Belief in causality is deeply held by many philosophers and scientists. Many say it is the basis for all thought and knowledge of the external world. The idea that every event has a cause leads to the view of causal closure or causal determinism. The simplest form of physical determinism is the Laplacian view that given the positions and velocities of all the fundamental particles in the world, together with the laws of nature, that there is only one possible future.

Reductionism is the view that material particles and the physical forces between them, which are supposed to be mathematically analytical, can explain all that happens in the world. Chemistry is thought to be reducible to physics, biology reducible to chemistry, psychology (via neuroscience) reducible to biology, and mind/brain (or cognitive science) reducible to psychology. All these causal relations are called "bottom-up."

The finest details of brain events are thought by some to be a consequence of motions of the material particles that comprise the brain. Reductionism implies that mind is an epiphenomenon, or worse, just an illusion. The reductionist idea that everything is the consequence of "bottom up" physical causes is often called eliminative materialism.

By contrast, downward causation is a kind of holism that denies reductionism. "Wholes" can enforce constraints on their "parts" to make them move in ways that may be unpredictable, even given the complete information about the parts (ultimately the atoms and molecules) along with the complete information about the state of the universe outside those parts.

Downward causation is closely related to the concepts of emergence, self-organization, and supervenience. It has become very popular in the study of complex physical systems which exhibit a kind of self-organization and emergence of visible structures when the systems are far from equilibrium conditions.

Most modern discussions of emergence and self-organization in hierarchical systems start with the early 1950's work of Ilya Prigogine on dissipative structures, physical and chemical systems that are far from equilibrium, through which there is a steady flow of matter and energy.

Despite the normal tendency to chaos (the second law of thermodynamics and increasing entropy), these dissipative systems develop relatively stable visible structures, such as Bénard convection cells and Turing autocatalytic reactions that show space-dependent, steady-state processes stable against perturbations. These visible structures reduce the entropy locally. Prigogine's discovery of such "order out of chaos" in physical systems is widely cited as evidence of emergent properties in complex adaptive systems. It lies at the heart of modern complexity theory and chaos theory.

The idea that the emergent structures exert downward causal control on their molecular components was perhaps first articulated by Roger Sperry in 1965. Sperry cites a wheel rolling downhill as an example of downward causal control. The atoms and molecules are caught up and overpowered by the higher properties of the whole. He says that he "worked the new mind-brain ideas into a discussion of holist-reductionist issues, emergent downward control. and ‘nothing but’ fallacies in human value systems, in a broad refutation of the then prevalent ’mechanistic, materialistic, behavioristic, fatalistic, reductionistic view of the 'nature of mind and psyche’."

But Sperry's downward control over atoms is a gross and "dumb" sort of control. What British emergentists were looking for, according to Brian McLaughlin, is a fine and "intelligent" control over individual atoms and molecules of the kind seen in biological systems, which Erwin Schrödinger called "order out of order."

In 1974 Donald Campbell coined the phrase "downward causation" and Campbell is widely cited in the current literature as the main source of the idea.

Some biologists (e.g., Ernst Mayr) have argued that biology is not reducible to physics and chemistry, although it is completely consistent with the laws of physics. Even the apparent violation of the second law of thermodynamics has been explained because living beings are open systems exchanging matter, energy, and especially information with their environment. In particular, biological systems have a history that physical systems do not, they store knowledge that allows them to be cognitive systems, and they process information at a very fine (atomic/molecular) level.

Information is neither matter nor energy, but it needs matter for its embodiment and energy for its communication.

A living being is a form through which passes a flow of matter and energy (with low or "negative" entropy, the physical equivalent of information). Genetic information is used to build the information-rich matter into an overall information structure that contains a very large number of hierarchically organized information structures. Emergent higher levels exert downward causation on the contents of the lower levels.

The problem of mental causation is a specific case of downward causal control that is central to the philosophy of mind.

The idea that minds have powers "over and above" the known physical, chemical, and biological laws is sometimes called "mentalism." It is related to the idea of "vitalism," that biology might involve new laws that cannot be reduced to "nothing but" the laws of physics.

But when biologists look closely at the "vital" processes going on in our cells, they see nothing but information processing and "self-assembly" going on at a rate that makes our best computers and manufacturing robots pale by comparison.