## 1. The historical origins of the concept



When levels of organization are understood as belonging to the broader
category of hierarchical depictions of nature, their history can be
traced back to the early days of western science and philosophy.
Aristotle suggested that living things could be arranged in a graded
scale, starting from plants at the bottom and ending with humans at
the top. This idea was further developed in Neoplatonism, and in
Medieval times, it transformed into the idea of "the Great Chain
of Being" (Lovejoy 1936). This *Scala Naturae* conveyed a
hierarchical conception of the world: at the top, there is God, and at
lower rungs, angels, humans, animals, plants, and so on. After the
scientific revolution, more scientific hierarchical classifications of
nature started to appear, the most famous ones being Linneaus'
taxonomical system and Auguste Comte's hierarchy of the sciences
(Comte 1830-1842 [1853]).



At the beginning of the twentieth century, several notions of
"levels" began to circulate in the philosophical and
scientific literature. Prominent among these was the idea of
"levels of existence" (alternatively: "being";
"complexity") developed by philosophers Samuel Alexander
(1920: 3, 45) and Charles Dunbar Broad (1925), which figured
prominently in the emergentist and emergent evolution literature of
the 1920's (Sellars 1926; McLaughlin 1992; Stephan 1999). This
introduced a number of themes that have become germane to general
levels-language. These themes include level-specific laws (Broad 1925:
77; cf. Woodger 1929) and the idea that 'levels' exhibit a
tendency in nature towards increasing complexity (Sellars 1917: 224;
cf. Needham 1937). Although most of these themes were severely
underdeveloped and divorced from the cutting-edge work in the life
sciences happening at the time (Needham 1937: 242 ff. 4), this
tradition has been an influential historical source for explicating
levels language in philosophy of mind (see the entries on
 emergent properties
 and
 supervenience;
 McLaughlin 1992; Beckermann, Flohr, & Kim 1992; Kim 1999,
2002).



The roots of the contemporary notion of levels of organization and the
associated hierarchical thinking are best linked to the efforts of
organicist biologists of the early-mid twentieth century (for primers
on the organicist movement, see Peterson 2014; Nicholson & Gawne
2015). Particularly important to the introduction and development of
the levels concept were Joseph Woodger (1929; 1930), Ludwig von
Bertalanffy (1928 [1933]; 1932), and Joseph Needham (1936b; 1937).
These individuals were in turn influenced by Alfred North
Whitehead's philosophy (see, e.g., Whitehead 1929; Needham
1941).



Organicism (alternatively "organismal" or
"organismic" biology) did not designate a clearly-defined
group or movement, but rather a loosely-threaded confluence of
scientists and philosophers distributed among many disciplines and
across Europe and the Americas. The organicists (like the
emergentists) were in part responding to the dispute between the
mechanists and the neovitalists of the early twentieth century. The
mechanists held that biological phenomena were "nothing over and
above" their physico-chemical components, meaning that
biological phenomena can in principle, and eventually will, be
exhaustively accounted for in chemical or physical terms (Loeb 1912;
1916). They also argued that biological phenomena, though seemingly
complex, are fundamentally similar to machines (Loeb 1912; 1916; see
also G. Allen 2005: 264; Bechtel & Richardson 1993 [2010: 17];
Nicholson 2012: 160). Neovitalists such as Hans Driesch and Henri
Bergson rejected these ideas, arguing that living things cannot be
explained solely based on their physico-chemical components, and that
biological phenomena therefore must involve non-physical forces or
entities. The organicists sought to offer a "third way"
into the mechanist-vitalist dispute that served as a middle ground
between the austerity of the mechanists and the extravagance of the
neovitalists (Peterson 2014: 286; Nicholson & Gawne 2015:
358).



Three major tenets of organicism were particularly congenial to the
development of the levels concept. Firstly, the central preoccupation
of organicist thought with *organization* laid down a
foundation for levels thinking (cf. Nicholson & Gawne 2015: 364).
For the organicists, organization marked the decisive feature for
demarcating living phenomena from non-living physico-chemical
phenomena (e.g., Woodger 1929: 290-1; von Bertalanffy 1928
[1933: 48]; Needham 1936b: ch. 1). Unlike the neovitalists, however,
the organicists did not hold organization to be inscrutable, but
rather an important explanandum of biological study (Nicholson &
Gawne 2015: 365). Moreover, living organisms exhibited a
*hierarchical ordering* among their distinct parts (Woodger
1930: 8; von Bertalanffy 1932: 83; Needham 1936b: ch. 5; see also
Nicholson & Gawne 2015: 366). This combined nicely with the focus
on organization as the major explanandum of biology in that the series
of elements that compose different organisms required specific and
contextualized treatment. This in turn complemented another main tenet
of the organicists, namely *the autonomy of biology* as a
natural science (cf. Nicholson & Gawne 2015: 366-7).



The levels concept proved to be a linchpin of these main tenets.
Joseph Woodger, combining these features, thus expresses the basic
thinking behind levels of organization:



> 
> 
> Two principal factors appear to have been responsible for the failure
> to take organization seriously in biology. First there is the vague
> belief that only atoms and molecules are 'real', and
> secondly the incautious use of the notion of chemical composition.
> Also the biological entities are found to be composite in the same
> sense, and moreover, some of the relata in the relation of composition
> in the physico-chemical objects are also relata in the biological
> objects. *But* the analysis of organisms as carried out by
> biologists reveal *other* relata in mutual relation of
> composition *in a different sense*, i.e., not in
> *chemical* composition, e.g., *the organism is analysable
> into organ-systems, organs, tissues, cells and cell-parts. There is a
> hierarchy of composing parts or relata in a hierarchy of organizing
> relations.* These relations and relata can only be studied at
> their own *levels* (cf. the quartz crystal) and not simply in
> terms of the *lower levels*. (Woodger 1929: 292-293,
> emphasis modified; see also Bertalanffy 1932: 100-101; Needham
> 1937 [1943: 234])
> 
> 
> 



Unlike the emergentists' "levels of existence", the
hierarchical view expounded by the organicists emphasized
differentiating the many distinct classes of matter's possible
forms to accommodate the diversity in biological entities. This in
turn led them to posit many more levels in the world than the four
'super levels' of the emergentists (the physical the
chemical, the vital, and the mental), and also implied an increase in
complexity as more levels of organization were brought to bear on
treating living systems. Woodger again remarks:



> 
> 
> If the parts [of an organism] were homogeneous then we should be able
> to call them units and there would only be one level of organization.
> But if each part were itself composed of parts forming in each an
> organized system, then clearly we should have two levels of
> organization and if the composing sub-parts of the first organization
> were intrinsically only numerically different we could speak of two
> homogeneous levels of organization. If, however, the sub-parts were
> intrinsically different then the first-order parts would be different
> and we should have a heterogeneous type of organization. By carrying
> out this process of subdivision further we could obtain very complex
> types of organization exhibiting a hierarchy of successive levels. And
> if now we consider one of the higher living organisms it is evident at
> once that its organization will belong to one of these heterogeneous
> hierarchical types. (Woodger 1929: 298)
> 
> 
> 



This idea that levels of organization of organization form
"nested" compositional hierarchies where there are wholes
at higher levels and their components at lower levels, and the
components themselves can be further decomposed into parts, remains
one of the core features of the notion of levels of organization up to
this day.



In the aftermath of organicism, development of the levels concept
splintered into different intellectual trajectories, with little
overall theoretical or conceptual coherence. One direction of
development flowed through the framework of general systems theory
(von Bertalanffy 1950, 1968), and particularly through so-called
hierarchy theory (Whyte, Wilson, & Wilson 1969; Weiss 1971; Pattee
1973). General systems theory sought to construct isomorphisms between
laws of different scientific fields based on their generic system
properties (von Bertalanffy 1950: 136), but nonetheless continued to
emphasize the hierarchical part-whole structure of systems (von
Bertalanffy 1968). 'Levels of organization' in this
tradition was ultimately given secondary consideration to the
articulation of system-generic properties and laws. Similarly, in
hierarchy theory, 'levels' tended to be treated as
derivative of the more general notion of a hierarchy, or the ordering
of a system into nested subsystems (Simon 1962 [1996: 184-5];
see also Weiss 1971). Hierarchy and levels alike continued to be
treated as generic structures of systems of all stripes (see also T.
Allen & Starr 1982; O'Neill et al. 1986). However, one
important distinction to come out of this tradition was the
distinction between levels of composition and levels of control (Simon
1962; Pattee 1973). The former speaks to the nested compositionality
typically identified with levels of organization, while the latter
refers to the idea that higher levels impose constraints on the
processes at lower levels, for example by limiting the degrees of
freedom of the system at a lower level (Pattee 1973: 85; see also
Umerez 2021).



Other early attempts at clarifying the concept of levels were carried
out by individual researchers working alone on similar or related
issues, such as reduction or multilevel selection. Mario Bunge (1960;
1977), Marjorie Grene (1969; 1987), and William Wimsatt (1976a; 1981;
1994) each developed several ground-laying observations concerning
levels. Bunge (1960) remarks on the plethora of meanings the term can
possess, then offers a long taxonomy of nine different meanings of the
term "levels", ranging from the innocuous (levels as
degrees or quantity, as in 'levels of stress') to the
sense of nature being ordered into an evolutionary series, with
several corollary senses of levels falling between (such as degrees of
complexity). In his 1977 paper, Bunge goes further, offering a
set-theoretic treatment of levels, and claiming that the leveled
structure of the natural world is what renders natural phenomena
intelligible to us (Bunge 1977: R82). Grene (1969) also discusses the
polysemic nature of the term "hierarchy", and later (1987)
distinguishes between two notions of hierarchy, one falling under a
mixed notion of levels as composition and control, and the other
exemplifying phylogenetic ranks. Wimsatt's work will be
considered in more detail below in
 section 2.3.


