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The word hierarchy stems from the Greek word hierarches, and early usage referred primarily to ecclesiastical structure and authority. The term is now widely used in a number of fields and generally denotes an inter-level relationship, usually conceived as a vertical layering of levels that implies higher value, power, or centralization at the top, and less of these qualities at the bottom.

History of the concept

The ancient Greek philosopher Plato has had an enormous influence on hierarchical thinking. In works such as the Republic and Phaedo, Plato argued that the world is divided into a lower, chaotic material reality, and a higher reality of forms that is the genuine source of truth, beauty, and the good. For Plato, this ontological distinction was necessarily related to epistemological and moral ones, for the realm of the forms are the source of true knowledge as well as being the ultimate good that all seek. Human beings were seen as a composite of the two worlds, the irrational world of matter and the rational world of the forms. In Plato's framework, the good person is one who shuns material things and pursues rational inquiry in accordance with one's true, nonmaterial nature.

During the Roman era, Plotinus (205–270) and other neo-Platonists expanded Plato's dualism into what twentieth-century philosopher Arthur Lovejoy (1873–1962) called the great chain of being. According to this view, God is the most real, out of which all other things emanate. Material reality is that which is most distant from the plenitude of God and, in a sense, the least real. As a composite of the different levels of reality, human beings stand at a halfway point, both material and spiritual. Neo-Platonism profoundly influenced the development of Christian theology, particularly through the writings of Augustine of Hippo (354–430), Pseudo-Dionysus (c. fifth century C.E.), and Bonaventure (1217–1274). In a Christian framework, angels naturally fit into a neo-Platonic framework as beings who occupied a higher level of reality. For Augustine in particular, evil could be explained as the absence of good, an irrational move from the most real (God) towards the unreal.

The rise of modern science played a significant role in the demise of hierarchical understandings of the world. Early scientific thinkers were influenced by philosophers such as William of Ockham (c. 1285–c. 1347), who denied Plato's theory of forms and hierarchical ontologies. This and other factors led to an understanding of the physical world that emphasized material causes alone, a tendency that seemed vindicated by the work of Galileo Galilei (1564–1642) and Isaac Newton (1642–1727). Such materialistic views were typically reductionistic in character. Materialist reductionists inverted and then rejected the neo-Platonic hierarchy of being, claiming not only that it is the material world that is most real, it is the only reality. Such materialism not only influenced scientists such as Pierre-Simon Laplace (1749–1827), but also the whole trajectory of nineteenth-century philosophy.

In the twentieth century, the legitimacy of ontological and moral hierarchies was intensely debated within specific fields of philosophy and theology. Debates about ontological hierarchies focused on questions of reductionism and emergence or holism, much of which centered on the status of the mind and human person. Reductionists emphasize that the material constituents of the world are all that there is, and that higher-order realities such as the human mind and culture can ultimately be explained by the laws of physics and chemistry. Reductionists often point to the success of neo-Darwinism and the discovery of DNA as justification for their approach. Likewise, categories of mind and the human person, so reductionists argue, can best be understood in terms of the activities of the brain. In the late twentieth century, reductionism was most associated with the popular writings of Richard Dawkins and Francis Crick in biology and the thought of Daniel Dennett, Paul Churchland, and Patricia Churchland in the philosophy of mind.

Modern opposition to reductionism has early roots in the movement of British emergentism, typified by the work of C. D. Broad. Opponents to reductionism have frequently endorsed the category of emergence, arguing that there are higher-order levels that emerge from, but are not reducible to, the lower levels of reality. Generally speaking, emergentists do not deny the validity of the lower-level sciences, only their sufficiency for explaining higher-order phenomenon. Emergentism came to be particularly important for the defense of biology as a legitimate and separate field of inquiry from physics and chemistry, and has been vigorously supported by such prominent thinkers as biologist Ernst Mayr and philosopher Karl Popper. Emergence has also been complemented by the concept of supervenience, which provides a philosophical framework for understanding the relation of different levels of reality. Philosophers such as Jaegwon Kim have argued, however, that supervenience ultimately leads to causal reduction of higher-level to lower-level physical properties. Within the paradigm of computational complexity theory, a similar suspicion has been raised against emergence by John Holland and others.

Hierarchy in the science-religion dialogue

Science and religion scholars have tended to support emergentist positions and reject reductionist ontologies. Both the physicist and theologian Ian Barbour and the biochemist and theologian Arthur Peacocke have strongly criticized reductionist interpretations of science. Both have noted that while science employs methodological reductionism in its attempt to analyze physical reality, such practice does not entail ontological reductionism. Going a step further, Peacocke has argued that the whole of reality should be understood as a complex hierarchy that begins at the bottom with physics and chemistry, and moves towards increasing levels of complexity, moving towards living organisms, human beings, cultures, and eventually God at the very top. Peacocke's analysis has had tremendous influence, and has been developed in different ways by philosophers Nancey Murphy and Philip Clayton.

Despite this, the value of hierarchical thinking has been much questioned in broader theological circles. Feminist theologians such as Sallie McFague have criticized traditional moral hierarchies because of their tendency to oppress women. Environmental theologians and philosophers have also criticized moral hierarchies as contributors to abuse of animals and destruction of ecosystems. Because traditional moral hierarchies have been justified by reference to ontological hierarchies, these too have come under attack. Serious dialogue between these differing theological perspectives has yet to occur and represents a likely step in the science-religion dialogue.

See also EMERGENCE; HOLISM; ORDER; PLATO; SUPERVENIENCE

GREGORY R. PETERSON