Reconciling the No Decoupling Approach and the Inbetweenness Approach to Constitutive Relevance Relation

In the biological sciences, the most powerful way to explain how something works is to describe the machine behind it. Scientists do not just say that a bird flies; they explain the flight by describing the beating heart, the contracting muscles, and the firing nerves that make it happen. This is known as a mechanistic explanation. But for this explanation to be true, every part mentioned must actually belong to the machine. If a researcher points to a random leaf on a tree and claims it is part of the bird's flight mechanism, the explanation fails. Philosophers of science have long struggled to define exactly what makes a part "relevant" to the whole. They ask: how do we know for sure that a specific component is a necessary piece of the puzzle, rather than just something that happens to be nearby?

For years, two different schools of thought have argued over the best way to find these parts. One group believes you should look at the whole phenomenon as a single, indivisible event and see if changing a part changes the whole. The other group argues that you should view the phenomenon as a chain of events, starting with a trigger and ending with a result, and look for parts that sit in the middle of that chain. These two views have been treated as rivals, with scientists and philosophers debating which one holds the correct answer. However, a new analysis suggests that this debate is based on a misunderstanding. The two approaches are not fighting over the truth; they are simply different tools designed for different moments in the journey of discovery.

The paper, written by Yichu Fan of the University of Edinburgh, proposes that these two methods are actually compatible and complementary. The author argues that the disagreement stems from how scientists choose to describe the phenomenon they are studying. Sometimes, a scientist looks at a complex biological rhythm, like the daily sleep-wake cycle of a fruit fly, and treats the entire cycle as one big, single event. In this view, the phenomenon is represented by a single variable. When researchers use this "single variable" picture, the method that works best is the "no decoupling" approach. This method checks if a specific part is so tightly linked to the whole that you cannot change the part without changing the whole. If the link is unbreakable, that part is a genuine component of the mechanism.

At other times, the same scientist might zoom in on a specific step within that cycle, such as how one protein stops another from working. Here, the phenomenon is no longer seen as a single block, but as a process with a clear beginning and a clear end. The researcher sees an input, a chain of events, and an output. When the phenomenon is viewed this way, as a "chain of nodes," the "inbetweenness" approach becomes the most useful tool. This method looks for parts that sit causally between the start and the finish. If a component is found to be the bridge that connects the input to the output, it is confirmed as a relevant part of the mechanism.

Fan demonstrates that these two approaches do not contradict each other; they simply apply to different ways of looking at the same reality. If a researcher switches from viewing a phenomenon as a single event to viewing it as a chain, they are not changing the facts of the biology, only the lens through which they are examining it. The paper shows that when both methods are applied correctly to the same biological system, they always point to the same components. They agree on what is essential, even if they use different logic to get there.

To prove this, the author turns to the real-world history of how scientists discovered the mechanism behind the circadian clock in fruit flies. This clock is the internal timer that tells the fly when to be active and when to rest, a rhythm that repeats roughly every twenty-four hours. The discovery of how this clock works did not happen all at once; it happened in two distinct phases, each requiring a different conceptual tool.

The first phase was the "localizing" stage. In the early days of this research, scientists knew the flies had an internal clock, but they did not know where it was or what it was made of. They treated the entire daily rhythm of the fly as a single, mysterious phenomenon. They could not easily point to a specific "start" or "end" for the rhythm because it was a continuous, self-sustaining loop. In this context, the "no decoupling" approach was the perfect guide. Researchers would manipulate a specific part of the fly, such as a gland or a gene, and see if the daily rhythm stopped or changed. If the rhythm remained unchanged, that part was not essential. If the rhythm broke, the part was a candidate for being a core component. This method helped scientists narrow down the search until they found the period gene, a specific piece of DNA that was essential for the rhythm to exist. At this stage, the phenomenon was too broad to be seen as a chain, so the single-variable approach was the only way to find the source.

Once the period gene was found, the research entered the second phase: the "enriching" stage. Scientists now knew they had a part of the machine, but they did not know how it worked. They needed to fill in the blanks of the mechanism. They began to look at the specific interactions between the proteins produced by the gene. Here, the phenomenon changed. Instead of looking at the whole day, they looked at a specific interaction: how the protein made by the period gene stops the gene from making more of itself. This interaction had a clear beginning (the production of the protein) and a clear end (the stopping of the gene's activity). Because there was a clear start and finish, the "inbetweenness" approach became the ideal tool. Researchers could now ask: what sits between the protein and the gene? They tested other proteins, like TIM and dCLOCK, to see if they acted as bridges in this chain. By confirming that these proteins sat causally between the start and the end of the interaction, scientists were able to map out the entire feedback loop that drives the clock.

The paper concludes that the history of this discovery shows that the two approaches are not rivals fighting for the title of "the truth." Instead, they are a toolkit. When a scientist is trying to find the location of a mechanism in a complex system, the "no decoupling" approach is the right tool. When the scientist has found the location and needs to understand the internal connections, the "inbetweenness" approach takes over. The author suggests that this pattern likely applies to many other areas of biology, not just the fruit fly clock. By accepting that different stages of discovery require different conceptual tools, scientists can avoid getting stuck in theoretical arguments and focus on the actual work of uncovering how life works. The paper does not claim to have solved every mystery of how mechanisms work, but it does clear the path for a more practical, flexible way of thinking about them, allowing researchers to choose the right method for the specific question they are asking at that moment.