## 2. Philosophical accounts of levels of organization



A recurring motif in the literature on levels of organization is to
lament the haphazard or unreflective way in which terms such as
'hierarchy' or 'levels' are applied, and to
call for more precise analyses (e.g., Beckner 1974; Bunge 1977; Grene
1987; Korn 2002; Valentine & May 1996). However, until recently,
developing substantial theories or accounts of levels of organization
has only occurred sparingly in the literature. In this section, we
first discuss three major accounts put forward in philosophy of
science to clarify or posit what exactly levels of organization are.
These are Paul Oppenheim and Hilary Putnam's
"layer-cake" account, the mechanist account developed and
advocated by Carl Craver (2007: ch. 5; 2015) and William Bechtel
(2008: ch. 4), and William Wimsatt's "local maxima"
account (1976a; 1994). After these, we also go through recent
skeptical approaches to the idea of levels of organization.


### 2.1 The layer-cake account



In the classic paper "The Unity of Science as a Working
Hypothesis" (1958; see also the entry
 The Unity of Science),
 Oppenheim and Putnam (hereafter O&P) put forward a system of
levels that became very influential, and is still implicitly present
in many references to levels. This "layer-cake" model
comprises three components. First, their conception of levels was
*comprehensive*, meaning they wished to subsume all instances
of where one could talk of 'levels' into their account.
Second, O&P posited that levels are related via compositional
relations that are structured in a *stepwise* fashion. That is,
all constituents of the objects of study of one branch of science, or,
the branch's "universe of discourse", are
exhaustively related as wholes to the parts located at the next
adjacent lower level, and as themselves parts to the constituents
occurring at the next adjacent higher level. This component is
probably most responsible for the "layer-cake" moniker.
Finally, O&P presumed a strict *correspondence* between the
constituents comprising a level and the predicates and theories linked
with these constituents, meaning that levels of science neatly map
onto levels of nature, so that for each level in nature there is
corresponding science or theory and vice versa (Craver 2007:
174-175; Brigandt 2010: 304-305).



O&P's usage of 'levels' can be broken down into
two roles within their framework. The first concerns their use of
'levels' in their explication of reduction, i.e.,
microreduction, which in turn aided in articulating their thesis of
unity of science (1958: 5-6). In this framework, O&P suggest
ordering the branches of sciences so that the major potential
reductions standing between the current situation and unified science
can be identified. For this purpose, they propose six "reductive
levels" (1958: 9): Social groups; (Multicellular) living things;
Cells; Molecules; Atoms; Elementary particles. The branch of science
with the things of a given level as its universe of discourse is a
potential "microreducer" of a branch with the things of
the next higher level as its universe of discourse (O&P 1958:
9).



O&P's second use of levels is to capture a natural epistemic
ordering of the sciences:



> 
> 
> The idea of reductive levels employed in our discussion suggests what
> may plausibly be regarded as a *natural order of sciences*. For
> this purpose, it suffices to take as 'fundamental
> disciplines' the branches corresponding to our levels. It is
> understandable that many of the well-known orderings of things have a
> rough similarity to our reductive levels, and that corresponding
> orderings of sciences are more or less similar to our order of 6
> 'fundamental disciplines'. (O&P 1958: 28, emphasis
> added)
> 
> 
> 



This shows that O&P assumed that the structure of the sciences
follows the structure of nature. Though they acknowledge the numerous
precedents to this hierarchical ordering of the sciences (e.g.,
Comte's pyramid of the sciences, 1958: 28), O&P considered
their layer-cake account to be superior to these other
"intuitive" senses of the order of sciences, since the
existence of levels themselves are grounded in the stepwise,
compositional continuity of nature.



The problems with the layer-cake account have been well-documented
(Craver 2007: 172-6; Kim 2002; Potochnik & McGill 2012;
Rueger & McGivern 2010). For one thing, if we look at contemporary
science, the strict correspondence between levels and scientific
fields breaks down immediately: Fields such as cognitive neuroscience
span multiple levels, and the level of multicellular organisms is
studied by a plethora of different scientific disciplines (Craver
2005; Bechtel 2008: 145). Likewise, the exhaustive stepwise condition
on compositional relations between natural constituents, requiring
that entities at one level are composed of only entities at the next
lower level, is largely a caricature (e.g., Kim 2002): think of blood,
nominally a tissue-level phenomenon, which is directly composed by
molecular constituents such as vitamins and water without
adjacent-level constituents, e.g., cells, playing any intermediary
role. Finally, and most importantly, the layer-cake account aspires to
a comprehensive notion of 'levels' that is wildly at odds
with the way that 'levels' is actually used in science.
Scientists often operate with a much more limited definition of the
levels concept that either (a) is restricted to a specific and local
context (see
 section 2.2)
 or (b) makes room for exceptions to what 'levels'
expresses (see
 section 3).
 Many biological sources in fact are completely aware that levels do
not capture a completely uniform reality, and sometimes remark on
important exceptions to the rules that are laid down by the levels
concept. This is most prominently seen when referring to
"organisms" in a piecemeal way so as to capture both
multicellular and unicellular forms of life (Mader 2010: 2).


### 2.2 Levels of mechanisms



The account of "levels of mechanisms" introduced by Bill
Bechtel (2008) and Carl Craver (2007) has in recent years become a
standard view of levels in philosophy of neuroscience. The context of
this account is the more general paradigm of "new
mechanism" (see the entry
 Mechanisms in Science).
 In levels of mechanisms, there are mechanisms at higher levels and
their components at lower levels. The mechanistic account proposes a
contextualized conception (as opposed to the comprehensive conception
of the layer-cake) that articulates levels in terms of constitutive
parthood within a mechanism (Craver 2007: 188-189; Kaplan 2015:
20; see also Cummins 1975 for an early expression of this idea). This
approach offers several advantages over the layer-cake account: For
one thing, it abandons the goal of providing a global or comprehensive
understanding of levels that applies homogeneously to all systems in
nature. Instead, the aim is to construe levels in a scientifically
informed manner whereby level demarcations only make sense on
case-based grounds (see also Love 2012 for a local approach to levels
that is not explicitly tied to the notion of a mechanism). The
mechanistic account also completely eschews any tidy correspondence
between the structure of the natural world and the structure of the
natural sciences: Levels of mechanisms are *levels in nature*,
and there is no straightforward mapping from these levels to theories
or fields of science (Craver 2007: 176). Finally, the mechanistic
account of levels supports a positive account of pluralistic,
multilevel explanation that emerges as an alternative to accounts of
reductionism in biology (Craver 2005; cf. Brigandt 2010: 297).



More specifically, levels of mechanisms have been defined as
follows:



> 
> 
> In levels of mechanisms, the relata are behaving mechanisms at higher
> levels and their components at lower levels. ... The interlevel
> relationship is as follows: *X*'s Ph-ing is at a lower
> mechanistic level than Ps-ing if and only if *X*'s
> Ph-ing is a component in the mechanism for S's Ps-ing.
> Lower level components are organized together to form higher-level
> components. (Craver 2007: 189)
> 
> 
> 
> Within a mechanism, the relevant parts are ... working
> parts--the parts that perform the operations that enable the
> mechanism to realize the phenomenon of interest. These may be of
> different sizes, but they are distinguished by the fact that they
> figure in the functioning of the mechanism. It is the set of working
> parts that are organized and whose operations are coordinated to
> realize the phenomenon of interest that constitute a level. (Bechtel
> 2008: 146)
> 
> 
> 



So far, these characterizations suggest that there are just two
levels, the level of the mechanism and the level of its components.
However, when we take into account that a component can also be
mechanism in itself, this picture is expanded into a multilevel
hierarchy: The components of that nested mechanism then form a third
level, which is two levels lower than the overall mechanism (Bechtel
2008: 147). This mechanistic decomposition can be repeated as many
times as necessary; there is no a priori limit to the number of levels
in a mechanism.



The standard example of levels of mechanisms is the case of spatial
memory and long-term potentiation (LTP; Craver 2007: 165-170).
In the spatial memory mechanism, four levels of mechanism can be
identified: the level of spatial memory, the level of spatial map
formation, the cellular-electrophysiological level, and finally the
molecular level. Entities at each lower level are components in the
higher-level mechanism. For example, an NMDA receptor at the molecular
level is a component of the LTP mechanism at the cellular level, and
the LTP mechanism is in turn a component of the hippocampal mechanism
of memory consolidation (at the level of spatial map formation). The
hippocampal mechanism of memory consolidation then contributes to the
overall mechanism at the level of spatial memory, which is the highest
level and involves, for example, behavioral tasks (e.g., navigating a
maze).



Levels of mechanisms share some key features with the traditional idea
of levels of organization: They are by definition compositional,
entities at higher levels are typically larger than entities at lower
levels, and levels of mechanisms can potentially amount to local peaks
of regularity and predictability (Craver 2007: 190; see next section).
However, levels of mechanisms are far more limited and minimalistic
than any other extant approaches to levels of organization. First of
all, as already mentioned, levels of mechanisms can only be identified
on a case-by-case basis, and different mechanisms can have entirely
different levels. For example, the set of levels in the mechanism of
protein folding is very different from the levels in the spatial
memory mechanism.



