Without denying the obvious similarities that motivated the initial categorization {salmon, guppies, sharks, dolphins, trout, ...}, there is more structure in the world: to maximize the probability your world-model assigns to your observations of dolphins, you need to take into consideration the many aspects of reality in which the grouping {monkeys, squirrels, dolphins, horses ...} makes more sense.
The old category might have been âgood enoughâ for the purposes of the sailors of yore, but as humanity has learned more, as our model of Thingspace has expanded with more dimensions and more details, we can see the ways in which the original map failed to carve reality at the joints âŠ
So the one comes to youâa-gainâand says:
Hold on. In what sense did the original map fail to carve reality at the joints? You donât deny the obvious similarities between dolphins and fishâbetween dolphins and other fish. Thatâs a cluster in configuration space! The observation that dolphins are evolutionarily related to mammals may be an interesting fact that specialized professional evolutionary biologists care about for some inscrutable specialist reason. But Iâm not a professional biologist. Choosing to define categories around evolutionary relatedness rather than macroscopic human-relevant features seems like an arbitrary ĂŠsthetic whim. Why should I care about phylogenetics, at all?
This one is going to take a few paragraphs.
Focusing on evolutionary relatedness is not an arbitrary ĂŠsthetic whim because evolution actually happened. Evolution isnât just a story that our Societyâs specialists happen to have chosen because they liked it; they chose it because it predicts what we see in the world. You canât choose a substantively different theory and make the same predictions about the real world. (At most, youâd end up with an isomorphic theory with additional epiphenominal elements, asserting that an allele rose in frequency âbecauseâ the angels willed it, without an account of why the angelsâ will happens to line up with what would have transpired if there were no angels.) Similarly, category definitions represent hidden probabilistic inferences; you canât âredrawâ the âboundariesâ of the categories your mind actually uses and still make the same predictions about the real world. Accordingly, it shouldnât be surprising that our knowledge of evolution turns out to have implications for how we should categorize organismsânot as an ĂŠsthetic choice, but for structural reasons that can be understood mechanistically.
One element of the evolutionary worldview is a âcontinuityâ postulate: all else being equal, creatures that are more closely related are more similar in general. Creationists sometimes try to discredit evolution by ridiculing the absurdity of the idea that a monkey could give birth to a person. But actually, evolutionary biologists agree on the absurdity of that specific scenario. Monkeys donât suddenly give birth to humans in a single generation; if they did, that would utterly falsify our understanding of evolution! Rather, monkeys and humans had a common ancestor forty million years ago, with the separate lines of descent leading to present-day monkeys and present-day humans each accumulating their own differences one mutation at a time.
In the language of causal graphs, conditioning on the âdolphin DNAâ node in the diagram d-separates the paths between the âblowholeâ and âflippersâ nodes that run through the âdolphin DNAâ node. That means thatâassuming there arenât any other paths between âblowholeâ and âflippersâ that donât go through âdolphin DNAâââblowholeâ and âflippersâ become conditionally independent given âdolphin DNAâ: when I see a creature with a blowhole, that makes me more likely to think itâs a dolphin, which makes me more likely to think it has flippers, but given that I already know something is a dolphin, learning more about its flippers doesnât change my predictions about its blowhole.
What space? What do the dimensions of this space represent? âFeaturesâ? But there are no pre-existing âfeaturesâ in the world. Assuming the existence of a âspaceâ up front is punting on most of the actual AI challenge. âThereâs conditional independence structure in the causal graphâ is a meaningfully deeper explanation than âThereâs a cluster in configuration spaceâ, because conditional independence is what what makes it possible to construct a âspaceâ such that there are clusters. (Though this isnât a complete explanation: we still need to figure out where the âvariablesâ in the causal graph come from.)
Going beyond the configuration space metaphor is important because it lets us understand how we can learn new things about dolphins that we donât already know. Dolphins are complicated! Dolphins are complicated in a very specific way. Dolphins are fragile: the shortest computer program that simulates a dolphin requires many bits of initial information, and if you changed some of the bits, you wouldnât have a dolphin anymore. Complex functional adaptations are universal within a species because each beneficial allele has to reach fixation before there can be selection pressure for the next incremental improvement. Thatâs why itâs possible to claim that there are 206 bones in âtheâ human skeleton, even if most humans havenât had their bones counted. I havenât been able to find a citation on how many bones dolphins have, but Iâm confident that itâs the same number for all or nearly-all members of a particular dolphin species.
