, in contrast to the order of chemical
events, the target variable of downwards causation could be in this case.
Would it make sense, for example, if this variable were the order of neural
events corresponding to thoughts?
On the other side of the divide which separates the living from the nonliving we have the fields of quantum physics and chemistry, and the distinction between elementary particles and atoms on the one hand and
molecules on the other. Does downwards causation account for this distinction? Since we have, as in the transition from molecules to living cells, a
relation between wholes and parts, the notion of downwards causation is
clearly applicable. The influence the relations between the parts have on the
order of part-events consists in the fact that when atoms or groups of atoms
are bound together in a molecule, these atoms are sometimes more and
sometimes less likely to combine with other atoms or molecules in contrast
15
See A. Premack, ‘The Codes of Man and Beasts’, Behavioral and Brain Sciences,  (),
pp. –, and Gavagai! Or the Future History of the Animal Language Controversy (MIT Press, ).
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with the situation in which they are not part of a molecule. Downwards
causation accordingly accounts for the difference between quantum physics
and chemistry.
As this criterion does better in two cases than the criterion of nondeducibility, it seems that strong emergence should be defined in terms of
downwards causation. The above discussion of the other distinctions between plants, animals and humans showed, however, that the criterion is not
sufficient to cover all the cases where we would like to see major differences
between things.
There is also a more serious problem. To be a genuine account of these
differences, downwards causation should not obtain in every case in which
there are relations between parts which form a whole. If this were the case,
the part–whole relation would be as good an account of emergence as
downwards causation. But then strong emergence would collapse into weak
emergence, because the difference between properties of the parts and
properties of the whole is just what characterizes weak emergence. If downwards causation is to be more than weak emergence in a special guise, it has
to be absent in those cases in which there is a part–whole relation but not a
major difference between things.
It helps to consider again a colony of unicellular organisms, such as
colonial flagellates, forming a certain kind of whole whose parts are not
specialized.16 Some aspects of part-events are different when a part is inside
the colony rather than outside. For example, there is a reduced likelihood
that a cell inside it will be destroyed by external impacts. Although there is
downwards causation, we would not here speak of emergent properties
(except for weakly emergent ones). Apart from such cases, it seems quite
generally that every relation that makes a whole out of some components
has some effects on the behaviour of these components. But if that is true,
we have exactly the situation where strong emergence is reduced to weak
emergence, and so the concept of downwards causation is not apt for the
work it was supposed to do, viz., to enrich the concept of weak emergence so
that it accounts for the levels of reality, for the borders between the sciences
or for the main distinctions in our common-sense world picture.
We shall not get what we desired, at least not with the concepts of nondeducibility and downwards causation. Yet it is not easy to find a concept of
emergence which is stronger than weak emergence, on the one hand, and,
on the other, is not bound up with our epistemic situation, in particular with
our theories and our computational abilities. On the contrary, since not
every transition from less to more complex wholes strikes us as alike, it seems
16
See L. Buss, The Evolution of Individuality (Princeton UP, ), p. .
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very likely that what we take to be emergent are more or less just the
properties of things which we find especially salient because the presence or
absence of these properties has profound consequences for our actions
towards the things. Emergence, finally, is what we did not want it to be, an
epistemic category.
Universität Heidelberg
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