Moreover, even within one mechanism, the question whether two items
are at the same or different levels often has no well-defined answer.
This is due to the fact that levels are defined solely in terms of
direct part-whole (or component-mechanism) relationships. For example,
NMDA receptors and synaptic vesicles are components of the cellular
LTP mechanisms, and thus can be said to be at the same level. The
components of NMDA receptors in turn include things such as glutamate
binding sites and glutamate ions, whereas the components of synaptic
vesicles include things such as transport proteins. However, as
glutamate binding sites and transport proteins are neither components
of one another nor direct components in the same (sub)mechanism, they
are neither at the same level nor at different levels (Bechtel 2008:
147). The question how they are related levels-wise has no answer in
the mechanistic framework. What this means is that even within a
specific mechanism, levels of mechanisms do not form horizontal layers
that span across the mechanism, but rather small islands formed by the
working parts of a (sub)mechanism. Another implication is that even
identical things (e.g., NMDA molecules) within the same overall
mechanism are often not at the same level (Eronen 2013). Craver (2015)
has suggested that the whole idea of being "at the same
level" is in fact unimportant or even meaningless in the context
of levels of mechanisms; it is sufficient that there is a clear sense
in which mechanisms are at a higher level relative to their components
(and the components are at a lower level relative to the mechanism as
a whole), and this does not require that the components or mechanisms
also form horizontal levels.



A further problem that the mechanistic account shares with the
layer-cake account is that it is embedded within a conceptual
framework where 'levels' are defined in terms of other
technical terms that are more foundational in that framework (Brooks
2017). The layer-cake account was originally embedded in Oppenheim and
Putnam's overarching project of arguing for the unity of
science, and their understanding of 'levels' was in part
conceived to explicate how microreduction works. The mechanistic
account for its part is embedded within the New Mechanism's
program of explicating mechanistic explanation and mechanisms; since
"levels of mechanisms" largely overlap with technical
terms that define what a mechanism is, it is a legitimate question
what distinct notion of 'levels' results at all that is
not directly derivable from the notion of a mechanism (cf. Eronen
2013).



In this light, it is clear that levels of mechanisms are only
distantly related to earlier attempts at elucidating levels in the
life sciences, and have many features that we intuitively would not
associate with the idea of levels (Eronen 2015). However, this is
compatible with the idea that the relationships of mechanistic
composition that levels of mechanisms track are as such crucially
important for understanding and explaining biological
organization.



Several authors have recently defended a "flat" view of
the levels in mechanisms (Bechtel 2017; Bickle, De Sousa, & Silva
2022; Craver, Glennan & Povich 2021; Fazekas 2022). For example,
Peter Fazekas (2022) suggests that what we call mechanistic
"levels" just identify modules, or sets of interacting
units, within a causal network. In this view, there are no
ontologically distinct levels in mechanisms at all, mechanisms are
just extremely complex systems of interacting parts all at one
"level". Bechtel (2017) and Craver et al. (2021) put
forward a similar flat interpretation of mechanistic levels, but argue
that this is compatible with the original non-reductive commitments of
the mechanistic approach (see also
 section 4).
 For further attempts at expanding or developing the notion of levels
of mechanisms, see Bertolaso & Buzzoni (2017), Harbecke (2015) and
Kaiser (2015).


### 2.3 Wimsatt's "local maxima" account



Both the layer-cake and the mechanistic approach to levels aim at
defining levels in the sense of giving at least necessary conditions
for what constitutes a level. William Wimsatt (1976a; 1994) takes a
different approach and sets out to characterize the key features that
levels of organization typically (but not necessarily) exhibit across
different instances. Originally, an important background motivation
for Wimsatt was to understand and characterize claims about
"higher-level" entities and phenomena that are ubiquitous
in biology and social sciences (Wimsatt 2021). The philosophical
discussions in the 1970s were focused on strong reductionism or
eliminativism (see the entry on
 Reductionism in Biology),
 which seemed inadequate to capture the way many scientists perceived
the organization of nature (Wimsatt 2021). Inspired by the work of
Simon (1962 [1996]), Wimsatt set out to develop a "tropical
rainforest ontology" in contrast to a Quinean "desert
ontology", and levels played a central role in this account.
First, according to Wimsatt, levels are compositional and form nested
hierarchical structures, so that wholes at lower levels function as
parts at higher levels. Second, levels of organization are a



> 
> 
> *deep, non-arbitrary, and extremely important feature of the
> ontological architecture of our natural world, and almost certainly of
> any world which could produce, and be inhabited or understood by,
> intelligent beings*. (Wimsatt 1994 [2007: 203])
> 
> 
> 



A further feature of levels of organization is that they are



> 
> 
> *constituted by families of entities usually of comparable size and
> dynamical properties, which characteristically interact primarily with
> one another*. (Wimsatt 1994 [2007: 204])
> 
> 
> 



A helpful metaphor that Wimsatt uses to illustrate this is that we can
think of theories or models of specific levels as sieves of different
sizes that sift out things with the appropriate size and dynamical
properties (Wimsatt 1976a: 237). Thus, theories at the level of
organisms sift out roughly organism-sized things that are capable of
reproduction and primarily interact with each other.



Finally, and perhaps most importantly,



> 
> 
> *[l]evels of organization can be thought of as local maxima of
> regularity and predictability in the phase space of alternative modes
> of organization of matter*. (1976a: 209; see also 1976a: 238)
> 
> 
> 



This point is "the closest that [Wimsatt] will come to a
definition" (1976a: 209), and requires some explanation. The
idea is roughly that patterns and regularities that can be used as a
basis for prediction and explanation are found clustered around
certain scales, and such clusters indicate levels of organization.
Thus, if we plot regularity and predictability against (size) scale,
then levels of organization will appear as peaks in the plot (see
Figure). The entities that we find at these "local maxima"
will exhibit stable regularities in virtue of the criteria (size,
rate, dynamic properties, etc.) with which they are grouped into
levels, and these regularities can also serve as the basis for
reliable predictability. Moreover, levels of organization comprise
"the *most probable* states of matter" (1976a:
239), meaning that if we could vary the initial conditions, under a
range of conditions natural selection or other selection processes
would result in the same levels (Wimsatt 1976a: 238-239). For
example, if the level of molecules is a level of organization in
Wimsatt's sense, then there should be a peak in regularity and
predictability at the scale(s) where molecules are located, and
molecules should be the most probable mode of organization of matter
under a range of conditions.



![a diagram: link to extended description below](wimsatt-maxima-levels.png)

Figure 1. Wimsatt's levels of
organization. Based on Wimsatt (1976a: 240), created in 1973; used
with permission. [An
 extended description of figure 1
 is in the supplement.]




Wimsatt points out many other features that levels may have: For
example, processes at higher levels tend to happen at slower rates
than processes at lower levels, higher level properties are typically
multiply realized by lower level properties, and higher-level causal
relationships are dynamically autonomous in the sense of being largely
independent of what happens at lower-levels. What differentiates
levels will also vary between instances, and may include part-whole
distinctions, the magnitudes of forces by which things interact, or,
pragmatically, considerations of size of different constituents.



Importantly, Wimsatt also argues that in contexts where part-whole
relationships become too complex, such as in many biological systems,
levels of organization break down, and in these cases a more
appropriate organizational notion is *perspective* (Wimsatt
1994 [2007, 227]). Perspectives are (incomplete) accounts of systems
based on a set of variables, and in contrast to levels, need not have
any part-whole structure. For example, we can approach organisms from
anatomical, physiological or genetic perspectives, each perspective
coming with a proprietary set of variables. When even the boundaries
of perspectives start to break down, and it is no longer clear to
which perspective a problem belongs to, we are facing what Wimsatt
calls a *causal thicket*. According to Wimsatt,



> 
> 
> neurophysiological, psychological, and social realms are mostly
> thickets, which are only occasionally well-ordered enough for local
> problems to be treated as perspectival or level-relative problems.
> (Wimsatt 1994 [2007: 239])
> 
> 
> 



Griesemer (2021) presents a more epistemic reading of these notions:
When scientists first approach a complex system, they encounter it as
a causal thicket, but they then proceed to "prune" it,
first to perspectives and then (possibly) to levels.



By including so many caveats and possible but not necessary
characteristics of levels, Wimsatt makes his account extremely
versatile and wide in scope, but at the same time open to charges of
vagueness or inconsistency (cf. Craver 2007: 182-183). It
contains a multitude of criteria that levels could but do not
necessarily have to satisfy, so that almost any set of entities that
are in some respect similar to one another could be said to form a
level. Moreover, the individual criteria as such also raise many
questions. For example, how should we understand the "local
maxima of regularity and predictability"? What kinds of
regularities should we include--only causal generalizations, or
also regularities describing associational or compositional relations
(Craver 2007: 182-183)? How do we count or estimate the number
of regularities on a given scale? How do these peaks match with the
other proposed criteria, such as the part-whole organization of
levels?



These worries are to some extent alleviated by taking into account an
important background idea in Wimsatt's approach: Levels of
organization and the entities that occupy them should be
*robust*, meaning that they should be detectable, measurable,
derivable, definable, and so on, in a variety of *independent*
ways (1981 [2007: 63-4]; 1994 [2007: 210]). In other words, they
should exhibit redundancy between independent means of accessing,
detecting or defining them, and therefore should not depend on any
single criterion or defining feature. Thus, if levels of organization
are robust, the choice of criteria, or the problems of some individual
criterion, will not be crucial. However, the extent to which there are
such robust levels of organization in nature remains an open question.
What is clear is that Wimsatt's account of levels of
organization is still highly relevant and continues to serve as the
backdrop for informed discussions of levels (see, e.g., the articles
in the collection Brooks, DiFrisco & Wimsatt 2021b; and DiFrisco
2021 for a process-based account of levels that is explicitly
Wimsattian in character).


