Naturalism or Metaphysical naturalism is the philosophical doctrine that the observable physical world is all there is.
At any rate, all scientists adopt the methodology of methodological naturalism when working, and this assumes that there is no way to contact or detect the supernatural. Because supernatural, intelligent forces - if they were to exist - are claimed to be be unpredictable and hence unrepeatable, scientists adhering to the scientific method by necessity must ignore the possibility of supernatural or magical intervention in the physical world. In order for it to be science everything must be explained by natural laws and principles.
Arrayed against naturalism are religious schools of thought which claim the the supernatural exists. In order to compete with science they have attempted to create "scientific theories" like Intelligent Design which aim to demonstrate signs of supernatural intervention in natural phenomena.
There is no single widely-accepted understanding of precisely what naturalism is in philosophical circles. Despite this, we will argue here for one definition of naturalism.
A key belief of naturalism is the causal closure of the universe or multiverse. It seems that all events in the universe are caused by purely physical means, maintaining strict physical causality. Naturalism in all its forms therefore denies any non-physical causality, including the causal power of purely-mental objects (souls), divine intervention, and so on. But this still leaves open the question of what "physical" means.
Some philosophers find that it is impossible in principle for mental things to be physical things - there seems to be a gap between the mental and the physical. This leads to certain naturalists that attempt to include not just the physical, but also things which are "supervenient upon the physical" as parts of existence (but without violating the causal closure of physical events). Similarly, some find that mathematical abstract entities really exist, due to certain mathematical theorems. Such views can be named "Pluralism" in regards to naturalism - they allow more than the physical.
Most naturalists, however, will maintain that there is only one reality, and therefore fundamentally only one substance. Even the ancient Greek philosophers embraced a single underlying substance, and modern philosophers often adopt monism (rejecting an independent mental existence). It should be noted that these include certain Pantheists, who nominally speak of God but effectively treat the word as meaning the underlying substance of existence. We will therefore focus on this, "Physicalist", naturalism.
We are left, then, with the question of what does "physical" mean. One way to parse it is that physical things are spatiotemporal things - they are energy-matter in space-time. In addition to being obviously limited by our modern scientific understanding of matter-energy and space-time, it isn't at all clear that "physical" should be restricted in this sense. Physicists often consider abstract systems that are not set against a spatiotemporal background, such as spin-networks. Indeed, quantum loop gravity posits that space-time itself is comprised of such non-spatiotemporal networks. All such systems, however, do obey certain abstract laws of nature.
Another way to parse "physical" is that something isn't physical if it isn't subject in principle to the scientific method. Since the scientific method is primarily concerned with the study of cause and effect, this implies, again, that the object isn't physical if it isn't subject to any abstract laws of nature (at least no laws related to cause and effect).
Either way, we return to the idea that naturalism is the assertion that all of existence is subject to a certain laws of nature. But what are laws of nature? Assuming that nature is consistent, at least formally all laws can be turned into a single law of nature. But what is that? There are two views on this, a "Necessetarian" view that sees the laws as indicating physical necessity, and a "Regulatory" view that sees the laws as describing regularities within nature. We shall adopt the regulatory view here, but in this case it seems that it isn't possible for something to be not describable by laws of nature - we seem to be at an impasse.
The solution is to realize that naturalism is not about whether elements outside reality affect reality in violation of necessary laws, but rather that reality should be described by particular kinds of descriptions, that the laws of nature should be of a particular kind. What distinguishes natural from non-natural is the character of the laws that describe reality.
We have already mentioned in passing the key characteristic of naturalistic laws of nature - that reality is, fundamentally, one. This is why naturalists generally abhor dualism, and are attracted to monism and to positing a single substance lies at the heart of all things. This is why naturalists are reductionists, claiming that sociology, biology, and everything else is fundamentally physics.
The core idea behind naturalism is hence that the description of reality can be reduced to a law which is uniform and universal, a law that recognizes no location or object that is fundamentally different from the others. Naturalism posits that there are no entities which are subject to special laws (like a God which is not subject to physical laws), no beings that are made up of special stuff (all objects are essentially made up of the same substance - energy in modern physics), no place or time that is special (like a Heaven where physical laws don't apply), and so on.
A corollary of this definition is that complex things, such as minds and living organisms, are complex structures of simpler constituents so that essentially complex things are reducible to fundamental particles obeying the law of nature. It specifically follows that mind is not a fundamental quality but rather an emergent structure and that it is physical.
In the language of modern physics, one can think of the universal law in term of the Lagrangian, which is a function describing the fundamental law of nature. The Lagrangian is a function of certain fields, each being some possible type of matter (matter obeying certain symmetries, having certain interactions, and so on). The principles of naturalism stated above indicate that the Lagrangian must not depend explicitly on the time or on any spatial dimension - otherwise, different places or times would be subject to different laws of nature. From this also follows that no object will enjoy a fundamentally special position in the grand scheme of things, as all different types of things (all fields) exist at any place. Physically, this requirement leads to the conservation of energy and momentum.
References and Notes
- For a good description of the history of this belief and its prominent role in naturalism, see the Standford Encyclopedia of Philosophy
- This is known as the mind-body problem, or "the hard-problem".
- See Keith Augistine's A Defense of Naturalism for an exposition on this and other points.
- See Wikipedia
- For a good description of naturalism in this context see the Catholic Encyclopedia