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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 why certain substances have the chemical properties that they do, and integrates these properties into a single explanatory framework, that of atomic structure.
We can usefully divide reductionism (the position) into three general areas – methodological, theoretical, and ontological – and reduction (the process) into two – theoretical and ontological.
Methodological reductionism is the position that the best scientific strategy is to attempt to reduce explanations to the smallest possible entities. Methodological reductionism would thus hold that the atomic explanation of a substance’s boiling point is preferable to the chemical explanation, and it that an explanation based on even smaller particles (quarks, perhaps) would be even better.
Theoretical reductionism is the position that all scientific theories either can or should be reduced to a single super-theory through the process of theoretical reduction. 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 was an ontological reductionist would presumably believe that everything is reducible to one of two substances.)
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 terrestral objects are reducible to Newtonian theories of mechanics, because all the explanatory power of the first is contained within the second. 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:
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. We now know, however, that all life forms are alive in 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. We therefore say that the property of life is reducible to the physical properties of an organism; being alive is simply nothing more than having certain physical properties.