### 2.4 Levels skepticism and deflationary accounts



Despite the familiarity of scientists and philosophers with the levels
concept, calls for its dismissal or de-emphasis in the scientific
lexicon are increasingly common (Eronen 2013; Guttman 1976; Ladyman
& Ross 2007; Potochnik 2017; 2021; Potochnik & McGill 2012;
Rueger & McGivern 2010; Thalos 2013). For one thing, nature may
simply be too messy to fit any layer-cake style picture. Consider as
an illustration the putative level of "organisms". Blue
whales and yeast cells are both clearly organisms and thus should
nominally be located at this level, but each comprises radically
different kinds of entities with radically different properties
(Potochnik & McGill 2012). Moreover, but when we consider the next
lower level, namely the one indicated by the components of these
organisms, the picture of levels as neat horizontal layers breaks down
completely. The components of blue whales include things such as
organs, tissues and cells, whereas yeast cells are composed of things
like the cell membrane, nucleus and mitochondria (cf. Potochnik &
McGill 2012). Furthermore, the whale is in part composed of various
symbionts, including gut bacteria which are at the same time
components of the whale and organisms in themselves. Thus, the
components of different kinds of organisms do not form any homogeneous
"level".



These problems are not just due to the difficulties of defining what
an organism is. Similar issues arise when we consider the components
of cells, such as the cell membrane and mitochondria, and especially
their subcomponents (e.g, lipid molecules and the outer mitochondrial
membrane respectively): they exhibit too much heterogeneity to form
neat levels (Eronen 2021). Moreover, the same kinds of things can be
parts in very different higher-level wholes: For example, hydrogen
ions are components of the lipid molecules that make up cell
membranes, but they can also be free-standing components in the
oxidative phosphorylation mechanism that involves the cell membrane as
another component (Bechtel 2008: 147).



More generally, Potochnik and McGill (2012; see also Potochnik 2021)
argue that 'levels' imposes a radically false, rigid
uniformity onto nature, and identify these problems with the basic
idea of the levels concept itself:



> 
> 
> Indeed, the very notion of stratified levels depends on not only the
> ubiquity, but also the uniformity, of part-whole composition. For
> strata to emerge, atoms must always compose molecules, populations
> must always compose communities, and so forth. But the uniformity of
> composition needed for stratified levels *simply does not
> exist*. (2012: 126; emphasis added; see also Guttman 1976: 113;
> Thalos 2013: 10)
> 
> 
> 



This comprehensive, uniform rigidity in turn undermines other features
attributed to levels of organization. One of these features is the
supposed epistemic merit (or fault) inherently exhibited by natural
constituents due to their position at a particular level (Potochnik
& McGill 2012: 129-30; Potochnik 2021). For instance,
reductionists frequently argue that molecular-level explanations are
generally more secure, more fundamental, or otherwise superior to
explanations citing higher-level structures like chromosomes or cells
(and conversely, anti-reductionists claim that higher-level
constituents are often necessary for producing adequate explanations).
However, as has been pointed out by many authors, blanket attributions
of significance like these are deeply misleading (see
 section 4).
 One reason for this is that particular levels exhibit epistemic merit
only in regard to what is being investigated. As such, what is taken
to be the relevant level will shift depending on what structures or
processes comprise the focus of scientists' investigative tasks,
and hence no level will have inherent or principled epistemic
superiority. Moreover, epistemic products like explanations and
theories in biology rarely exhibit a monolevel structure, but rather
include multiple levels simultaneously (e.g., Schaffner 1993:
97-8, 387; Mitchell 2003: 147; Craver 2007).



The basic conclusion that Potochnik and other levels skeptics draw
from this is the following. The levels concept precludes a
sophisticated discussion of philosophical and scientific issues by
imposing an overly simplistic representation of science and nature.
So, although perhaps no one would deny the attractiveness of
'levels' in seeking to make complex natural systems
tractable to analysis, depicting these systems using the concept seems
to do far more harm than good. Burton S. Guttman is very clear on
this, arguing that "if it is stated in any but the sloppiest and
most general terms, [the concept of levels] is a useless and even
misleading concept" (Guttman 1976: 112). Similarly, Miriam
Thalos colorfully names "the conceit of levels" before
stating that "the notion of levels provides no useful
philosophical ideas whatsoever" (Thalos 2013: 13).



This levels-skepticism also possesses a more constructive side.
Several authors have suggested replacing or reinterpreting levels talk
in terms of other concepts (Eronen 2015; Noble 2012; Potochnik &
McGill 2012; see also P.S. Churchland & Sejnowski 1992; McCauley
2009; Rueger & McGivern 2010). The motivation behind this
"deflationary" approach (Eronen 2013) is that the problems
associated with levels of organization can be avoided if more
well-defined notions are applied instead. For instance, the notion of
*scale* arguably does not lead to the same kinds of problems as
the notion of levels of organization. All that is needed for arranging
things on a scale is measuring some quantitative property of those
things. Scales also have the advantage of being entirely continuous,
while levels require placing things at distinct and discrete positions
in the hierarchy (Potochnik & McGill 2012). The scale that is the
most obviously relevant one in this context is the size scale, which
is based on how big things are: Organisms are usually (though not
always) bigger than cells, which are bigger than molecules, which are
bigger than atoms, and so on.



However, the time scale is also crucially important for understanding
biological organization (Simon 1962; DiFrisco 2017). Time scale can
refer to the rate at which a process occurs, but also to the frequency
of a process, or to the rate of interactions between processes or
entities, all of which are relevant scales in the context of biology
(Baedke 2021). For example, interactions between organisms (e.g.,
sexual reproduction) take place at much slower rates than interactions
between cells (e.g., synaptic communication), which again are slower
than interactions between molecules (e.g., receptor binding). The idea
of ordering biological processes based on time scale has a long
history, going back at least to organicism (see
 section 1;
 Baedke 2021). Understanding levels in terms of scales is also
consistent with the idea that at some scales we may find peaks of
regularity or predictability (Wimsatt 1994 [2007]), or clusters of
causal relationships (Potochnik & McGill 2012). Besides scales,
also other notions have been proposed to replace levels, such as
networks (Potochnik 2021; see also Bechtel 2017) or explanatory styles
(Potochnik & Sanches Oliveira 2020).



Although these skeptical and deflationary approaches, where the notion
of levels is reduced or deflated to more well-defined concepts,
ostensibly increase conceptual clarity, they may not be sufficient for
accounting for the role of levels of organization in biological theory
and practice (see also
 section 6).
 For example, the cellular level is a canonical level organization,
but cells come in many different sizes, and it is hard to identify a
size (or time) scale that would play a similar role as the notion of a
cellular level. Moreover, skepticism regarding specific philosophical
conceptions of levels, such as layer-cake-style levels or levels of
mechanisms, leaves open the possibility that more piecemeal or
context-specific approaches to levels may still be useful.


## 3. Levels as a fragmentary and heuristic notion



In the wake of skeptical arguments against the levels concept, recent
work on the notion has sought to address the pessimistic appraisals
outlined in the previous section by rejuvenating the discussion
surrounding levels. One key component in reestablishing the potential
of levels for philosophical insight has been to address the gap
between philosophical expectations placed upon the concept in
skeptical treatments, and the usage dynamics informing scientific
applications of the concept (Brooks 2021b; see also DiFrisco 2017 and
Brooks, DiFrisco & Wimsatt 2021a).



In particular, philosophical expectations placed upon accounts of
levels tend to depart from scientific applications of the concept both
in characterizing how the levels concept is used and in the
motivations behind applying the concept in scientific contexts. This
is especially visible in the way that levels are conceptualized by
critics. For instance, Potochnik characterizes the objectionable
consequences of the levels concept as stemming from "the very
idea" of levels (2017: 170), where this "very idea"
takes the concept to impose (i) universality of composition, (ii)
uniform relations between putative levels, and (iii) total
discreteness of level-bound units (2017: 170; see also Potochnik &
McGill 2012: 126). Similarly, Thalos claims that to even postulate a
leveled ontology is to be committed to a reductionist "master
ontology" grounded in physics from which all other higher levels
derive (2013: 7).



Both of these characterizations rest on attributions to the levels
concept stemming from philosophical sources rather than scientific
usage of the concept. For one thing, Potochnik's idea of levels
seems to presume a layer-cake conception, an account that has few
contemporary advocates and whose fidelity to science has arguably
always at least been questionable. Likewise, the commitment to a
reductionist ontology purported by Thalos rests decidedly on the
further postulation of reductionist physicalism, a stance to which the
levels concept is at best neutral (see
 section 6).
 Though these sources for making levels intelligible as an object of
criticism may enjoy intuitive appeal widely in play in general
philosophy of science and philosophy of mind, it is plausible that
assessing the merits (and demerits) of the concept requires extraction
from base intuition and exposure to further scientific-philosophical
scrutiny (Brooks, DiFrisco, & Wimsatt 2021a).



For these reasons, efforts to rejuvenate levels and their significance
have turned to different sources to enable scientific input into the
conceptualization of levels. One important consequence of this has
been the rejection of any neat and tidy definition or unitary account
of levels, instead treating the concept within multiple facets of
reconstruction that scientific usage engenders. That is, engaging with
the notion is seen as a widely open-ended endeavor (Brooks 2021a).



One such front of approaches to levels focuses on the roles of the
concept in scientific reasoning. For instance, Brooks and Eronen
(2018) locate a heuristic element within usage of levels of
organization. In this approach, instead of focusing primarily on what
levels are ontologically speaking, or attempting to give a unitary
definition of levels, the focus is to recover first the significance
attributed to levels when applied in science (and parts of philosophy
of biology). In particular, depicting biological systems requires
emphasizing some details over others, as incorporating all details of
an investigated system is not possible due to the cognitive and
physical constraints placed on human agents (in line with the focus on
heuristic reasoning). Thus, scientific uses of levels involve
descriptive abstractions figuring into the decisions researchers make
in selecting features of a system intended for the attending analysis.
A second use Brooks and Eronen identify concerns the postulation of
levels as guiding scientific inquiry in that different putative
levels, though perhaps not well-characterized at the outset, serve as
points of departure for new fields of inquiry in laying out landscapes
of problems and questions that lie in front of these fields (such as
systems biology in the 2000s). Eronen (2021) identifies a further
heuristic in the basic idea that in levels of organization
"similar things are composed of things that are similar to one
another", which may often work as an abstraction. For example,
both proteins and genes (at a higher level), which are similar in
important ways, are composed of monomers such amino acids and
nucleotides (at a lower level), which are again similar to one another
in important ways.



