What is Reductionism?

Last week, we spoke about emergence, this week, we will speak about the opposite: Reductionism. Reductionism, loosely speaking, is the idea that you can understand things by taking them apart into smaller things. This definition of reductionism, as we will see, is not quite correct, but it’s not too far off. Before we get to the details, however, a few words about how enormously important reductionism is for scientific understanding.


A lot of people seem to think that reductionism is a philosophy. But it most definitely is not. That reductionism is correct is a hypothesis about the properties of nature and it is a hypothesis that has so far been supported by every single experiment that has ever been done. I cannot think of *any scientific fact that is better established than that the properties of the constituents of a system determine how the system works.

To be sure, taking things apart into pieces to understand how they work is not always a good idea. Even leaving aside that taking apart a living organism typically kills it, the problem is that the connection between the theory for the constituents and the theory for the whole system may just be too complicated to be useful. Indeed, this is more often the case than not, which is why figuring out how an organism works from studying its components is not a fruitful strategy. Studying the living organism as a whole is dramatically more useful, so this is what scientists normally do in practice.

But if you really want to *understand what an organism does and how it does it, you will look for an explanation on the level of constituents. Like this part sends a signal to that part. This part stores and releases energy. This piece produces something and does this to another piece, and so on. If we want to really understand something, we look for a reductionist theory. Why? Because we know from experience that reductionist theories have more explanatory power. They lead to new predictions rather than just allowing us to reproduce already observed regularities.

Indeed, the whole history of science until now has been a success story of reductionism. Biology can be reduced to chemistry, chemistry can be reduced to atomic physics, and atoms are made of elementary particles. This is why we have computers today. But, again, this does not mean it is always practical to use a theory for the constituents to describe the composite system. For example, you would not use the standard model of particle physics to predict election outcomes. And why not? Because that would not be useful. The computation would take too long. So what’s the use of reductionism then? The use is that at each level of reduction that scientists have discovered, they gained new insights about how nature works and that has enabled us to make both intellectual and technological progress.

But here is the important point. There are two different types of reductionism. One is called methodological reductionism, the other one theory reductionism. Methodological reductionism is about the properties of the real world. It’s about taking things apart into smaller things and finding that the smaller things determine the behavior of the whole. Theory reductionism on the other hand means that you have levels of theories where the higher – emergent – levels can be derived from the lower – more fundamental – levels. But in this case, a high level does not necessary mean the theory is about large things, and a low level does not necessarily mean it’s about small things.

So what type of reductionism is it that has been so successful in the history of science? The funny thing is that it’s a combination of both. Methodological reductionism has so far gone hand in hand with theory reductionism. As we have looked at smaller things we have found more fundamental theories.

But this does not necessarily have to remain this way. There is no reason to think that the next better theory of nature will be found by studying shorter distances. Just because the two types of reductionism have been tied together for a while does not mean it will remain this way.

Indeed, some of the biggest currently open problems in physics manifest themselves on large scales, not on small scales. Besides dark energy and dark matter there is also the measurement problem in quantum mechanics. I have told you about those problems in some earlier videos. They are not in any obvious way short-distance phenomena.

So the next time a particle physicist tries to tell you that we need higher energies to probe shorter distances because that’s where progress will come from, remind them that methodological reductionism is not the same as theory reductionism.