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Plenty of better qualified people than myself have made this point, but it's cropped up a few times in both Twitter and face to face conversations, where 'reductionist' has been used in a generally dismissive way, as if to be identified as such signifies the end of a discussion. So, partly because I want to clarify for myself, and also to get feedback in case I'm missing something, I thought I'd set down my take on reductionism.
In general, I think reductionism is a sound approach. Anyone with a child will know that we are natural reductionists. When your child goes through the 'why?' phase what they want is a sub-explanation to a seemingly simple question ('Why is it cold?'). That nearly all children do this naturally should tell us it's a pretty good knowledge gathering approach. You look for answers to a higher level phenomenon by breaking it down into its constituent parts.
Reductionism works well – all around you are the products of reductionism: that computer you're using could have a sticker on it 'Powered by reductionism'; that book on evolution 'Authored by reductionism'; that medicine that is curing your disease 'Brought to you by reductionism'.
So why are people so down on reductionism? Dan Dennett puts it well in his book Darwin's Dangerous Idea:
"The term that is most often bandied about in these conflicts, often as a term of abuse, is 'reductionism'… [reductionism is made to look] philistine and heartless, if not downright evil… There are bland and preposterous readings of [a reductionist] claim. According to the bland readings, it is possible (and even desirable) to unify chemistry and physics, biology and chemistry, and yes, even the social sciences and biology… According to the preposterous readings, reductionists want to abandon the principles, theories, vocabulary, laws of the higher-level sciences, in favor of the lower-level terms. A reductionist dream, on such a preposterous reading, might be to write "A Comparison of Keats and Shelley from the Molecular Point of View"… Probably nobody is a reductionist in the preposterous sense and everybody should be a reductionist in the bland sense, so the 'charge' of reductionism is too bland to merit a response"
The problem is that of 'greedy reductionism' as Dennett terms it, whereby too much is claimed, too quickly and overly simplistic explanations are given which miss, or ignore some of the complexity. In reaction to this it seems that sentiment has swung too far the other way and declared all reductionism wrong.
There are also phenomenon that reductionism is not good at explaining, for example chaos theory, and emergence. Some scientists will argue that emergence can still be explained in reductionist terms we just don't have sufficiently complex models, and others that it can't and there are emergent properties that are greater than the sum of the parts. This is a debate worth having and whether different approaches are viable to different phenomenon, but surely the conclusion is not that reductionism never works (we have plenty of evidence to the contrary).
Other people have a general mistrust about reductionism, that it is mechanistic and they want to retain some sense of the whole. This can be a misunderstanding about what reductionism seeks to do, as Dennett suggests. As both Marr and Pylyshyn have argued there are different levels of explanation: if you want to know why I was running down the street you expect a psychological explanation ('I was late for a meeting') not a biological one ('my muscles were contracting causing my legs to move in response to messages from my brain'), or a chemical or physical one.
The concern I have about those who wish to dismiss reductionism in favour of more holistic responses is that it is often a pseudo-explanation or even a dead-end. For centuries the church provided this pseudo-explanation along the lines of 'It's God's will', which effectively means don't look any further. But being good little reductionists, people did look further and we found subsequent explanations at every level we looked. This didn't stop people feeling emotions, producing art or making jokes. So I am always wary when reductionism is rejected but nothing offered in its place other than vague ideas of complexity or emergence (or even mysticism). We've been here before. (There is also a slight implication that reductionism is in some sense 'right wing'. I'm probably naive in this sense, but I simply don't understand this at all.)
I fully concur that reductionism is a) not always the best way to explain things b) may miss some subtlety between the components and c) may not be the correct approach as we look at emergent properties in complex systems. But that doesn't mean it offers nothing, so rather than the rejection of reductionism being an endpoint in an argument ('that's so reductionist!') it should be the start, in that you need to have a good reason why it won't work and what you are replacing it with.