Ontology rests on the idea that reality is not perfectly uniform – that it has discontinuities. To develop a sense of these “contours”, hierarchy theory provides a crude and attractive account that makes for a good beginning. It may seem unscientific, but it efficiently summarizes many of the concepts that scientists shamelessly and consistently use in their thoughts and writings, and form a kind of tacit metaphysical schema without which science would fall apart. We may therefore indulge in it with good conscience.
Hierarchy theory pictured our reality as one bathing in a primeval soup of pixels, structure-less at its finest scales, but rippled by the occasional whirl of activity resilient enough to persist over time. Such resilience sometimes emerges when elements interact with each other at a much higher rate than with others, and, given appropriately tuned observer criteria, it translates to a discontinuity in the fabric of reality. It is helpful to think of such contained dynamic as a “system” – an entity with a well-defined boundary – but dangerous to forget that reality is nowhere near as neatly partitioned as we believe it to be.
When one whirl encounters another one, they threaten to dissolve each other and to both be engulfed by the surrounding chaos. To avoid this, they agree to modify each other, to adapt, in a bid to retain at least some semblance of their former selves. As a consequence of this co-evolutionary process of mutual modification (called “complicity” by some) they merge into a super-whirl that from future encounters can proceed to build recursively upon itself. The previous system boundaries are not erased, but fossilized in how the super-whirl is recognizably multi-levelled. We may dub this tendency “arborization”, and the horizontal interaction across such systems “reticulation”. And so what crystallizes in this arborizing brew, when left to seethe, is an underwater coral reef of interlocking hierarchies, not entirely decomposable (but nearly so) and teeming with desire to self-complicate
An important feature of this imagery is that any discontinuity, any “system”, is simultaneously a part and a whole – equally dependent participants in a higher-order relation and self-contained entities themselves. Arthur Koestler, in his classic work “The Ghost in the Machine”, called these two-faced nodes of near-decomposable hierarchies “holons”. Through a parts-within-parts architecture, the superordinate holon imposes negative constraints on behavior, while the activity of subordinate holons drives positive self-assertive behavior. The result is a kind of polarity between centripetal and centrifugal tendencies that underpins the coordination into complex action patterns.
Through their history of mutual modification, each holon has evolved an interface that specifies what behavior will follow a certain kind of interaction. We may view this as signals instructing a slot machine to respond in a certain manner, or as a rule determining whether an event will take place, which evolutionary feedback has made sure sustains both superordinate and subordinate holons. Like a combination of locks in descending order, an input signal in the first holon triggers an input signal to an internal holon, thus unleashing a cascade of sub-unit activation down to the simplest parts, from which intelligent and adaptive behavior emerges.
The most obvious examples of hierarchy are those that are structurally nested, with small entities that, to reap the benefits of symbiosis, join others into larger conglomerations. For example, according to the endosymbiotic theory, organelles like mitochondria were originally free-living bacteria that combined with other prokaryotic cells to form eukaryotic cells. Another example is how anthropologists view humans to have undergone three distinct stages of organizational complexity, from hunter-gatherer bands, via chiefdoms, to states. Nested hierarchies therefore evolve from specific to general.
But hierarchies need not be nested and system boundaries may be more subtle than cell membranes and house walls: in what is called the “multiplier effect”, a general system evolves into a more specialized one. For example, a biochemical cycle may use one single enzyme (rule) to accelerate the conversion of a protein (signal) based on whether it fits the active site (signal tag). It is very slow, so it starts experimenting with parallel, intermediate stages in which one enzyme makes a few changes, then ensures that the product fits the active site of another enzyme (etc.). An analogous example is the drive for specialization in economic markets. The root node of the hierarchy may be that “humans need shoes”, but production becomes more efficient if it is decomposed into “sole-maker 1 needs leather from producer 1 and cloth from producer 2” and “lace-maker 2 needs thread from producer 3 and plastic from producer 4”. The shoemaker needs then only deal with the sole-maker and the lace-maker, and this arrangement lowers production costs for everyone involved.
Through the lens of hierarchy theory, what distinguishes the inert and inanimate from the living and self-propelled is their hierarchical depth – the extent to which they have managed to adapt to challenges posed by other systems, and thus increased its complexity. An atom or a crystal lattice have relatively low depth, and accomplish nothing impressive singlehandedly as a result. Meanwhile, living organisms, the paragons of complexity, exhibit baffling degrees of freedom. This complexity is a result of the replication of DNA and associated machinery, which guarantee that a “sameness” is maintained over time, since by virtue of being imperfect replicators, they agree to incorporate random errors via natural selection. The key to system longevity and complexity thus is adaptation. But the most important insight carried by hierarchy theory is that the continuum between simple systems and complex systems – between rocks and organisms – makes ontology applicable to them all. Each holon can be said to have its own ontology, its own set of “real” categories!