This being said, the heuristic approach offers an incomplete account
of levels, an intentional feature meant to invite further work on the
notion. The heuristic approach thus seeks to capture some minimal
features that a more committed body of work should account for. A core
motivation of this approach is to construct a rapprochement between
advocates of the levels concept and more skeptical theorists (see,
however, Potochnik 2021 for criticism of the heuristic approach).
Particularly, supporters of the heuristic approach argue that a more
nuanced appraisal of the levels concept aids to reveal its flexibility
and usefulness in science. Most especially, it emphasizes that, like
other heuristics, the concept may work in some contexts but not in
others. Such scattered records of success, failure, and neutral
accomplishments are in turn "mundane to scientific concepts in
general" (Brooks & Eronen 2018: 41).



Using the heuristic appraisal of levels as a springboard, a more
systematic reconstruction of the levels concept in scientific
reasoning points to a "fragmentary approach" whereby more
overarching usage profiles begin to take shape. Brooks (2021a) for
instance articulates two such profiles associated with the levels
concept in scientific usage. One of these profiles identifies the
levels concept as a tool (*sensu* Griffiths & Stotz 2008;
Feest 2010) that scientists use to, for example, construct material
inferences in specific experimental systems and to aid in providing
projectibility to these inferences by conferring generality to the
structures and processes involved. This profile of usage
arguably captures practical scientific work operative in research
articles published for specialist researchers. A second usage profile
associated with levels appears as a kind of "doctrinal"
notion in the vein of cell theory or the germ theory of disease. This
profile draws its inspiration from the notion of doctrine, in the
sense of something that is widely taught or adhered to, and is visible
in scientific textbooks (see
 section 6.7)
 and in broad statements of position by grant-funding agencies and
major theoretical works in biology. That is, the notion of levels of
organization comprises a core theoretical component in the edifice of
biology in serving to introduce or advocate for a basic view of
biological science. Both of these usage profiles, seen as tool and
doctrine, complement each other in conferring (i) fine-grained content
to the concept in specific instances (tool) and (ii) general warrant
to the notion as an established confluence of insights acquired
throughout history (doctrine). Moreover, both suggest a unifying
rationale to the notion, identified in its potential ability to
structure scientific problems (Brooks 2021b).


## 4. Reductionism, pluralism, and levels



As we saw above, Oppenheim and Putnam introduced their highly
influential view of levels in the context of the reductive unity of
science, and Wimsatt's account was partly motivated by the need
to respond to reductionism. Up to this day, levels have continued to
play an important role in debates on reductionism (see the entry
 Reductionism in Biology).
 Questions of reductionism are often explicitly formulated in terms of
levels: Do higher level properties, theories, or explanations reduce
to lower level ones?



In the classical Nagelian account (Nagel 1961), reduction amounts to
deriving a theory that is to be reduced from a more fundamental theory
on the basis of two conditions: deducibility and connectability (see
the entry
 Scientific Reduction).
 The standard example is the reduction of thermodynamics to
statistical mechanics. Nagel thought that from the laws of statistical
mechanics one an derive the laws of thermodynamics (deducibility),
with the help of "bridge principles" that connect the
terms of these two theories (connectability; Nagel 1961: ch. 11;
Schaffner 1967).



Nagel's original approach to reduction has been criticized from
many angles, but in the present context, the most relevant criticism
is that the classical model ignores the distinction between
*interlevel* and *intralevel* reductions in science
(Wimsatt 1976a; 1976b; McCauley 1996). Intralevel (or successional)
settings involve competing theories within a particular science and
within a particular level, for example the phlogiston theory and the
caloric theory of heat. Interlevel settings, on the other hand,
involve theories at different levels of organization that thereby have
differing explananda, terminology and methodology: for example,
cognitive psychology and molecular neuroscience. Nagel's model
in its classic form treats these different contexts equally. For this
and many other reasons (see, e.g., Walter & Eronen 2011), it is
widely agreed that the Nagelian model fails to reconstruct reductive
relations as they appear in actual science (Wimsatt 1976b; 1979), and
that the model in its original form is inapplicable to actual
biological cases (Kaiser 2012).



Problems of the Nagelian model led to the development of alternative
accounts based on the same basic idea, which actively incorporated the
idea of levels into their framework (Hooker 1981; P.S. Churchland
1986; Schaffner 1993; Bickle 1998; 2003). This culminated in the
"New Wave" model of reduction, where intertheoretic
relations form a continuum ranging from complete elimination to
retentive Nagel-style reduction (Bickle 1998). For a summary of this
model, see the entry
 Scientific Reduction
 SS2.6.



Other accounts of reduction also countenance the levels concept,
albeit in different ways. In his "ruthless" account of
reductionism, Bickle (2003) abandons the idea of reduction progressing
stepwise from level to level, and argues that cellular and molecular
neuroscience can directly replace psychology (see also Barwich 2021).
If one can intervene onto the molecular level to change variables at
the behavioral level, then, Bickle contends, one has discovered the
"genuine" causal mechanism for those variables (Bickle
2003). Here 'levels' figure into ruthless reductionism as
useful fictions, where intermediary levels such as tissues, organs,
and the like provide a scaffolding for emerging molecular-level
explanations. These intermediary levels exhibit "merely"
heuristic value, only in the service of constructing
behavior-to-molecules reductions (Bickle 2003: 130). Recently, Bickle
has defended an even stronger and explicitly "levels-less"
version of reductionism, where levels no longer have even a heuristic
role (Bickle et al., 2022).



A different reconceptualization of reduction is offered by
Huttemann and Love (2011), who note that part-whole reduction in
particular has become a widely-endorsed alternative to the Nagelian
framework (see also Brigandt 2013: 77). They put forward
*compositionality* and *causality* as two aspects of
reductive explanation (Huttemann & Love 2011: 4), which are
both expressed in levels-laden language. Compositionality, for
instance, is couched in 'levels' in that the very notions
of part and whole in biological contexts correspond to respectively
lower- and higher-level status (cf. Kaiser 2015, 192; Winther 2006).
Kaiser (2015) goes further than this approach in claiming that a
"lower-level character" is a necessary condition for an
explanation to even be considered reductive (2015: 188).



The idea of levels has also figured prominently into arguments for
antireductionism and pluralism. In his classic treatise, Philip
Kitcher (1984) elevates the level concept to a centerpiece of the
antireductionist position. According to Kitcher, the reductionist
aspiration of reducing classical Mendelian to molecular genetics will
not pan out because molecular constituents do not capture the causal
relations between Mendelian constituents (e.g., chromosomes) and
heritable traits. Importantly, Kitcher grounds this in the structure
of the world as captured by the levels concept:



> 
> 
> Anti-reductionism construes the current division of biology not simply
> as a temporary feature of our science stemming from our cognitive
> imperfections *but as the reflection of levels of organization in
> nature*. (1984: 371, emphasis modified; Rosenberg [1985: 119]
> makes a similar statement)
> 
> 
> 



Elevating the levels concept even further, Kitcher makes levels (of
the layer-cake variety) part of the conception of antireductionism
itself, concluding that



> 
> 
> to the extent that we can make sense of the present explanatory
> structure within biology--that division of the field into
> subfields corresponding to levels of organization in nature--we
> can also understand the antireductionist doctrine. (Kitcher 1984:
> 373)
> 
> 
> 



In a more recent defense of antireductionism, Robert McCauley (1996;
2007) expands on the intra-interlevel distinction to plead for a
multilevel explanatory pluralism between higher- and lower-level
theories. McCauley targets the New Wave model where intertheory
relations result in a spectrum of consequences, ranging from identity
to elimination, for higher-level theories (P.M. Churchland 1979; P.M.
Churchland and P.S. Churchland 1990; Bickle 1998). He argues that a
single spectrum of reductive consequences misrepresents the nature of
intertheory coevolution. Intralevel reductions, focusing on reduction
in terms of replacement, do not capture the historical dynamics
involved in evaluating interlevel cases, and elimination is virtually
nonexistent in interlevel cases. Moreover, eliminating or reducing
higher-level theories would also undermine the status of the
higher-level's scientific institutions (McCauley 1996). As this
rarely happens, and attributing epistemic, ontological, or
methodological superiority to the lower levels is at best a claim that
needs to be substantiated, the New Wave model appears to be wildly at
odds with actual scientific practice. According to McCauley, we
observe intertheoretical coevolution instead, where higher-level
theories can offer lower-level theories criteria for external
coherency, evidential constraints, and novel problem solving
strategies (McCauley 1996).



Other arguments for pluralism also take their start from levels of
organization. Sandra Mitchell (2003), for instance, argues for
*integrative pluralism* that is suggested by the multilevel and
multicomponential "ontology of complex systems" (2009:
109). The basic idea of integrative pluralism falls out of the fact
that biological systems evolve in a contingent manner, thus setting
the stage for multilevel evolutionary scenarios (Mitchell 2003). This
in turn highlights the preponderance of 'emergent
properties' in biological systems, which appear at novel levels
of organization out of the interactions of simpler components. Both of
these factors inflate the overall number of explanatory sources that
could contribute to explaining a given phenomenon (2003; 2009).
Consequently, discovering unilateral causes for complex phenomena will
not be forthcoming (2003, 160). Likewise, Ingo Brigandt (2010; 2013)
emphasizes the pursuance of investigation at multiple levels of
organization as a fact about scientific practice, but also as cause to
articulate the localized significance of explanatory contributions to
particular problems, as in the case of evo-devo explanations.



