The world appears to contain diverse kinds of objects and systems—planets, tornadoes, trees, ant colonies, and human persons, to name but a few—characterized by distinctive features and behaviors. This casual impression is deepened by the success of the special sciences, with their distinctive taxonomies and laws characterizing astronomical, meteorological, chemical, botanical, biological, and psychological processes, among others. But there’s a twist, for part of the success of the special sciences reflects an effective consensus that the features of the composed entities they treat do not “float free” of features and configurations of their components, but are rather in some way(s) dependent on them.Consider, for example, a tornado. At any moment, a tornado depends for its existence on dust and debris, and ultimately on whatever micro-entities compose it; and its properties and behaviors likewise depend, one way or another, on the properties and interacting behaviors of its fundamental components. Yet the tornado’s identity does not depend on any specific composing micro-entity or configuration, and its features and behaviors appear to differ in kind from those of its most basic constituents, as is reflected in the fact that one can have a rather good understanding of how tornadoes work while being entirely ignorant of particle physics. The point generalizes to more complex and longer-lived entities, including plants and animals, economies and ecologies, and myriad other individuals and systems studied in the special sciences: such entities appear to depend in various important respects on their components, while nonetheless belonging to distinctive taxonomies and exhibiting autonomous properties and behaviors, as reflected in their governing special science laws. (The point might be generalized yet further to include human artifacts which are not the object of any natural science, but whose conditions of individuation are tied to human language and practice. But artifacts are set aside in this entry, as these raise distinctive issues that are discussed in the entry on [material constitution](https://plato.stanford.edu/entries/material-constitution/). Whether there are composites that are neither artifactual nor amenable to scientific analysis is controversial, and if there are, they plausibly will not meet candidate autonomy conditions on emergence. But this will not be explored further here.)The general notion of _emergence_ is meant to conjoin these twin characteristics of dependence and autonomy. It mediates between extreme forms of _dualism_ , which reject the micro-dependence of some entities, and _reductionism_ , which rejects macro-autonomy.* * *## 1. IntroductionAlthough debates concerning the reality or precise nature of emergence are largely driven by contemporary scientific theorizing, the basic notion has quite a long history stretching back at least to Aristotle (384–322 BC). On Aristotle’s view, human beings, like other “secondary” substances, arise from a distinctive arrangement of the four material elements. While the mental powers of human beings require and are necessitated by such an arrangement, these powers are distinct from, and downwardly causally efficacious with respect to, any non-mental powers. Furthermore, Aristotle’s form/matter compound conception of material substances is consonant with the standard emergentist stance between substance dualism and reductionism. For detailed discussion, see Caston (1997). Among Peripatetic philosophers, Alexander of Aphrodisias (late 2nd–early 3rd century AD; in _On the Soul_) and Galen (129–c.200; in _On the Elements According to Hippocrates_ and elsewhere) develop distinctive versions of Aristotle’s basic emergentist picture to apply to chemical compounds and other non-living phenomena as well as living beings. (For discussion of several texts, see again Caston 1997: 347–353.)Aristotle’s philosophy of nature is re-appropriated in the medieval era, first by Persian and Arabic philosophers such as Avicenna/Ibn Sina (980–1037) and then, with the translation of many of Aristotle’s key texts into Latin, by philosophers in the West, including Thomas Aquinas (1225–1274). In the Latin West, Aristotelian ideas, once introduced, become pervasive throughout the era. In particular, Aristotle’s matter/form conception of substance and his concomitant rejection of atomism become axiomatic points of departure in theorizing about the nature of particular kinds of bodies and the distinctive kinds of processes associated with them. (On Aquinas, see Pasnau 2001 and Stump 2003, Part II; Pasnau 2011 examines in detail a wide range of views of substance in the post-Aquinas scholastic period.)The consensus around the Aristotelian philosophy of nature was dismantled by the Scientific Revolution, with Aristotle’s physics being the first casualty. René Descartes advances an austerely mechanistic and reductionist conception of material bodies, and this broad outlook becomes widespread. However, Descartes argues that the human mind or soul is a non-material substance, and so endorses a substantial form of mind-body dualism. For those also accepting the reductionist conception of the physical world, the alternatives to Descartes’ substance dualism are stark: idealism (on which matter is a mere “phenomenon” to be analyzed in terms of sensations, as advocated by George Berkeley) or reductionist materialism, as exemplified by Julien de la Mettrie’s _L’homme Machine_ (_Man the Machine_ , 1747).This menu of options is rejected in the nineteenth century by the so-called British Emergentists (with Lewes [1875] first using the term “emergence” for the philosophical position). In “On the Composition of Causes” (_A System of Logic_ , 1843: Ch. 6), [John Stuart Mill (see entry)](https://plato.stanford.edu/entries/mill/) argues that the behavior of living beings involves a failure of aggregativity or linearity of influence among their elements. He proposes an account that distinguishes “homopathic” and “heteropathic” laws and effects involving organized phenomena, maintaining that the latter laws (governing emergent phenomena) supplement without supplanting basic physical laws of more general scope. [Samuel Alexander (see entry)](https://plato.stanford.edu/entries/alexander/) suggested that life was a “new quality” that emerges from physico-chemical processes and brings with it “special laws of behavior” and which must “be accepted with the ‘natural piety’ of the investigator. It admits no explanation” (1920: vol.2, 46–47). Even so, he insists that he is endorsing “a species of the identity doctrine” (1920: 9), which suggests that he is seeking to articulate a more intimate relationship between levels of the natural world, perhaps very much akin to weak emergence accounts discussed in [section 3](https://plato.stanford.edu/entries/properties-emergent/#WeakEmer) below.Finally, British Emergentism reaches its most developed form in [C. D. Broad’s (see entry)](https://plato.stanford.edu/entries/broad/) _The Mind and Its Place in Nature_ (1925). Broad uses an epistemological criterion for what he intends to be a metaphysical condition of emergent autonomy: