Reductionism refers to several related but different philosophical positions regarding the connections between phenomena, or theories, ""reducing"" one to another, usually considered ""simpler"" or more ""basic"". The Oxford Companion to Philosophy suggests that it is ""one of the most used and abused terms in the philosophical lexicon"" and suggests a three part division:Ontological reductionism: a belief that the whole of reality consists of a minimal number of partsMethodological reductionism: the scientific attempt to provide explanation in terms of ever smaller entities Theory reductionism: the suggestion that a newer theory does not replace or absorb the old, but reduces it to more basic terms. Theory reduction itself is divisible into three: translation, derivation and explanation.Reductionism can be applied to objects, phenomena, explanations, theories, and meanings.In the sciences, application of methodological reductionism attempts explanation of entire systems in terms of their individual, constituent parts and their interactions. Thomas Nagel speaks of psychophysical reductionism (the attempted reduction of psychological phenomena to physics and chemistry), as do others and physico-chemical reductionism (the attempted reduction of biology to physics and chemistry), again as do others. In a very simplified and sometimes contested form, such reductionism is said to imply that a system is nothing but the sum of its parts. However, a more nuanced view is that a system is composed entirely of its parts, but the system will have features that none of the parts have. ""The point of mechanistic explanations is usually showing how the higher level features arise from the parts.""Other definitions are used by other authors. For example, what Polkinghorne calls conceptual or epistemological reductionism is the definition provided by Blackburn and by Kim: that form of reductionism concerning a program of replacing the facts or entities entering statements claimed to be true in one area of discourse with other facts or entities from another area, thereby providing a relationship between them. Such a connection is provided where the same idea can be expressed by ""levels"" of explanation, with higher levels reducible if need be to lower levels. This use of levels of understanding in part expresses our human limitations in grasping a lot of detail. However, ""most philosophers would insist that our role in conceptualizing reality [our need for an hierarchy of ""levels"" of understanding] does not change the fact that different levels of organization in reality do have different properties.""As this introduction suggests, there are a variety of forms of reductionism, discussed in more detail in subsections below.Reductionism strongly reflects a certain perspective on causality. In a reductionist framework, the phenomena that can be explained completely in terms of relations between other more fundamental phenomena, are called epiphenomena. Often there is an implication that the epiphenomenon exerts no causal agency on the fundamental phenomena that explain it. The epiphenomena are sometimes said to be ""nothing but"" the outcome of the workings of the fundamental phenomena, although the epiphenomena might be more clearly and efficiently described in very different terms. There is a tendency to avoid taking an epiphenomenon as being important in its own right. This attitude may extend to cases where the fundamentals are not clearly able to explain the epiphenomena, but are expected to by the speaker. In this way, for example, morality can be deemed to be ""nothing but"" evolutionary adaptation, and consciousness can be considered ""nothing but"" the outcome of neurobiological processes.Reductionism does not preclude the existence of what might be called emergent phenomena, but it does imply the ability to understand those phenomena completely in terms of the processes from which they are composed. This reductionist understanding is very different from emergentism, which intends that what emerges in ""emergence"" is more than the sum of the processes from which it emerges.