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In philosophy, reduction is the process by which one object, property, concept, theory, etc., is shown to be explicable in terms of another, lower level, entity. In particular, a concern of philosophy is as to the scope of physical theory, and whether, for example, all events are ultimately physical events, a discussion closely related to the topic of causal closure.
In science, such reduction is generally desirable, because it explains why and how the thing which is being reduced exists, and because it promotes conceptual and theoretical economy. Reducing chemical properties to properties of atoms thus explains these properties and integrates them into a single explanatory framework, that of atomic structure. For example, we say that physical properties such as the boiling point of a substance are reducible to that substance’s molecular properties, because statistical mechanics explain why a liquid boils at a certain temperature using only the properties of its constituent atoms. Thus we might also describe reduction as a process analogous to absorption, by which one theory (or concept, or property, and so on) is wholly subsumed under another.
Types of reductionism
Methodological reductionism is the attempt to reduce explanations to the smallest possible entities. Methodological reductionism would result in the atomic explanation of a substance’s boiling point, and perhaps in an explanation based on even smaller particles (quarks, perhaps).
Theoretical reductionism is reductionism applied to scientific theories, the goal of reducing the present multiplicity of theories to a single super-theory through the process of theoretical reduction, the theory of everything.
Finally, ontological reductionism is the belief that reality is composed of a minimum number of kinds of entities or substances. This claim is usually metaphysical, and is most commonly a form of monism, in effect claiming that all objects, properties and events are reducible to a single substance. (A dualist who is an ontological reductionist would presumably believe that everything is reducible to one of two substances.)
Types of reduction
The distinction between the processes of theoretical and ontological reduction is equally important. Theoretical reduction is the process by which one theory is absorbed into another; for example, both Kepler's laws of the motion of the planets and Galileo’s theories of motion worked out for terrestrial objects are reducible to Newtonian theories of mechanics, because all the explanatory power of the former are contained within the latter. Furthermore, the reduction is considered to be beneficial because Newtonian mechanics is a more general theory — that is, it explains more events than Galileo's or Kepler's. Theoretical reduction, therefore, is the reduction of one explanation or theory to another — that is, it is the absorption of one of our ideas about a particular thing into another idea.
By contrast, ontological reduction is the process of reducing things themselves to one another. For example, it was once believed that life was an irreducible property of objects. An ontology of such properties might therefore have read:
- extension in space
- location in space
- is alive
- and so on.
All the other properties of an object, such as its shape, color, or mobility are considered to be nothing more than the effects of these irreducible properties. Shape, for example, is a function of in what way the object is extended in space, as is color, since it is determined by how light bounces off a surface, which is in turn determined by how that object is extended in space.
Science now considers that all life forms are alive by virtue of the fact that they are physically organized in such a way that they can reproduce themselves, not because they possess a special property distinct from and in addition to their physical organization. Biologists therefore say the property of life is reducible to the physical properties of an organism; being alive is simply nothing more than having certain physical properties.
Benefits of reduction
An ontological reduction reduces the number of ontological primitives that exist within our ontology. Philosophers welcome this, because every ontological primitive demands a special explanation for its existence. If we maintain that life is not a physical property, for example, then we must give a separate explanation of why some objects possess it and why others do not. This is more often than not a daunting task, and such explanations often have the flavor of ad hoc contrivances or deus ex machina. Also, since every ontological primitive must be acknowledged as one of the fundamental principles of the natural world, we must also account for why this element in particular should be considered one of those underlying principles. (To return to an earlier example, it would be extremely difficult to explain why planets are so fundamental that special laws of motion should apply to them.) This is often extremely hard to do, especially in the face of our strong preference for simple explanations. Pursuing ontological reduction thus serves to unify and simplify our ontology, while guarding against needless multiplication of entities in the process.
At the same time, the requirements for satisfactorily showing that one thing is reducible to another are extremely steep. First and foremost, all features of the original property or object must be accounted for. For example, lightning would not be reducible to the electrical activity of air molecules if the reduction explained why lightning is deadly, but not why it always seeks the highest point to strike. Our preference for simple and unified explanations is a strong force for reductionism, but our demand that all relevant phenomena be accounted for is at least as strong a force against it.
In metaphysics, the Bundle theory says that objects can be reduced to collections of properties; so whenever we talk about objects, we can be understood to be talking about bundles of properties. Does this mean that the bundle theory says that objects do not exist? Perhaps not objects as we had thought of them, but the theory is trying to give an account of what objects are; namely, they are bundles of properties. So, the bundle theorist is not denying that objects exist; he or she is affirming that objects are the same as bundles of properties. The only reason one would have for maintaining, then, that the bundle theory holds that objects do not exist is if you think that, according to our ordinary concepts, something simply cannot both be a bundle of properties and an object.
Philosophers mean about the same thing when they talk about what exists ultimately. For example, the bundle theory says that ultimately, properties and bundles thereof exist, rather than objects. The things that exist "ultimately" are precisely the things to which other things are reduced.
- Routledge Encyclopedia of Philosophy