Antioxidants -> reduced free radicals -> less DNA damage -> less cancer

Unfortunately, these biologically plausible-sounding claims usually don’t work when rigorously tested.

Are mechanistic arguments useless?

No. They are a great source of *hypotheses*. While most of these hypotheses fail, some eventually lead to important new treatments.

Unfortunately, health gurus, podcasters, and even sometimes (though they should know better) doctors and scientists use mechanistic arguments to convince the public about treatments for which we have little evidence.

Mechanistic arguments in health sound scientific and impressive. They make the speaker seem authoritative and knowledgeable. And they *seem* very hard to argue with. However, there is one general argument that works for most of them: “That sounds nice, but let’s look at randomized experiments in humans to check if it works.”

Why is it so common that health-related mechanistic claims don’t work when rigorously tested in randomized trials?

What goes wrong with X->Y biological thinking?

1) Causality: The first issue is that an X->Y claim may be true in terms of associations, without the links being *causal*. It’s typically a lot easier to establish that when people have higher X, they also have higher Y than to show that increasing X causes higher Y.

Alzheimer’s research seems to be experiencing this problem in a major way. The hypothesis:

Amyloid plaques -> Alzheimer’s

Seems to be oversimplified or perhaps mostly associational (rather than causal), as drugs that reduce brain plaques have had disappointing results.

2) Multiple mechanisms: even if it’s true that X is in the causal chain for Y, it may also be true that Y is also highly influenced by other mechanisms, and so changing X may not change Y that much, even if you control X completely.

3) Other effects: even if the mechanism is completely correct, there may be alternative effects of the treatment. These could undermine the original benefit through other pathways, or cause other forms of harm that mean the benefit is not worth it.

4) Equilibrium: even if mechanistically X->Y, the body may work hard to maintain a balance of Y (much as it does to keep your core body temperature roughly constant regardless of whether you’re drinking a hot beverage or standing outside in the cold). Hence, the effects of intervening on X may not create lasting impacts on Y because your body works to restore a homeostasis.

5) Evaluability: unlike arguments based on empirical evidence (we gave patients this treatment in a study and here’s how their outcomes differed from the control group) and logical arguments, which a reasonably knowledgable non-expert can understand and assess to at least some degree, biological mechanism based arguments can’t be evaluated at all by non-experts. Take this claim, for example. Is this sound? See if you can tell:

“Subcutaneous WPP9 injections activate orexinergic neurons via Gq-coupled receptor agonism in the lateral hypothalamus, which increases daytime cortisol rhythm, leading to increased alertness.”

So, is this a valid mechanistic argument about human biology? Well, no, but I only know that because I had an LLM AI make this argument up by prompting it to generate a biologically plausible sounding but made-up argument. An expert on the topic may immediately identify it as implausible, but anyone else is going to have no realistic way of evaluating its soundness without help.

So, what’s the takeaway here? Well, when a podcaster or health guru tells you that we know a treatment works because [insert biological mechanistic argument here], remember that it isn’t strong evidence, no matter how impressive it sounds. We need careful randomized experiments (or other high-quality evidence) to be confident it’s true. Mechanistic arguments are for generating hypotheses; they give us a reason to collect more data and run studies to see if an idea pans out – they don’t themselves serve as strong evidence for what’s true.

Of course, we don’t always need strong evidence to try a treatment if it is worth it. If a treatment isn’t expensive and is low risk, we would be able to tell if it is working, and we don’t have more evidence-backed alternatives, then experimenting with the treatment (even if it only has weak evidence) can still be a good idea. But we shouldn’t mistake “worth experimenting with” for “having strong evidence for”.

This piece was first written on May 8, 2025, and first appeared on my website on May 15, 2025.

Think of something you value that:

A. Multiple other people you know are capable of achieving, but that…

B. You assume you would not be capable of achieving, even though…

C. You have never actually tried to do this thing well before.

Now, suppose for a moment that you have no choice but to do the thing. That is, everything you care about in the world will be destroyed if you do not achieve it in X months. Here, X could be 1 if it’s a very small thing, or X could be 100 if it’s a much larger thing.

Under those circumstances, do you STILL believe you would fail to achieve it?

I think this sort of thought experiment can help us distinguish between things that we don’t believe we are capable of merely because we aren’t motivated enough, versus things that we ACTUALLY believe are impossible for us.

