One version is simply the idea that all of nature can eventually be described scientifically; that there are no inherently un-knowable facts.
Sometimes it is used to describe science (particularly physics) as a basis for ontological reductionism[?]--the idea that everything that exists can be explained as the interactions of a small number of simple things (such as matter and energy) obeying physical laws. It is this idea, for example, that Sir John Eccles criticizes in his book Evolution of the Brain, Creation of the Self when he says: "I maintain that the human mystery is incredibly demeaned by scientific reductionism, with its claim in promissory materialism to account eventually for all of the spiritual world in terms of patterns of neuronal activity. This belief must be classed as a superstition....we have to recognize that we are spiritual beings with souls existing in a spiritual world as well as material beings with bodies and brains existing in a material world."
Another attack against this form of reductionism, which is popular among solid-state physicists, argues that it is incorrect to regard the laws which govern the components of structures to be more fundamental than the laws which govern the structures. For example, it has been argued that a traffic jam contains patterns of behavior which cannot be reduced to the behavior an individual car. Similarly metals under go collective behavior and interactions that are not reducible to the behavior of an individual atom within that metal, and it has been argued that the laws which describe this collective behavior are no less fundamental than the laws that describe the atoms themselves.
Yet another attack against the idea of reductionism comes from supporters of the anthropic principle. Some believe that the laws of physics may be randomly determined and explain the fact that we observe certain physical laws by postulating that only a small subset of laws allow for conscious observers. Seen this way, consciousness does not arise from the laws of physics, but rather the observed laws of physics exist because of consciousness.
Daniel Dennett defends this basic kind of reductionism, which he says is really little more than materialism, by making a distinction between this and what he calls "Greedy reductionism": the idea that every explanation in every field of science should be reduced all the way down to particle physics or string theory. Greedy reductionism, he says, deserves some of the criticism that has been heaped on reductionism in general because the lowest-level explanation of a phenomenon, even if it exists, is not always the best way to understand or explain it. Richard Dawkins describes the alternative as "hierarchical" reductionism: organisms can be described in terms of DNA, DNA in terms of atoms, atoms in terms of sub-atomic particles; but there is no need to deal with details of sub-atomic particles to explain animal behavior if one can make adequate explanations and predictions at a higher level. Some physicists argue that large structures undergo collective behaviors which are not most usefully described in terms of the behavior of their constituents (see for example emergence) and therefore there is no reason to label the lower level behaviors as more fundamental.
Both Dennett and Steven Pinker argue that too many people who are opposed to science use the words "reductionism" and "reductionist" less to make coherent claims about science than to convey a general distaste for the endeavor. Furthermore, these opponents often use the words in a rather slippery way, to refer to whatever they dislike most about science. Dennett suggests that critics of reductionism may be searching for a way of salvaging some sense of a higher purpose to life, in the form of some kind of non-material / supernatural intervention. Dennett terms such aspirations "skyhooks," in contrast to the "cranes" that reductionism uses to build its understanding of the universe from solid ground. He writes :-
As Pinker puts it,
In light of this, it might be wise to make some effort to distinguish between rabble-rousing uses of these words, and efforts to make serious claims with them.