Methodological naturalism is the view that science (as a tool) should restrict itself to only allow physical or natural explanations. Ontological naturalism on the other hand is generally the view that all that exists is what the physical sciences reveal, although there are many varieties of naturalism and pinning down a precise definition is not easy. There is, however, a common core to all varieties. This article will be about ontological naturalism, not methodological naturalism.
II. Making a Causal Difference
Thanks partially to empirical science, the central doctrine of naturalism is that all physical effects have purely physical causes, which is called "causal closure." The project of naturalism is thus centered around fitting the following apparently non-physical entities into the physical picture, so that they can have physical effects:
- Mental properties - Mental properties appear to be different from physical properties: representation, first-person experience, free will, and rationality are all properties of the mind that are not properties of physical matter.
- Moral facts - If moral facts exist, they would seem to be something other than particles, forces, and the like.
- Mathematical and abstract objects - Numbers, modal propositions, etc. are abstract, not concrete physical objects.
A brief rundown of the history of the causal closure thesis:
- Descartes and Leibniz - 17th Century - The founding fathers of modern science, wanting to have a purely mathematical description of nature, said that any material body can only change velocity or location if caused to do so by another material body.There didn't seem to be any room for mental causes, hence the source of Descartes' dualism, and also the source of the primary argument against it: how can non-physical mental causes have any effect on physical matter?
- Newton - Newtonian physics, involving such forces as gravitation and magnetism, changed the Cartesian picture. Matter could now be affected at a distance, and perhaps mental causes could be brought back in.
- Law of Conservation of Energy - The discovery of this law in the 19th Century once again seemed to limit causes to only physical ones.
- 20th Century - Further empirical scientific research (such as neuroscience) supports the thesis that only physical causes are involved in physical effects.
Attempts to describe mental events as physical events have generally fallen into two categories: A) attempts to describe the mind as identical to physical brain events (i.e., there is only one thing, not two), and B) attempts to describe the mind as arising from physical events (i.e., the whole is greater than the sum of the parts). One major goal of both theories is to avoid concluding that physical events cause the mind but that the mind does not cause physical events.
- Token-identity theory: Mental events are physical brain events, while not being any particular brain event. For example, "happiness" in one person consists of the firing of neurons, and "happiness" in another person consists of the firing of other neurons. There is no law-like correlation between the mental event and the firing of specific neurons. The problem with this is that if true, then the physical event would be enough to explain the physical effects, and the mental event would just be a dangler: it wouldn't play any role in causing anything. We want to be able to say that our mental properties can cause physical events (like writing down your thoughts).
- Type-identity theory: Mental events are specific types of brain events. For example, "happiness" is just the firing of "happiness" neurons, and anybody who is happy will be so because their happiness neurons are firing. All persons who are happy have the exact same neurons firing. The problem with this view is that mental properties are not properties of anything physical (representation, subjectivity, etc), and so it seems implausible that they are strictly identical with physical properties. Also, you would have to then say that aliens future AI computers will not be able to have feelings like "happiness", because they don't have "happiness neurons".
- Non-reductive materialism: Mental events are physical events, but they are "substrate independent". They can be realized by many different physical systems, but are not reducible to or strictly identifiable with any of them. The most common term for this is called "supervenience." A physical system, when arranged correctly, "brings about" mental properties, which can then be said to supervene on the physical substrate. The problem with this view is that it ends up with every effect of a mental cause having two causes: the physical cause from the brain, and the mental cause that is supervening on the brain.
V. Limitations of Supervenience of the Mind
If the mind is supervenient (or emergent) on the brain, then this alone is not a strong or problematic claim. However, naturalists need to say that the supervening mind causes physical effects, which is a much stronger claim. And a stronger supervenience ends up being strong overdetermination as well: the raising of your beer to your lips is caused both by the physical event (firing neurons in the brain) and by the mental event (a desire to drink some beer). And this is problematic at best.
V. Mental Properties and Causal Closure
Some say that conscious causation is irreconcilable with causal closure, and so the do not have physical effects. The physical brain causes non-physical consciousness, but not vise versa. This leads to epiphenomenalism, a one-way causal form of dualism (the brain causes the mind, but the mind has no ability to cause anything).
Some think that consciousness is not reducible to the physical, and others think that it is. If it is, then physicalism is preferable to epiphenomenalism, as we would like to be able to say that our minds can cause our bodies to do things. Not to mention, there are no other examples of this type of one-way causal relationship in nature. So while epiphenomenalism is on the table, general principles of theory choice would prefer physicalism, so that we can keep mental causation.
The primary case against interactionist dualism (as opposed to epiphenomenalism) is the causal closure thesis.
Sometimes, it is suggested that physicalism can be argued for because of the close correlation between mind and physical brain events.
Moral values are facts that might be prima facie immaterial (you can't weigh "justice", for example). So attempts to fit these into the causal closure principle have been of numerous types:
- Moral non-naturalism: Not to be confused with supernaturalism. This theory says that moral facts are their own species of facts, alongside natural facts (e.g., the sky is blue, murder is wrong). But if they are non-physical, then how can we know about them? Our senses are physical, and thus input would have to come from a physical source. So this theory may not work well with the causal closure thesis.
- Moral non-realism: Moral facts either do not exist, or they are all false.
- Moral realism: Moral facts do exist, but are equivalent to or reducible to natural facts (e.g., "moral goodness" = "happiness for the greatest number of people").
Mathematical facts, like numbers and sets, and other abstract objects are at least apparently immaterial, and so like with moral facts, naturalistic theories attempt to fit them into the causal closure thesis as well. Similar to moral facts, they fall into three main categories:
- Non-natural realism: Numbers and abstract objects really exist, but alongside natural things. This again conflicts with causal closure; if numbers and such are non-physical, then how can we know about them?
- Fictionalism: Mathematical non-realism. Numbers and such do not exist at all. They are just a useful fiction. Not a very popular theory, as most of our best scientific theories depend on math.
- Naturalist realism: Mathematical (and other abstract) objects are equivalent to natural facts. This is not a widespread view, since it is difficult to explain how infinite sets, perfect geometrical concepts, and so on that are said to exist outside of time and space can be equivalent to anything in time and space.