This kind of multilevel pluralism is also defended by the supporters
of the New Mechanism (Bechtel 2008; Craver 2005, 2007; Brigandt 2010:
297; see also
 Mechanisms in Science).
 As McCauley (2007) points out, mechanistic explanation can be seen as
explanatory pluralism "writ small": In each localized
mechanistic context, several levels (understood as levels of
mechanisms) are needed to fully explain the phenomenon. However,
authors such as Fazekas (2022), Fazekas and Kertesz (2011), and
Soom (2012) argue that the commitments of New Mechanism are in fact
incompatible with robust antireductionism.


## 5. Levels and downward causation



Levels of organization have traditionally provided the framework for
the debate on downward causation, where the question is whether
higher-level entities or properties can exert causal influence on
lower-level ones (e.g., Bechtel 2008; Campbell 1974; Emmeche et al.
1997; 2000; Kim 1992; 1999; Craver & Bechtel 2007). The idea of
downward causation is closely connected to emergentism, and is often
seen as one of its core tenets (Emmeche et al. 1997; 2000; Kim 1999;
see also the entry
 Emergent Properties).
 Examples of putative downward causation include psychological states
causing physical behavior, the activity of an organism causing changes
in the tissues and cells of that organism, and cell-level processes
such as synaptic communication causing molecular changes. On the one
hand, downward causation seems *prima facie* to be common or
even ubiquitous in experiments and explanations in the life sciences,
and many scientists and philosophers consider it to be an important
feature of nature (Campbell 1974; Ellis 2008; Love 2012; Noble 2012).
On the other hand, in philosophy downward causation has often been
approached with skepticism (e.g., McLaughlin 1992; Kim 1992; 1999;
2005; Craver & Bechtel 2007).



Although the discussion on downward causation relies heavily on the
notion of levels, this notion itself has not received much explicit
attention in this context. However, to what extent downward causation
is possible or plausible crucially depends on what notion of levels is
assumed (Eronen 2021). Most of the critics of downward causation have
in mind causation across *compositional* levels, where a whole
exerts causal influence on its own parts. This is also what is at
stake in the influential discussion of Craver and Bechtel, who apply
the framework of levels of mechanisms to analyze top-down causation
(Bechtel 2008; Craver & Bechtel 2007; Craver 2015).



Craver and Bechtel argue that genuine top-down or downward causation
does not exist; there are just normal same-level causal relationships
that sometimes have "mechanistically mediated" effects
downward in the mechanism. What is meant by "mechanistically
mediated" is that changes in the higher levels of the mechanism
immediately result in changes in the lower levels of the mechanisms
due to the constitutive relationship between the higher-level
mechanism and its lower-level components. Mechanistically mediated
effects may have the initial appearance of downward causation, but the
relationship between the mechanism and its components is constitutive,
not causal, and therefore mechanistically mediated effects are not
causal.



To clarify this, consider the example of Hal's glucose
metabolism (Craver & Bechtel 2007: 559-560). Hal is playing
tennis, and as he keeps running around and swinging the racket, the
cells in Hal's body start taking in and using more glucose. It
seems that the cause of the increase in Hal's glucose metabolism
at the cellular level is Hal playing tennis, which suggests causation
from a higher level (the level of the whole organism playing tennis)
to a lower level (the cellular level). However, according to Craver
and Bechtel, this would violate one central assumption regarding
causation, namely that causes must be distinct from their effects, and
must occur prior to the effects. Instead, they see this as just
regular same-level causal relationships at the cellular level: Nerve
signals to the muscle cells cause a cascade of events that results in
increased glucose metabolism. Hal's tennis playing activity is
partly *constituted* by these cellular mechanisms, and this
constitutive relationship is responsible for the appearance of
downward causation. As Hal starts to play, there are changes in these
mechanisms, but only in virtue of them partly making up the tennis
playing activity.



This approach is appealing, but faces several challenges. First, it
has been argued that there are no clear methods of distinguishing
between causal and constitutive relationships in mechanisms (e.g.,
Leuridan 2011); if this is correct, there seems to be no reason to
posit that "mechanistically mediated" effects are
non-causal. Second, the idea of "normal" intralevel
causation is problematic in the framework of levels of mechanisms. As
we have seen in
 section 2.2,
 the idea of "being at the same level" has little if any
significance in this framework, so same-level causation cannot be the
default or normal type of causation. The idea seems to be rather that
there is no causation from a mechanism as a whole to the components of
that same mechanism, but this is just one very specific form of
downward causation, and how other forms should be treated is left open
in this account (Eronen 2013). Third, it is not clear whether
higher-level causal processes in mechanisms can be distinct from
causal processes at lower levels of the mechanism--if not, there
are no higher-level causal processes that could have mechanistically
mediated effects downwards in the mechanism to begin with (Fazekas
& Kertesz 2011; see also Fazekas 2022; Craver et al.
2021).



Although levels of mechanisms have been prominent in recent
philosophical discussions, a far more common way of approaching
downward causation in the biological literature has been to explicate
it in terms of higher-level *constraints* or boundary
conditions. The idea is that the behavior of lower-level things is
constrained by the higher-level whole that they are a part of, and
therefore explaining the behavior of these lower-level things is not
possible exclusively at the lower level, but requires appealing to
higher-level factors (e.g., Bishop 2008; Campbell 1974; Ellis 2008;
Emmeche et al. 1997; 2000; Green 2018; 2021; Kistler 2009; Noble 2012;
Pattee 1973; Sperry 1969). In a classic article, Donald Campbell
expressed this idea as follows: "processes at the lower levels
of a hierarchy are restrained by and act in conformity to the laws of
the higher level" (Campbell 1974: 180). For example, according
to Noble (2012), the higher-level cellular property of membrane
potential constrains the activity of ion channels at a lower level,
and this amounts to downward causation. Similarly, Green (2018; 2021)
argues that properties of tissue structure (e.g., resistance) set
boundary conditions for the propagation of action potentials,
resulting in a form of downward causation.



These cases do not seem to involve causation across compositional
levels, that is, causation from a whole to its own parts: for
instance, the membrane potential and the ion channels do not stand in
a part-whole relationship (Woodward 2021). Instead, the underlying
notion of levels seems more akin to scales (as is also explicitly
stated by Noble 2012 and Green 2021). More generally, Woodward (2021)
has argued that (downward) causation should not be seen as involving
things (such as wholes and their parts) as relata, but rather
variables, such as a variable representing the membrane potential, and
a variable representing whether an ion channel is open or closed. When
downward causation and levels are understood in this way, the worry
that the causes are not sufficiently distinct from their effects in
downward causation seems to dissolve, as the higher-level variables
(e.g., membrane potential at a higher scale) are clearly distinct from
the lower-level effect variables (e.g., the ion channels at a lower
scale; see Woodward 2021 for more).

 One unsettled question is
whether all putative cases of downward causation involve
*causation* in a substantial sense, or whether it would be more
appropriate to designate, for example, constraints or boundary
conditions as some form of downward determination or explanation (see,
e.g., Hulswit 2005). Another remaining problem is how to reconcile
downward causation with the much-debated *causal exclusion
argument* (e.g., Kim 1992; 1999; 2005). To put it very roughly,
the core of this argument is that if we accept the plausible
assumption that all physical occurrences must have sufficient physical
causes, it is not clear how there could be room for additional
higher-level causes of lower-level physical effects. Some philosophers
have argued that this problem arises even for the relatively weak and
minimalist interventionist theory of causation that is often assumed
in debates on downward causation (Baumgartner 2013; Gebharter 2017),
whereas others have defended the coherence of downward causation
(Eronen & Brooks 2014; Raatikainen 2010; Woodward 2015; 2021).

## 6. Levels in biological theory and usage



Besides the more philosophical debates discussed above, levels of
organization also play an important conceptual role in biological
research and theory. Interestingly, this growing body of literature on
levels in biology has (until recently) been almost entirely
disconnected from the debates on levels in philosophy of science
discussed above.


### 6.1 Levels and units of selection



In the debate on levels of selection (Lewontin 1970; Brandon 1982),
the hierarchical organization of nature into levels is an important
background assumption, as the aim is to find out at which level(s) of
the biological hierarchy natural selection is taking place (Griesemer
2001; Okasha 2006). Although Darwin's original account was
focused on evolution at the level of organisms, the conditions for
natural selection can be formulated abstractly without referring to
any specific kinds of entities, which allows for natural selection to
operate at any level where the conditions are satisfied (Griesemer
2001; Lewontin 1970). Since the 1970s, the debate on levels of
selection has kept on growing and extending to different areas, though
no precise consensus has been reached. Positions range from the
gene-centered view, where natural selection is taken to operate almost
exclusively at the level of genes (e.g., Dawkins 1976; Williams 1966),
to the pluralistic multilevel selection theory, which allows for
natural selection to operate on any level of the biological hierarchy
where we find the right kind of units (e.g., Sober & Wilson 1998;
Wilson & Wilson 2008). For more on the levels of selection debate,
see Okasha (2006) and the entry
 Units and Levels of Selection.