And I think it’s important to distinguish between these two cases, because if something is in the first category, we may actually be able to get ourselves to succeed just by finding ways to increase our motivation!

I also suspect that for many people, a number of the things that they view as being impossible for them would seem more possible in the face of carrying out this thought experiment. In other words, it is easy to confuse “I’m not motivated enough to try really hard” with “I’m incapable.”

As an example, suppose you believe you are just inherently bad at math, and that no matter how hard you try, you couldn’t understand calculus. Well, what if the fate of the world rested on your understanding of calculus in 6 months? I think under those circumstances, you would very likely find a way to learn it, with plenty of time to spare.

This piece was first written on April 26, 2018, and first appeared on my website on September 22, 2025.

How to Test a Theory Without Doing An Experiment:

Think of as many things as you can that are predicted by your theory, and try to identify at least one prediction that has the following four properties:

(1) your theory implies it is very likely to be true.

(2) it seems very unlikely to be true if your theory is false (i.e., there is no reason you can see to make that prediction other than if you believe in your theory). You may want to ask others if they agree that the prediction seems very unlikely to be true from the perspective of those who don’t believe your theory; otherwise, you might be deceiving yourself.

(3) you don’t already know the answer to whether it is actually true or not (you are merely guessing it is true because it is implied by your theory). This is important because if you already know the answer, you may be tricking yourself one way or another (e.g., by having already used this evidence in the construction of your theory, or by choosing this particular test because on some level you know it will make your theory look good).

(4) you can reliably look up whether that prediction is true or not. Then all you have to do is look up whether the prediction is true!

If it turns out to be true, that provides evidence to support your theory. More specifically, the more likely that true result is given your theory, and the less likely that true result is (to someone who doesn’t know your theory or if your theory is invalid), the more evidence the test provides. To explore the math for only a moment: that test of your theory should change your prior odds (i.e., your estimated probability that your theory is true, divided by the probability that it is not true) by using what’s known as the “Bayes factor,” namely, your estimation probability. Looking up the answer to that question would yield the result it did, if your theory is true divided by the probability it would yield that result if your theory is not true.

In other words:

odds your theory is true = ( probability of that result if your theory is true ÷ probability of that result if your theory is false) × prior odds your theory is true from before the test

Odds can be confusing to think about (remember that it is the probability that something is true divided by the probability that it is false). For instance, odds of 2 would mean that a thing is twice as likely to be the case then it is to not be the case.

It’s worth noting that the above procedure can work really effectively to test your hypotheses for yourself, but it’s NOT a good way to provide evidence that your theory is true to other people that don’t trust you (i.e., your honest pursuit of truth or your competence). The reason is that it’s easy to claim you conducted this fair test of your theory, when in fact, you already knew the answer to how the test would turn out (and therefore were incentivized to choose this particular test of your theory rather than another). Or perhaps you interpreted your theory in a weird way or tweaked it after the fact to make it look like it correctly predicted the outcome when it actually didn’t. Or maybe you screwed up one of the steps accidentally. In other words, the evidence this procedure produces is only trustworthy if you really trust the rigor and character of the person who produced the evidence.

This is why experiments are, at least in theory (but unfortunately too often not in practice), a better way to gather evidence when you need to communicate those results to others who don’t trust you, especially if you pre-register your study and analysis plan, and if you release your materials and the data, you collect.

If you’re interested in the concepts underpinning the approach I’ve described here, you may want to check out our program on Bayesian thinking (which you may or may not be happy to know uses almost no math):

http://programs.clearerthinking.org/question_of_evidence.ht…

Here are 24 different methods you could use to better understand the way humans work. In each case, I use “trying to figure out new things about post-traumatic stress disorder (PTSD)” as an example. What techniques am I leaving out here?

1. Papers: Reading academic studies (e.g., from social psychology, consumer psychology, cognitive psychology, neuroscience, etc.)

Example: reading the newest academic papers on treatments for PTSD.

2. Products: Carefully examining products and apps that have been successful (as well as, potentially, those that have failed) to understand what human needs they were or weren’t satisfying and why they succeeded or failed.

Example: looking at what mental health apps related to PTSD are popular in the app stores.

3. Outliers: Investigating extreme case studies of real-life events (e.g., case studies of people with extreme psychological conditions who may have been reported about in the news or true stories about what actually happened when people were placed into really extreme conditions).

