We recently wrote a post about Maciej Cegłowski’s essay Scott And Scurvy, a fascinating account of how the cure for scurvy was discovered, lost, and then by incredible chance, discovered again. At the time we said that this essay is one of the most interesting things we’ve ever read, and that we hoped to write more about it in the future. It was, we do, and here we go.

In the other post, we talked about what the history of scurvy can teach us about contradictory evidence — stuff that appears to disprove a theory, even though it doesn’t always. In this post, we want to talk about something different: the power of concepts.

First we’re gonna show you how bad it can be if you don’t have concepts you need. Then we’re going to show you how bad it can be if you DO have concepts you DON’T need.

Diseases of Deficiency

As Cegłowski puts it:

There are several aspects of this ‘second coming’ of scurvy in the late 19th century that I find particularly striking … [one was] how difficult it was to correctly interpret the evidence without the concept of ‘vitamin’. Now that we understand scurvy as a deficiency disease, we can explain away the anomalous results that seem to contradict that theory (the failure of lime juice on polar expeditions, for example). But the evidence on its own did not point clearly at any solution. It was not clear which results were the anomalous ones that needed explaining away. The ptomaine theory made correct predictions (fresh meat will prevent scurvy) even though it was completely wrong.

We’re not quite sure if he’s right about the concept of “vitamin” — even James Lind seems to have thought the cure was something in certain foods, maybe the fact that they were so tart and acidic. More critical might be the problem of focusing on the noticeable aspect of citrus (they are very tart) and missing the hidden reason it actually cures scurvy (high in vitamin C). Not sure what advice we could give there except “don’t mistake flash for substance”, but that’s easier said than done.

But we do wonder about the concept of a deficiency disease in the first place. Even James Lind thought that scurvy was actually caused by damp air, and vegetable acids were just a way to fight back. Vegetable acids were thought to be cures, not essential nutrients. They were “antiscorbutic” like “antibiotic”. The concept of a deficiency disease doesn’t seem to have existed before the 1880s and got almost no mention until 1900, at least not under that name:

Without this concept, it does seem like doctors of the 19th century were missing an important tool in their mental toolbox for fighting disease. 

This reminds us of other problems in global medicine — maybe we should introduce the idea of a “contamination disease”. People are already familiar with this concept to a point — lead poisoning, arsenic poisoning, etc. — but people don’t look at a disease and think, maybe it’s from a contaminant. In fact, they often look at the symptoms of exposure to contaminants and think, that’s an (infectious) disease.

A good example is so-called Minamata disease. In 1956, in the city of Minamata, Japan, a pair of sisters presented with a set of disturbing symptoms, including convulsions. Soon the neighbors were showing signs as well. Doctors diagnosed an “epidemic of an unknown disease of the central nervous system”, which they called “Minamata disease”. They assumed it was contagious and took the necessary precautions. 

But soon they started hearing about mysterious cases of cats and birds showing similar symptoms, having convulsions or falling from the sky. Eventually they figured out “Minamata disease” was not contagious at all — the disease was methylmercury poisoning, the result of mercury compounds a local Chisso chemical factory was leaking into the harbor.

You might say, “Well it was not a disease at all; they were poisoned. If SMTM are right, then obesity isn’t a disease either; everyone has just been low-grade poisoned all at once.” We think this highlights the need for a deeper discussion about our categories!

“Disease” really does come from just “dis” “ease”. If you’re a doctor and someone comes to you, and they are not at ease, they are diseased, and that’s what you should care about. The disease might ultimately be bacterial, or viral, or an allergy, or a parasite, or the result of a deficiency, or the result of exposure to a harmful contaminant or poison, but it’s still a disease. For more discussion of this particular point, see here, also coincidentally about obesity, we didn’t stack the deck on this one it’s from 2010.

(If we were being really strict, we would say that obesity is a symptom, because conditions like Cushing’s Syndrome and drugs like Haldol can cause it too. If one or more contaminants also cause obesity, then the result of that exposure is a contamination disease, with obesity as a symptom. For more discussion of THIS particular point, see here.)