In this debate, the notion of levels of organization is typically used
as a background notion, the exact meaning of which is left implicit,
or illustrated with just a list of standard levels (Eronen &
Ramsey forthcoming; Griesemer 2005). A notable exception is Okasha
(2006), who puts forward a proposal for understanding levels of
organization in natural selection. Okasha considers the hierarchical
organization of nature to be one of the key factors that give rise to
the levels-of-selection problem, and takes as a starting point the
traditional picture where cells are nested within organs and tissues,
which in turn are nested within organisms, and at yet higher levels we
find groups, colonies, and so on (Okasha 2006: 10). However, Okasha
observes that this standard picture is inadequate, as it is not clear
which levels should be included in the context of levels of selection.
Therefore, following McShea (2001) and Sober and Wilson (1998), Okasha
argues that the entities at a level should interact in a way that
affects their individual fitness (usually defined in terms of the
number of offspring an individual produces). In other words,
*fitness-affecting interactions* among parts is taken as a
defining feature that distinguishes genuine levels from arbitrary or
nonrelevant sets of parts. In addition, according to Okasha we need to
rule out entities such as tissues and organs from genuine levels (of
selection). To this end, he proposes (referring to McShea 2001) that
entities at a level should be *homologous* with organisms in a
free-living state, either extant or extinct. For example, the cells
that compose organisms interact with each other and are homologous to
free-living unicellular organisms, and therefore constitute a level in
this account.



This proposal has been criticized for being too restrictive: (Eronen
& Ramsey forthcoming): Most importantly, if we include the
homology requirement, it is not clear how things like genes or groups
of organisms could be seen as levels of selection, as they do not seem
to be homologous with extant or extinct organisms. As an alternative,
Eronen and Ramsey propose to characterize levels of selection directly
in terms of fitness-affecting interactions (or some notion that plays
a similar role, such as evolutionary individuality): A level is formed
by any entities that are sufficiently similar and engage in
fitness-affecting interactions. This results in a very deflationary,
case-specific and localized conception of levels of selection, and
conceptually detaches levels of selection from the broader idea of
levels of organization. In general, the relationship between levels of
selection and levels of organization remains a topic that requires
further philosophical attention (see also
 section 6.5).


### 6.2 The Hierarchy Theory of Evolution



A different approach to the interplay of levels and evolution is
provided by the "hierarchy theory of evolution" developed
by Niles Eldredge and colleagues (Eldredge 1985; Eldredge &
Salthe, 1984; Eldredge et al. 2016; Salthe 1985; Vrba & Eldredge
1984; see also Eldredge et al. 2016). In this theoretical framework,
levels and hierarchies are taken synonymously to be fundamentally
important ontological features of nature:



> 
> 
> Biological evolutionary theory is ontologically committed to the
> existence of nested hierarchies in nature and attempts to explain
> natural phenomena as a product of complex dynamics of real
> hierarchical systems. (Temkin & Eldredge 2015: 184)
> 
> 
> 



In the hierarchy theory of evolution, a distinction is made between
two types of hierarchies and the corresponding levels (Eldredge 1996;
Vrba & Eldredge 1984): The *ecological* and the
*genealogical* hierarchy. In both kinds of hierarchies,
higher-level things are formed through specific interactions among
lower-level things. In the ecological hierarchy, these interactions
are exchanges of matter and energy, such as consuming and gathering
resources, and in this hierarchy, we find things such as cells,
organisms, and ecosystems. In the genealogical hierarchy, the defining
activity through which levels are formed is the transmission of
information through replication, and the hierarchy includes things
such as cells, organisms, demes and species. (Note that cells and
organisms appear in both hierarchies, but in the ecological hierarchy
they are seen as interactors, whereas in the genealogical hierarchy
they are seen as replicators.) Work on this hierarchy theory of
evolution is ongoing (see, e.g., Eldredge et al. 2016; Temkin
2021).


### 6.3 Evolutionary transitions



The issue of evolutionary transitions is closely connected to the
debates on levels of selection and levels of organization. Here the
focus is on the emergence of *new* levels of organization
through evolutionary processes (Buss 1987: ch. 5; Griesemer 2001;
Maynard Smith and Szathmary 1995; Okasha 2006). The background
idea is that the complex hierarchical organization of nature that we
observe today must itself be a result of evolution, and therefore
requires an evolutionary explanation. For example, somehow prokaryotes
evolved to eukaryotic cells, single-celled organisms evolved to
multicellular organisms, individual animals evolved to colonies, and
so on (see also
 section 6.5).
 In their highly influential book, Maynard Smith and Szathmary
(1995) proposed that the characteristic feature of major evolutionary
transitions is that entities that were capable of replicating
independently before the transitions are only capable of replicating
as parts of higher-level wholes after the transitions (see also Buss
1987). For example, after a single-celled organism has evolved into a
multicellular organism, the cells of the organism can no longer
replicate independently of the organism as a whole.



Detailed models of these processes of evolutionary transitions have
been developed, such as the evolutionary transitions of individuality
(ETI) account of Michod (1999; 2005). The basic idea of this model is
that evolutionary transitions involve a decline in the fitness of
individuals (e.g., cells as they engage in cooperative behavior) and a
corresponding increase in the fitness of the collectives that they
form (e.g., multicellular organisms). This eventually results in
fitness being completely transferred to the collectives, which thereby
form a new level of organization (see also Bourrat 2015 for criticism
and discussion).


### 6.4 The evolution of complexity



Levels of organization are also invariably tied to discussions of
complexity in biology. One of the most prominent appearances of levels
in the investigation of complexity occurs in observations of the
apparent increase in structural hierarchies in organisms over
evolutionary time. This increase of "nestedness", where
parts become located within other systems, appears to be a clear trend
in the history of life (McShea 1996; 2001). Daniel McShea (2001)
points to three examples of this trend, including the origin of
eukaryotic cells from prokaryotes via endosymbiosis, the emergence of
multicellular organisms, and the appearance of colonies formed by
multicellular individuals. Yet it is not clear what precise
relationship exists between levels of organization and complexity, and
the explication of complexity remains an important task in its own
right. On this point, though there exists no consensus among
biologists, the two clearly inform each other.



Beginning with complexity, several measures for complexity have been
proposed, many of which are intimately tied with the idea of levels.
For instance, John Tyler Bonner (1988: 220) draws attention to the
relations of size and complexity, identifying a trend in the upper
limit of body size and an increase in complexity. This overall trend
accompanies both actual increases as well as decreases in body sizes
over evolutionary time. Bonner entertains two measures of complexity,
including (1) the correlation of body size and number of cell types,
and (2) the relation between genome size and number of cell types.
Although he (and later researchers) concluded that there is no clear
pattern indicating that the increase in the number of genes in a
genome is accompanied by an increase in cell types, the former
correlation appears to recover robust inference patterns, and has
become a mainstay of informing uses of complexity (Valentine, Collins,
& Meyer 1994; Valentine 2002; McShea 2021). Another proposed
measure of complexity, offered by Hinegardner, and Engleberg (1983),
characterizes the notion in terms of the minimal description necessary
to capture the system's behavior. These two approaches are
related (Valentine, Collins, & Meyer 1994), though have not been
systematically integrated. Finally, McShea (2021) summarizes and
contrasts "horizontal" complexity, understood
approximately as the number of cell types, and "vertical"
complexity, which refers to the number of hierarchical partitionings
(understood as nestedness) that are found within a system.



These characterizations of complexity and their relations to levels,
like many discussions concerning the latter, leave much room for
continued discussion and further scholarship. For one thing, though it
appears clear that complexity and levels remain intimately related,
the authors summarized above are not without discrepancies regarding
the importance, or even place, of levels of organization in talking
about complexity. For example, Bonner (1988) sets out his
investigation by noting that it is not levels *per se* that is
of interest but rather how levels themselves come into existence.
Relatedly, McShea (2001) notes that levels of organization are a
"secondary theme" (ibid., 406) to notions of nested
hierarchies in biology and measuring complexity.


### 6.5 Levels and experimental biological research



Although the levels concept may at first glance appear an abstract
idea poised orthogonally to empirical matters (or interpretations
thereof), the constituents of nature that the concept orders also
comprise targets of experimental investigation. In this way, the
levels concept can be conceptualized not only as local maxima of
regularity and predictability, but also as local maxima of
observability and manipulability. Here we describe several
experimental practices where levels play an important role, focusing
on observation and manipulation.



Tracking the various objects and processes occurring at different
scales of nature is a centerpiece to conducting observations and
observational studies in biology. That these constituents then also
frequently make up central units of study for biology (molecules,
cells, tissues, organs, and so on) is one notable element of the
levels concept's basic expression (Wimsatt 1994; 2021). The
systematicity of these constituents' relationships is an
especially noteworthy consequence of reasoning with the levels
concept: When we shift between scale resolutions (be it temporal or
spatial), different units of study can be identified and investigated.
Oftentimes, the units generically involved in an area of research
(molecules, cells, tissues) are basically known, but their specific
dynamics comprise targets for further study and characterization
(Brooks 2021b).



In addition to observational techniques, experimental manipulation of
different scale-bound units is another hallmark of daily practices in
biological research. Here the levels concept contributes to our basic
reasoning of these practices by aiding our efforts in testing
hypotheses and building explanations. In this vein, Alan Love (2021)
argues that applying the levels concept is not merely a matter of
abstract representation, but also of articulating well-defined
experimental targets in laboratory settings. Love considers examples
such as mixed cell aggregates, a well-established and classical
experimental technique where tissues are first physically separated
into their constituent cells, mixed together in a medium, and then
segregate and regrow into tissue aggregates of similar cell-type (see
especially Moscona 1959). Such targets of manipulation, Love reasons,
align with the explicit aims of researching biologists to describe,
predict, and explain biological phenomena such as morphogenesis (and
biological development in general; see Love 2021: 139).