Example: reading about cases of extreme cults and whether or not each cult produced PTSD-like symptoms in its members.

4. Society: Examining the large-scale trends and organization of societies, including potentially human cultures around the world (American culture, traditional cultures, etc.) and how they are similar or different, and which societal structures tend to be created and what the effects of these structures are, etc.

Example: investigating which sort of societies PTSD appears to be much more prevalent in.

5. Conversations: Talking to smart and insightful people about what they think is true of human psychology. You can also show them the models or theories you have so far to get their criticism and suggestions for improving them.

Example: asking really smart (non-experts) who had PTSD what their current theories about PTSD are.

6. Reading: Reading books, blog posts, media articles, etc., containing information from smart or insightful people about psychology (including potentially writing from life coaches, relationship experts, marketers, thoughtful bloggers, user experience designers, game makers, etc.)

Example: seeing what theories bloggers have proposed regarding PTSD that many academics may not have considered.

7. Experts: Reaching out to experts and talking to them to understand their models of psychology (e.g., psychologists, psychiatrists, game developers, etc.). You can also show them the models and theories you have so far to get their criticism and suggestions for improving them.

Example: talking to world-class experts in PTSD.

8. Textbooks: Reading academic textbooks about psychology, cognitive science, etc.

Example: Linehan’s textbook on DBT and thinking about possible applications of DBT in PTSD treatment.

9. Models: Looking at what models or frameworks of psychology others have created (e.g., in social psychology, theoretical cognitive neuroscience, gamification research, persuasion research, etc).

Example: looking at the model of PTSD that is used in Cognitive Behavioral Therapy treatment.

10. Intuition: Our own intuition for what’s true about humans (presumably based on our first-hand experience interacting with others, as well as our first-hand, ingrained experience of just being a human).

Example: We may have the intuition that after experiencing a traumatic event, people will tend to start having frequent disruptive thoughts about that event, even when doing unrelated things.

11. Interviews: talking to individuals or groups about their thought processes, feelings, behaviors, etc., to understand their individual psychology and then attempting to extrapolate that knowledge to humans more generally.

Example: interviewing 20 people with PTSD about what their experience of it is like and what does or does not seem to help them.

12. Application: attempting to put into practice a psychological theory (e.g., by coaching individuals using a psychological technique or by trying to build an app that implements it) and paying attention to the ways it seems to work or not work.

Example: creating a tool for people with PTSD and making it publicly available, then monitoring how people respond to it.

13. Anecdotes: psychologically surprising or interesting anecdotes that we’re confident actually happened (e.g., because they happened to us or were reported on by a trustworthy source) that may have a bearing on human psychology.

Example: a story a friend told you about how they (believe) they fully cured their own extreme PTSD in a matter of days.

14. Self-observation: careful (and honest) observation of our own thoughts/beliefs/behavior/emotions and internal processes.

Example: observing the mental processes in your own mind that seem to occur after a very upsetting event has happened.

15. Self-experimentation: simply trying techniques and paying close attention to what effects they appear to have and what it feels like to apply them.

Example: try the technique of writing down for a week every upsetting thought that you notice yourself having, as well as writing down everything you observe or learn about the experience of doing this.

16. Deduction: combining propositions we already believe to produce new propositions we didn’t realize before that may be true.

Example: knowing that women with PTSD are much more likely to have associated depression than men with PTSD, and knowing that depression is a strong risk factor for attempting suicide, we might predict that women with PTSD are more likely to attempt suicide than men with PTSD (even though they may not be more likely than men to actually commit suicide).

17. Experiments: Conduct your own online randomized controlled trials, surveys, longitudinal studies, or fMRI studies, etc.

Example: looking at which variables that don’t seem obviously PTSD related are, in fact, strong predictors of whether someone has PTSD.

18. Statistics: looking up known statistics about a phenomenon.

Example: looking up whether younger people or older people are known to be more likely to get PTSD in any given 12-month period.

19. Data: finding existing data sets (e.g., large longitudinal surveys or government data sets) and running your own statistical analyses to test hypotheses.

Example: measuring whether it is true that after someone gets a pet, they are more likely to recover from PTSD than similar people who do not get a pet. Running studies can also be a good way to search for new hypotheses. For instance, examining which of many variables in the data are most associated with a rapid recovery from PTSD.