But ânumber of bonesâ wasnât one of the dimensions of the âspaceâ that we originally noticed the dolphin cluster in! Thatâs what the âcarving reality at the jointsâ metaphor means: genetic relatedness is an underlying generator of similarities, that includes the âfinned swimmy animalsâ properties that dolphins and fish have in common, but also includes many more high-dimensional details: how dolphins are warm-blooded, how dolphins have eyelids, the way female dolphins nurse their live-born young, the way male dolphins sometimes gang-rape female dolphins, the way dolphins sleep with only half their brain at a time, the specific bones in the (the!) dolphin skeleton (however many there turn out to be), the way dolphins swim in a circle to trick fish into jumping and being eaten, &c.
In contrast, âfinned swimmy animalsâ is an intrinsically less cohesive subject matter: there are similarities between them due to convergent evolution to the aquatic habitat, and it probably makes sense to want a short word or phrase (perhaps, âsea creaturesâ) to describe those similarities in contexts where only those similarities are relevant.
But that category âfalls apartâ very quickly as you consider more and more aspects of the creatures: the finned-swimmy-animals-with-gills are systematically different from the finned-swimmy-animals-with-a-blowhole, in more ways than just the ârespiratory organâ feature that Iâm using in this sentence to point to the two groups.
A âdefinitionâ is just a description that helps someone else pick out âthe sameâ natural abstraction in their own world-model: you canât pack everything there is to know about dolphins into the definition of the word âdolphinâ, in part because we donât know everything there is to know about dolphins as an empirical regularity in the real world. The âfinned swimmy animalsâ category less useful to the extent that it fails to compress more information than is contained in its definition. Blood is thicker than water (that is, the similarities induced by shared blood cluster in a âthickerâ (higher-dimensional) configuration space than the similarities induced by living in the water).
The one replies:
But what if I donât need to compress any more information than âfinned swimmy animalsâ? If Iâm watching a nature documentary, I donât think Iâm being done any favors by having word-structures that group lungfish and lamprey while excluding sea turtles. In general, the concepts I find useful respond to my immediate needs. I care more about âwould be at home atop a fruit pizzaâ rather than âeverything anatomically analogous to an appleâ. When a child points at a whale and says âlook, a fishâ, and youâre like âhaha no, its tail flaps horizontally and its grandma had hairâ, whoâs in the wrong here?
In some sense, sure: ignorance isnât better than knowledge if you donât care about knowing things. If you live in human civilization and donât need to carve up the world of aquatic life in much detailâif your use-case for thinking about aquatic animals is watching a nature documentary (for entertainment??) rather than living and working with them every day, then you might think the deeper causal structure isnât buying you anything. And for you and your extremely limited use-case, maybe it isnât. But you would likely change your mind if you were a veterinarian or a zoologist who actually had skin in the game in robustly describing this part of the world.
When people have skin in the game, they care about the underlying mechanisms and want short codewords for them, because the underlying mechanisms sometimes have decision-relevant implications. If you hurt your ankle while running, you would probably be interested to know whether it was a sprain or a stress fracture because that affects your decisions about how to recover. You wouldnât say, âWell, all I know is that my ankle hurtsâthatâs all a child would knowâso Iâm going to call it a hurtankle; I donât care about anatomy.â
You may not be intrinsically curious about anatomy, but even if the only thing you care about is relief from pain and recovering your mobility, you still benefit from living in a Society whose shared ontology distinguishes sprains and stress fractures being different things in the territory, even if they compress to the same point in your map of how much your ankle hurts right now. And you probably also benefit from living in a Society that can stabilize a shared map of living things based on the facts of evolutionary history, which we can all agree on in the limit of good science, unlike the vagaries of what I personally think tastes good on pizza.
When you think about it, it makes sense that our shared language ends up being optimized for robustly describing reality, rather than catering to the ignorance of people who donât have reasons to care about whether a particular distinction is actually robust. Personally, I confess I donât know the difference between alligators and crocodiles, and I donât particularly need to know: Iâm not likely to encounter either outside of a zoo or a nature documentary. But precisely because I donât need to know, you donât see me demanding that the rest of the world redefine one of these words as a hypernym that includes both. The people who write encyclopedias seem to think thereâs a difference, and since they probably know what theyâre doing, it makes sense for their opinion to have more weight on English language common usage than mineâat least until I were to start regularly ending up in situations where I need to point to an alligator-or-crocodile in my environment and I still didnât notice any differences.