Lemon Mold Lime Mold

One of the weirdest things Cegłowski describes is how back in the day, people used the words “lemon” and “lime” interchangeably to describe any citrus fruit, which they thought of as a single category:

The scheduled allowance for the sailors in the Navy was fixed at I oz.lemon juice with I + oz. sugar, served daily after 2 weeks at sea, the lemon juice being often called ‘lime juice’ and our sailors ‘lime juicers’. The consequences of this new regulation were startling and by the beginning of the nineteenth century scurvy may be said to have vanished from the British navy. In 1780, the admissions of scurvy cases to the Naval Hospital at Haslar were 1457; in the years from 1806 to 1810, they were two. 

(As we’ll see, the confusion between lemons and limes would have serious reprecussions.)

This ended up making a huge difference in the tale of the tragedy of scurvy cures:

When the Admiralty began to replace lemon juice with an ineffective substitute in 1860, it took a long time for anyone to notice. In that year, naval authorities switched procurement from Mediterranean lemons to West Indian limes. The motives for this were mainly colonial – it was better to buy from British plantations than to continue importing lemons from Europe. Confusion in naming didn’t help matters. Both “lemon” and “lime” were in use as a collective term for citrus, and though European lemons and sour limes are quite different fruits, their Latin names (citrus medica, var. limonica and citrus medica, var. acida) suggested that they were as closely related as green and red apples. Moreover, as there was a widespread belief that the antiscorbutic properties of lemons were due to their acidity, it made sense that the more acidic Caribbean limes would be even better at fighting the disease. 

In this, the Navy was deceived. Tests on animals would later show that fresh lime juice has a quarter of the scurvy-fighting power of fresh lemon juice. And the lime juice being served to sailors was not fresh, but had spent long periods of time in settling tanks open to the air, and had been pumped through copper tubing. A 1918 animal experiment using representative samples of lime juice from the navy and merchant marine showed that the ‘preventative’ often lacked any antiscorbutic power at all.

It’s worth focusing on one part of this passage in particular: “Both ‘lemon’ and ‘lime’ were in use as a collective term for citrus.” This seems to be the case. As far as we can tell, the word “citrus” wasn’t really used prior to 1880. It was probably introduced as a scientific term for the genus before slowly working its way into common usage. Before then, “lemon” dominated the conversation, and “lime” dominated ten times over:

Maybe it’s not surprising that it took the language a while to sort itself out, but it still seems surprising that your great-great-grandfather didn’t think to distinguish between two fruits that you can tell apart at a glance. Even so, we think there are a couple of reasons to be sympathetic.

The name stuff is confusing, but swapping out one citrus fruit for another seems understandable, even if it ended up being misguided. To Europeans at the time, the thing that stood out about limes AND lemons was how tart they were, so it’s not surprising that they thought that the incredible tartness of these fruits was critical to the role they played in treating scurvy. But sourness in citrus fruits generally comes from citric acid, not vitamin C / ascorbic acid (incidentally, this is ascorbic as in “antiscorbutic”). Unfortunately, they had no way of knowing that. 

The second reason to be sympathetic is this: People mixed up limes and lemons in the 1800s. You may laugh but actually you are mixing up citrus right now.

The lemon is a single species of fruit, Citrus limon. It’s a specific species of tree that gives a specific yellow fruit that is high in citric acid and high in vitamin C. If you go to the store and buy a lemon, you know what you’re getting.

(Well, mostly. The Wikipedia page for lemons has a section called “other citrus called ‘lemons’”, which lists six other citrus fruits that are also called lemons, like the rough lemon and the Meyer lemon. But besides this, a lemon is a lemon.)

In comparison, the Wikipedia article on limes says,

There are several species of citrus trees whose fruits are called limes, including the Key lime (Citrus aurantiifolia), Persian lime, Makrut lime, and desert lime. … Plants with fruit called “limes” have diverse genetic origins; limes do not form a monophyletic group.