20. Literature: looking at portrayals of psychology in literature, art, myths, stories, philosophical writings, etc.

Example: What hypotheses do Kafka’s novels give us about the nature of trauma?

21. Crowdsourcing: Request suggestions for hypotheses on a social media platform like Facebook or X or on a question-answer site like Quora.

Example: posting to Facebook asking people to suggest theories for why PTSD sometimes does and sometimes doesn’t happen after extreme trauma.

22. Social experiments: bringing a group of people together under a new set of chosen social rules or guidelines and observing what happens.

Example: getting a small group of people together with the idea that it will be a time to discuss traumas that members of the group have experienced.

23. Evolution: considering what possible evolutionary function different psychological phenomena might have.

Example: consider ways that aspects of PTSD responses might be evolutionarily adaptive.

24. Practitioners: examine how people “in the field” who benefit from being able to understand this aspect of psychology do their work.

For example, at veterans hospitals where rates of PTSD are high, investigate what techniques and approaches the therapists have developed to work successfully with their patients.

This piece was first written on November 16, 2017, and first appeared on my website on July 22, 2025.

It feels, to nearly everyone, on a gut level, that I am ‘me’ and you are ‘you’, and consciousnesses are distinct from each other and easy to separate. Moreover, the vast majority of people accept that you 1 minute from now is still the same “YOU” in a meaningful sense as YOU right now; that it makes sense for you right now to try to prevent harm to you one minute from now (even if acting purely selfishly – not altruistically trying to prevent harm coming to any being).

But imagine the following hypothetical scenario taking place far, far in the future, when our understanding of the brain, our ability to perform brain surgery, and our technology, in general, is drastically more advanced than today.

You’ve voluntarily elected to undergo an operation (for science!) where your brain will be slowly but completely replaced with a replica brain.
The steps of the surgery are as follows:

(1) you are temporarily put into a deep slumber (e.g., using something like general anesthesia) for a few hours.

(2) surgeons very carefully remove some tiny portion of your brain and then analyze that portion of the brain (e.g., using some sort of scanning device)

(3) based on this analysis, they then construct a replica brain piece (whether robotic or made of organic material, it doesn’t really matter) that will behave essentially just like the piece they just removed (e.g., maybe using some kind of 3d cell printer)

(4) they then insert this replica piece in the place where the original brain piece used to be and affix all of its connections to the surrounding brain tissue just as the original brain piece was affixed so that it will operate within the greater brain essentially just like that missing brain piece used to

(5) the original piece removed from the brain is then carefully preserved elsewhere, and its original position in the brain is carefully recorded

(6) finally, you are woken up, and you get to spend the day doing whatever you like before returning the next morning for further surgery

Now, after each time that steps 1-6 occur, you should feel almost exactly the same (minus any nuisances of having undergone brain surgery, but with this future technology, those are minor). After all, only a very small portion of your brain was removed each time, and that piece was replaced with a piece that was designed to be functionally identical. If the portion removed was small enough, and the replacement piece was a sufficiently accurate replica, you shouldn’t even be able to tell that you’ve changed at all.
What’s more, you remember your life before surgery, you remember being put to sleep on the lab table, and you remember waking up. You have no more gap in your life than you would after a sleep where you don’t remember your dreams.

So it seems that after steps 1-6, you are still “you” in any meaningful sense (or if not EXACTLY you, you are still really, really close to you).

Okay, but suppose now that steps 1-6 are repeated many times (replacing one tiny brain region each morning and then spending the rest of each day living your normal life).

At some point, 50% of your brain has been replaced (whether measured by mass, volume, or “functionality,” it doesn’t matter). Are you still “you” in a meaningful sense?

In my experience, at this point in the thought experiment, most people will say that it’s definitely still you. With each tiny replacement, you couldn’t tell the difference, and as long as those replacements were each accurate enough, you shouldn’t even be able to tell the difference with 50% of your brain replaced. What’s more, there was never even any large gap in your consciousness.