As the example of mixed cell aggregates implies, observation and
manipulation co-occur in experimental settings: Tissues (higher level)
are physically separated into individual cells (lower level) and
placed in a medium (that is, are manipulated), and their subsequent
behavior is observed and measured. Such transitions to and from higher
and lower levels demonstrate the structuring capacities of the levels
concept for interpreting the significance of experimental efforts.
Consider another example from experimental evolution (for a primer
see, e.g., Kawecki et al. 2012), where the transition from cellular to
multicellular individuals is experimentally induced in different
species of unicellular life. In one experiment, Ratcliff, Denison, et
al. (2012) induced multicellular snowflake-like clusters in the yeast
*Saccharomyces cerevisiae* using a simple selection regime
(i.e., gravity; multicellular clusters sink more quickly in a medium,
and were collected after various intervals and placed in a new
medium). These clusters began reproducing via post-division adhesion
(rather than aggregation) across several generations, thus creating
its own novel life history (reproducing uniclonal clusters rather than
aggregates containing individual cell genomes), developing a division
of labor among cell constituents (see also Simpson 2012), and even
exhibiting regulation of cluster phenotype behavior, seen in the
emergence of programmed cell death in individual cells unable to
contribute to the supracellular individual (see also Ratcliff,
Fankhauser, et al. 2015; Herron et al. 2019).



Focusing on the practical and experimental significance of levels of
organization also stands to contribute to the philosophical discourse
on levels more generally. Love (2021), for instance, argues that the
practices of working biologists in manipulating levels warrant an
inference from successful practice to scientific metaphysics, thereby
supporting a "modest realism" regarding levels of
organization.


### 6.6 Levels and theory building in biology



The insights and hypotheses based on leveled thinking, in combination
with manipulations and observations performed in the lab, provide one
instance of synthesis between conceptual and practical matters of
investigation. This synthesis often results in conceptually guided and
empirically driven theory. Explicitly attending to different levels
and the distinct scale-relative dynamics supports hypothesis and
theory building by making intelligible the scale-bound practices of
observation and motivating manipulation of well-defined targets in
manifestly complex systems.



Instances of such synthesis are widely available in contemporary and
twentieth century biological science. For example, conceptualizing
cancer as primarily a tissue-level disease (albeit with multiple
levels playing roles) has long been closely connected to interpreting
and reinterpreting laboratory findings based on a leveled conceptual
landscape. Already in the 1930s, organicist biologists were
hypothesizing that cancer be understood as deviations in tissue
competence that disrupt maintenance of healthy tissue, enabling
uncontrolled growth (Waddington 1935; Needham 1936a; see also
Abercrombie, Heaysman, & Karthauser 1957; Sonnenschein & Soto
1998). More recent conceptually guided work on cancer has reiterated
this insight, with many postulating that cancerous growth comprises a
reversal from the multicellular state to the unicellular state,
amounting essentially to a dissolution of a tissue by disrupting
tissue-level forces (Chen et al. 2015; see also Trigos et al. 2018).
This transition from multicellular to unicellular directly mirrors the
experimentally induced emergence of multicellular individuals
described in the previous section, a pattern noted by the authors
above in the case of the snowflake yeast clusters (Ratcliff, Denison,
et al. 2012).



Another example of synthetic theory involving levels more generally
concerns the dynamics of collective behaviors in tissue. Busby and
Steventon (2021) argue that "tissue tectonics", that is,
tissue-level mechanical forces and associated timing, provide a
mechanism for coordinating morphogenetic events across multiple levels
of organization (see also Xi et al. 2019; Zinner, Lukonin, &
Liberali, 2020). One striking example of tissue tectonics concerns the
sliding of tissue sheets containing populations of cells within the
respective sheets, which allows intercellular signaling to occur
between these populations, thus enabling a crucial timing event to
induce changes in cell states. One key feature of such examples is the
preponderance of claims that causality free moves between levels via
concrete interactions, both in "upward" and
"downward" fashions (Busby & Steventon 2021; Zinner,
Lukonin, & Liberali 2020; see also
 section 5).


### 6.7 Usage of levels in the education of biology



The concept of levels of organization plays a prominent role as a
pedagogical resource in the education of biology. Beginning with the
notion's ubiquitous presence in textbooks and other introductory
statements in biology (see Schneeweiss & Gropengiesser
2019 for a review; examples include, e.g., Begon, Townshend &
Harper 2006; Lobo 2008; Mader 2010; Urry et al. 2016; O'Neill et
al. 1986), depictions of levels often dominate the first few pages of
major biology textbooks with large centerfold illustrations stretching
across whole pages. In this capacity, the levels concept serves as an
organizational principle that communicates basic themes of the study
of biology to aspiring scientists early in their training (Brooks
2023).



![Box with blue onion cells labelled: Organelles: The nucleus, dyed blue in these onion cells, is an example of an organell; arrow to box with b/w pic of toroidal shapes labelled: Cells: Human blood cells; arrow to box with cut-away pic of human skin showing many layers labelled: Tissues: Human skin tissue; arrow to box with cut-away pic of cartoon human with orange line from mouth to stomach, yellow stomach, orange small intestines, green colon labelled: Organs and Organ Systems: Organs, such as the stomach and intestine, make up the human digestive system; arrow to box with pic of forest labelled: Organisms, Populations, and Communities: In a forest, each pine tree is an organism. Together all the pine trees make up a population. All the plant and animal species in the forsest comprise a community; arrow to box with pic of forest by the shore of a body of water labelled: Ecosystems: This coastal ecosystem in the southeastern United States includes living organisms and the environment in which they live; arrow to box with world map in an oval labelled: The Biosphere: Encompasses all the ecosystems on Earth.](bio-levels-org.png)

Figure 2. Diagram of levels of
organization from *General Biology* (Boundless),
 SS1.8;
 CC BY-SA 4.0




Some of the usage patterns one finds in support of these basic themes
within contemporary textbooks include, e.g., (i) the apparent
preponderance of emergence in living systems, (ii) the need to be
aware of the relative success of reductive and non-reductive,
"systemic" approaches to analyzing biological phenomena,
and (iii) the need to actively embed choices of investigation within
appropriate methods that are more-or-less useful at different scale
resolutions. These uses, present already in early historical textbook
references to levels, in turn has been offered as a partial
explanation for the rapid uptake of the concept in the textbook
literature in the mid-twentieth century.



The efforts of Eugene P. Odum (see especially Odum 1959; 1963; cf.
Odum 1953) in particular imbued the levels concept with a fundamental
character attached to the study of the life sciences. In his two
successful series of textbooks *Fundamentals of Ecology* (five
editions) and *Ecology* (two editions), the levels concept was
foundational to the structure of the books themselves, following the
different scale-bound units of ecology. This "levels
approach" in turn was applied as a means of teaching students to
search for "distributive adequacy" in the investigative
practices of scientists (Brooks 2023). That is, Odum presented one
core lesson of the levels concept to be that no one level was
privileged over others in delivering the "right" answer to
a given scientific question. Instead, investigating multiple levels
would be needed to adequately gather results that would satisfactorily
answer scientific questions.



![Rectangle with 'matter' inside on the left and 'energy' inside on the right, connected by wavy lines; from left to right, outside the box, there are the following labels connected to the rectangle: 'genes'/'genetic systems', 'cells'/'cell systems', 'organs'/'organ systems', 'organisms'/'organismic systems', 'populations'/'population systems', 'communities'/'ecosystems'; all of this is labelled 'Biosphere' and there is an underbracket covering organismic systems, population systems, and ecosystems that is labelled 'area of major ecological concern'](odum-levels-spectrum.svg)

Figure 3. Levels of organization diagram
redrawn from Odum 1971, 5.




To be sure, while scientific textbooks often only allude to
cutting-edge science, preferring to display established knowledge
accessible to novices, this already enables viewing the levels concept
as exhibiting usefulness for later stages of scientific training,
particularly when seen as a scientific doctrine (see Brooks 2021a:
47-50, for a discussion). Specifically, and especially when
viewed with the motivation to teach students "how to think like
a biologist" (Urry et al. 2016), the centrality of the levels
concept in textbooks can be seen as representative of the expressive
power of the concept in reconstructing scientific reasoning.



In addition to active usage as an organizing principle in textbooks,
the levels concept has also attracted increasing attention from
scientific educators for improving and innovating new learning and
teaching strategies in biology. Noting the concept "as central
to the study and practice of science" (1999: 3), Wilensky and
Resnik argue that focusing on the levels concept as a pedagogical
device is "critically important to the understanding of many
scientific phenomena and many foundational philosophical
questions" (1999: 17; see also Jordens et al. 2016). More
recently, Knippels and Waarlo (2018) have reviewed the implementation
of the so-called "yo-yo strategy" in biology classrooms as
one explicit means of applying the levels concept to introduce and
"promote coherent conceptual understanding of various biological
phenomena" (Knippels & Waarlo 2018: 1). The yo-yo strategy,
characterized as a "heuristic of systems thinking"
(ibid.), was explicitly developed to aid grasping complicated
biological phenomena (such as Mendelian genetics or biological issues
associated with understanding sickle cell anemia), for which more
linear forms of learning face difficulties in securing student
comprehension (see Knippels & Waarlo 2018 for a discussion). This
strategy works by first localizing the contributions of different
leveled components to primary units of the system (e.g., organism,
cell, molecule), and then shifting "up and down" between
these units to gather insight into the specific contributions that
each unit expresses for more general biological lessons. A related
approach to implementing levels into active learning strategies has
been developed by Schneeweiss and Gropengiesser (2022; see
also Schneeweiss, Molgen, & Gropengiesser 2022).
Their "zoom map" encourages students to not only explore
different levels of organization in the construction of explanations,
they also place a marked emphasis on interacting with the units
perched at various levels to encourage learning how different levels
contribute to answering different questions posed of the same
phenomenon.


## 7. Concluding remarks



Although 'levels of organization' has been a key concept
in biology and its philosophy since the early twentieth century, there
is still no consensus on the nature and significance of the concept.
In different areas of philosophy and biology, we find strongly varying
ideas of levels, and none of the accounts put forward has received
wide acceptance. However, emerging work on levels of organization
suggests that the insights garnered from the concept are far from
exhausted.