The very first section of the article is called, “plants known as ‘lime’”, which gives you a sense of how vague the name “lime” really is. The list they give includes the Persian lime, the Rangpur lime, the Philippine lime, the Makrut Lime, the Key Lime, four different Australian limes, and several things called “lime” that are not even citrus fruits, including the Spanish lime and two different plants called the wild lime. They also say: 

The difficulty in identifying exactly which species of fruit are called lime in different parts of the English-speaking world (and the same problem applies to synonyms in other European languages) is increased by the botanical complexity of the citrus genus itself, to which the majority of limes belong. Species of this genus hybridise readily, and it is only recently that genetic studies have started to shed light on the structure of the genus. The majority of cultivated species are in reality hybrids, produced from the citron (Citrus medica), the mandarin orange (Citrus reticulata), the pomelo (Citrus maxima) and in particular with many lime varieties, the micrantha (Citrus hystrix var. micrantha).

This means there is not even a straight answer to a question like “how much vitamin C is in a lime?” — there are at least a dozen different fruits that are commonly called “limes”, they all contain different amounts of vitamin C, and many of them are not even related to each other.

The British Admiralty seems to have switched from lemons grown in Sicily to West Indian limes. You probably know these as Key limes, and in case the nomenclature isn’t complicated enough, they’re also called bartender’s limes, Omani limes, or Mexican limes. We’ll stick with “Key lime” because that’s probably the name you know because it makes me think of pie. Mmmm, pie. 

The kind of limes you buy at the store, Persian limes, are a cross between Key limes and lemons. We can’t find any actual tests of the amount of vitamin C in Key limes, so we think all the published estimates of the amount of vitamin C in limes are probably from Persian limes.

We generally see numbers of about 50 mg/100 g vitamin C for lemons and about 30 mg/100 g for limes, presumably Persian limes. Since Persian limes are a cross between lemons and Key limes, Key limes probably have less than 30 mg/100 g vitamin C. Genetics isn’t this simple, but if we were to assume that Persian limes are the average of their forebears, then Key limes would contain about 10 mg/100 g vitamin C, less than a tomato. You need about 10 mg of vitamin C per day to keep from getting scurvy, so already we can see why this might be a problem.

Cooking reduces the vitamin C content of vegetables by about 40% (though of course this varies widely with specific conditions), so the 50 mg or so in a lemon would become about 30 mg after boiling, but the 30 mg in a lime would become about 18 mg after boiling. Lemon juice would be as good of an antiscorbutic after boiling as Persian lime juice would be fresh, and Key limes seem like they would have less vitamin C than either, boiled or not.

Persian limes also turn yellow as they ripen — you only think of them as green because farmers pick them and send them to the grocery store before they change colors. And of course, lemons are green in their early stages of growth. So like, so much for the “limes are green and lemons are yellow” thing.

All these issues pale in comparison to the fact that citrus taxonomy is insane. Not only are limes not limes, it seems like nothing is really anything, or maybe anything is everything.

You walk into a supermarket and you think you recognize a bunch of Platonic fruits — oranges, clementines, lemons, limes, grapefruits, and so on. But when you do a Google image search for “citrus genetics”, you get diagrams like:

And this diagram:

And this diagram:

And this diagram, in which the Bene Gesserit attempt to breed the Kumquat Haderach. No, really.

The written material on the subject is, if anything, even more disheartening. Let’s look at a couple passages from the Wikipedia article on citrus taxonomy:

Citrus taxonomy is complex and controversial. Cultivated citrus are derived from various citrus species found in the wild. Some are only selections of the original wild types, many others are hybrids between two or more original species, and some are backcrossed hybrids between a hybrid and one of the hybrid’s parent species. Citrus plants hybridize easily between species with completely different morphologies, and similar-looking citrus fruits may have quite different ancestries. … Conversely, different-looking varieties may be nearly genetically identical, and differ only by a bud mutation.

The same common names may be given to different species, citrus hybrids or mutations. For example, citrus with green fruit tend to be called ‘limes’ independent of their origin: Australian limes, musk limes, Key limes, kaffir limes, Rangpur limes, sweet limes and wild limes are all genetically distinct. Fruit with similar ancestry may be quite different in name and traits (e.g. grapefruit, common oranges, and ponkans, all pomelo-mandarin hybrids). Many traditional citrus groups, such as true sweet oranges and lemons, seem to be bud sports, clonal families of cultivars that have arisen from distinct spontaneous mutations of a single hybrid ancestor. Novel varieties, and in particular seedless or reduced-seed varieties, have also been generated from these unique hybrid ancestral lines using gamma irradiation of budwood to induce mutations.

For more on using radiation to make new fruit, please refer to these talking dinosaurs.

In case that isn’t weird enough for you, there’s even a graft chimera citrus called the Bizzaria (really), which produces fruits that look like this: 

On limes in particular, this page says: 

Limes: A highly diverse group of hybrids go by this name. Rangpur limes, like rough lemons, arose from crosses between citron and mandarin. The sweet limes, so-called due to their low acid pulp and juice, come from crosses of citron with either sweet or sour oranges, while the Key lime arose from a cross between a citron and a micrantha.

All of these hybrids have in turn been bred back with their parent stocks or with other pure or hybrid citrus to form a broad array of fruits. Naming of these is inconsistent, with some bearing a variant of the name of one of the parents or simply another citrus with superficially-similar fruit, a distinct name, or a portmanteau of ancestral species.

…

While most other citrus are diploid, many of the Key lime hybrid progeny have unusual chromosome numbers. For example, the Persian lime is triploid, deriving from a diploid Key lime gamete and a haploid lemon ovule. A second group of Key lime hybrids, including the Tanepao lime and Madagascar lemon, are also triploid but instead seem to have arisen from a backcross of a diploid Key lime ovule with a citron haploid gamete. The “Giant Key lime” owes its increased size to a spontaneous duplication of the entire diploid Key lime genome to produce a tetraploid. [Editor’s note: uhhhhh]

Pretty much every citrus page on Wikipedia has shit like this, truly enough to drive a man mad. You wander onto Wikipedia trying to find out what in god’s name a lumia is, and soon you are reading this: “A recent genomic analysis of several species commonly called ‘lemons’ or ‘limes’ revealed that the various individual lumias have different genetic backgrounds. The ‘Hybride Fourny’ was found to be an F1 hybrid of a citron-pomelo cross, while the ‘Jaffa lemon’ was a more complex cross between the two species, perhaps an F2 hybrid. The Pomme d’Adam arose from a citron-micrantha cross, while two other lumias, the ‘Borneo’ and ‘Barum’ lemons, were found to be citron-pomelo-micrantha mixes.” Lovecraft, eat your heart out. 

This is the much deeper problem that the history of scurvy reveals. In science, you need tools you can trust. You need to have the right equipment, the right study design, and the right analysis techniques — but you also need the right concepts.

Most of us are trained to calibrate our equipment and to double-check our experimental designs, but how often do we reconcile our concepts? Back in the 1800s, they trusted the terms “lemon” and “lime” to be relevant, to be reliable, to be meaningful — and to be interchangeable, to mean the same thing as each other. But they were all of them, deceived. 

This will continue to be a problem forever. We distinguish between lemons and limes today, and we’re better off for it, but we aren’t safe and can’t afford to forget this problem. “Lime” is still considered a perfectly good tool, and if you were going to do a study on whether limes are good for your heart or something, no one except for citrus geneticists would think anything of it.

But “lime”, as we have hopefully convinced you today, is not a good category at all! It’s not a good tool. You can’t trust it. Yet the assumption that “lime” is a perfectly normal category is so deeply embedded that you never realized it was an assumption.

Evaluating simple propositions like “limes cure scurvy” depends on accepting that “limes”, “scurvy”, and even “cure” are coherent and meaningful concepts. But they may not be!

The TRUE way that reality is very weird is that words and concepts that you use every day and take entirely for granted may be just as incoherent as the term “lime”. Concepts you think of as normal may some day seem as crazy as using the words “lemon” and “lime” interchangeably for all citrus fruits. We can pretty much guarantee that this will happen for something.

In our last post we described “splitting” as the practice of coming up with weird special cases or new distinctions between categories in the face of contradictory evidence. Splitting concepts is especially risky, in part because concepts are so powerful. If there is a confusion of categories, then all the research up to that point will be hopelessly confused as well, entirely muddled.

But if you split the categories in a better way, you will suddenly be left facing nothing but low-hanging fruit — be they lemons, limes, other limes, grapefruits, other other limes, clementines, pomelos, lumias, etrogs, etc.