But here’s the twist: as you realized a few months into the experiment (well before 50% of your brain was swapped), the surgeons had actually lied to you originally. You thought that they had been analyzing each piece of your brain bit by bit as they removed it in order to make replicas of each piece.
What they had ACTUALLY done (while you were asleep that very first time on the operating table) is scan your entire brain right away, building a nearly perfect, functioning, living replica, which they keep in a vat. This functioning replica is able to think just like you, has all your memories, and even BELIEVES that it is you. The replica is so accurate that, if the replica was hooked up to a machine that lets it talk, a person who knows you really well wouldn’t be able to tell which of you it was talking to (other than due to the fact that the replica is disembodied – which of course would change its behavior, but your own behavior would change in an equivalent way if you were disembodied).

So when the surgeons were removing tiny portions of your brain, they were also removing the closest possible corresponding tiny portion of the replica brain (after putting it to sleep in the same manner that they put you to sleep) and simply swapping it with yours. So the replica got your tiny portion, and you got the replica’s corresponding (and incredibly close to equivalent) tiny portion.

This lie from the surgeons doesn’t much change what happened to your brain FUNCTIONALLY since, in either case, each tiny portion of your brain is replaced with a very similar tiny replica portion. However, since the replica brain and your brain are having different experiences each day, there are genuinely irreconcilable differences between the tiny parts of the two brains (e.g., your brain may have some new connections in that tiny region that the replica doesn’t). The surgeons simply do their best to always swap the tiny part from your brain with the closest, most identical portion of the replica brain, even though the match isn’t 100% perfect. By making the tiny portions sufficiently tiny, you should still notice very little difference upon waking after each surgery.

Now, most people agree that when the replica of your brain is first created (before any swapping occurs), it is just like you, but it is not actually YOU. One way to think about this is that you still control your original body, and you still experience the thoughts of your original brain, whereas the replica separately experiences its own thoughts. In fact, you didn’t at first even know that a replica was ever created, you just went on having your normal experiences, so it seems that the replica being created had no effect (initially) on your identity.

But now that we’re at the halfway point, with half of the portions of your brain swapped out into the replica and half of the replica brain swapped into yours, personal identity becomes genuinely confusing. Both copies are now half original and half replica. In fact, both brains have a consistent series of memories, with no weird gaps (beyond the sort that one has from sleeping at night). The replica remembers going to sleep that first time on the surgery table just like you do; the only difference (as far as it is concerned) is that when it woke up, it didn’t have a body. Each day you and the replica both underwent surgery, and after each surgery, you both woke up being very close to identical to what you were prior to surgery.
While I think at this point most people still think that the original brain that slowly had 50% swapped out is more YOU than the replica that slowly had 50% swapped in, it’s gotten a lot harder to explain WHY. Both brains now have an equal amount of original material, and both feel equal, like they are the original you.

The thing is, the surgeons aren’t done at 50%. They decide to go all the way, continuing steps 1-6 until 100% of your brain has been swapped with the replica brain.

The really weird thing now is that the body walking around that you used to identify as YOU now has the replica brain in its head! And the vat where the surgeons keep the other brain has your original brain in it! In other words, the two brains are now just back to what they started at, with the only difference being that during the interim period, they were temporarily mixed up (continuing to gain experiences while mixed up), and “your” brain happened to end up in the vat whereas the replica ended up in your body.

Compare this whole thought experiment to the surgeons just (all in day one) removing your brain and putting it into the vat, then making a replica that they put into your body. The actual thought experiment is very similar to this, except that with the slow process of swapping a tiny portion each time, your two identities get more mingled (since those brain regions are all slightly changing each day as the two brains have experienced, one in a body, one in a vat).

At this point, people tend to be much more confused, but I think most would say that the brain in the vat is now the real YOU. After all, it is essentially entirely your original brain.

If this all sounds very abstract asking “which one is really YOU?”, then how about asking this more concrete question: now that you know the full details of the experiment, if one of the two brains (either the one in your body or the one in the vat) is going to be tortured in the future (after all the surgeries are fully complete) and you get to pick which of the two brains it happens to (you pick prior to the experiment beginning, of course), which one do you choose to be tortured (assuming here that you’re a selfish person that doesn’t want to experience torture)? You can also ask the same question (again, choosing before the experiment begins), assuming that the torture occurs after 50% of your brain is swapped with the replica (instead of 100%). In the 50% case, which brain would you choose to be tortured (the one in your body, or the one in the vat)?

Another way to think about what we mean by “YOU” in this thought experiment: suppose the brain that ends up in the vat and the brain that end up in the body both are properly hooked up to working eyes at the end of the experiment so that they can both see, and furthermore that when the experiment has totally finished the brain in the vat will get to see a picture of Tom Hanks, whereas the brain in the body will get to see a picture of Bob Saget if you (before the experiment has started) are asked “Later when the experiment is over will you see a picture of Tom Hanks or a picture of Bob Saget?” what would you answer? If you say “Tom Hanks,” then you expect to have the experiences of the brain that ends up in the vat, whereas if you say “Bob Saget,” then you expect to have the experiences of the brain that ends up in the body. Note that if, for someone reason, this question lacks meaning for you, then note that outside of weird philosophy of mind thought experiments, this sort of question is not only meaningful but has an obvious answer. For instance, if you knew someone was going to show me a picture of Bob Saget tomorrow and show you a picture of Tom Hanks tomorrow and someone asked which picture you expected to see tomorrow, you would say “Tom Hanks” – duh!

How can this scenario be interpreted?

(A) We could say that the brain in the body started out as YOU, and at some discrete point, things flipped so that YOU end up in the vat. But what seems to make no sense about that is where to draw the line on that flip. The only way this could happen is if there was exactly one surgery (which, remember, only after swaps out a tiny, functionally nearly identical brain region) which caused the brain in the body to flip to no longer being you and the brain in the vat to being fully you. If there was some special “soul” molecule in your brain which determined what was you, then this would make sense (since the moment that soul molecule was transferred, the brain in the vat would become you), but nobody seems to believe in such a soul molecule (neither scientists nor believers in souls).

(B) We could say that in the end, neither of the two brains are YOU in a meaningful sense. This seems pretty weird since, after the first small surgery, it seems nearly certain that the brain in your body is YOU in a meaningful sense (or very, very nearly you), so what operation prevents the being in the vat (which has all your original brain matter essentially, and has all your memories, and has the daily experience of being you daily) from being you? What exactly about this whole procedure fully eroded your identity?

(C) We could say that both brains are about equally much you (say, each close to 50% you). This seems weird because at which point did you split into two? It also seems weird because the brain in the vat is actually made of essentially all of your original brain material, so how did this slow process of taking piece by piece somehow make the replica brain have equal claim to being you? After all, if all the surgeons had done is take your brain out and move it to the vat immediately, we’d all agree that the vat brain was you, not the replica.

(D) We could say that BOTH brains end up as fully you. This seems even weirder because it’s not like what one of those brains thinks will causally change the behavior of the other. And the brain in the vat definitely doesn’t control your original body. So they can’t both be you in the sense of both being one indistinguishable consciousness. But maybe they could both be you in the sense of having equally strong claims to being you?

(E) We could say that this thought experiment is physically impossible. In order to work (as a thought experiment), it doesn’t actually matter how incredibly hard or unlikely this scenario is (the point is simply asking how we should interpret it if it WERE to occur). But if something about the thought experiment actually violates the laws of physics, then we can dismiss it, saying that it couldn’t take place in our universe, so the question being asked is not actually a question about our universe. But what aspect of the thought experiment violates the laws of physics?

(F) We could argue that although it doesn’t violate the laws of physics, the only way to do it without violating those laws would be to have the tiny swapped brain regions be a lot more different from each other than I imply. In other words, doing swaps of tiny brain regions is not impossible, but it’s impossible to make them extremely similar (e.g., because the brain is changing too fast each day, or because of error in such a procedure that is impossible – it’s directly implied by physical laws). This conclusion might push us a bit towards (B), thinking that neither brain is really you (they get too messed up in the process, so both drift from you in a meaningful sense).

(G) You’re still [the brain in] the body. You “stay in the body (fully and determinately) with every small exchange, so you stay in the body after the full exchange process is completed.” (David Chalmers takes this view, as he indicated in a comment on my original post of this essay)

(H) We are so deeply confused about identity that when we say it is or isn’t YOU, we don’t realize that we aren’t actually saying anything sensible/meaningful; we only think we are.

(I) We could say that YOU started out in the body, then for a while you were mingled with this other being merging your YOUNESS, but, by the end, you have separated again with YOU ending up in the vat (for much the same reason that we would say you are in the vat if your brain had originally just been cut out immediately and put in the vat – only in this case it was done bit by bit instead of all at once causing a temporary period of being merged).