Some animals that I do see in my local environment sometimes are cats and dogs, because people often keep them as pets. I benefit from having separate words (in my map) for cats and dogs, because I can see that cats and dogs are actually different (in the territory). If my pen pal from a faraway land that had no cats were to visit America and encounter a cat for the first time, he might remark, âWhat a strange dog!â If I were to reply, âActually, thatâs a cat; theyâre not the same thing as dogsâ, it would be pretty obnoxious if he were to snap back, âWhat kind of definitional gymnastics is this? Itâs a four-legged furry animal with a tail! As far as Iâm concerned, itâs a dog.â
Itâs true that dogs and cats are both four-legged furry animal with a tail. If you had never seen a cat before, or you didnât spend much time around four-legged furry tailed animals at all, it might not be immediately obvious why someone might want to allocate two words for these subcategories, or why anyone might oppose just using dog to refer to the supercategory. And yet thereâs some sense in which my countrymen who think cats and dogs are different things know what theyâre doing. My âActually, thatâs a catâ claim represented an attempt to convey information about the statistical structure of creatures in the real world, and my foreign friendâs insistence that he can define a word any way he wantsâto suit his ignorance, to avoid challenges to his current ontologyâfunctions to shut down that transfer of information.
But if you donât know what a better ontology can buy youâif you donât know that there are mathematical laws governing the use of categories in a rational mindâyou may not know what youâre missing. As part of a review of a book on post-traumatic stress disorder, psychiatrist Scott Alexander casually mentions the American Psychiatric Associationâs âphilosophical commitment to categorizing by symptoms rather than causeâ: â[w]hen the APA decides not to [recognize developmental trauma disorder], theyâre not necessarily rejecting the seriousness of child abuse, only saying itâs not the kind of thing they build their categories around.â
In a sane world, this would be utterly discrediting to the APA. The cognitive function of categories is to group relevantly similar things together in order to make similar predictions and decisions about them. But for the decisions involved in treating a condition, causes are of supreme relevance! Medical doctors understand this: we consider bacterial and viral infections to be different categories of disease even when they cause similar symptoms, because antibiotics can treat the former but not the latter. No matter what words are used to describe it, at some point your decision algorithm needs to categorize by cause in order to compute the correct treatment: for example, to give antibiotics to the patients with bacterial diseases and antivirals to the patients with viral diseases. If the authoritative body of professional psychiatrists has a âphilosophical commitmentâ against this, that means we donât have a science of psychiatry.
In short, if you care about making high-quality decisions, mechanisms matter and causality matters, and mechanisms and causality arenât necessarily pinned down by whatever particular high-level surface analogy happens to seem most salient to a particular human.
The one replies:
Okay, youâve convinced me that phylogenetics isâpotentiallyâof more than just specialist interest. But âfishâ are a paraphyletic category: descended from a common ancestor, but not including all the descendant groupsâin this case, excluding the tetrapods (amphibians, reptiles, mammals, birds, &c.). If youâve decided that you want to use phylogeny as the basis for your definitions, shouldnât you have the courage of your convictions and only admit monophyletic clades that include all descendants of a common ancestor?
But itâs not that weâve âdecidedâ that we âwantâ to define animal words based on phylogeny. Definitions are uninteresting; you canât change reality by choosing a different definition! When we find structure in the distribution of animals in the world, and we want to come up with a âdefinitionâ of a category in order to efficiently point to the structure to someone who doesnât already know what the words in our language refer to, weâre likely to end up talking about phylogenetics as a convenience, because the creatures that are actually all-around similar are actually related to each other for non-accidental reasons. But there is no principle that it would be hypocritical to betray, that definitions need to be monophyletic clades.
Itâs true that paraphyletic groups like fish are evolutionary non-events: thereâs no inherited feature that all fish share, that isnât also shared by the tetrapods. That doesnât mean we somehow canât or shouldnât talk about fish! Paraphyletic categoriesâdescendants of a common ancestor, but excluding one or more monophyletic groupsâcan make sense when the excluded groups have picked up some salient features not shared by the other âbranchesâ of the family. Tetrapods picked up a lot of adaptations specific to living on land; itâs not crazy to want to talk about their cousins that didnât do that, even if that means that some fish are more recently related to some tetrapods than they are to some other fish.
Noticing the relevance of evolutionary relatedness to optimal categorization doesnât mean being slavishly committed to taking âyears since last common ancestorâ as our only criterion for which creatures are relevantly similar. âYears since last common ancestorâ correlates with overall similarity, all other things being equal, but sometimes not all other things are equal, and people who arenât committed to the fallacy that words need to have a simple definition can take the other things into account.
If someone handed you a phylogenetic tree diagram of the development of life on some alien planet, and the diagram was only labeled with years and species names, without any other information about these alien creatures, you wouldnât have enough information to âcarve it at the jointsâ. You wouldnât spontaneously invent a paraphyletic groupingâbut you also also wouldnât know which monophyletic groups are most significant.
In contrast, when classifying life on Earth, weâre not in the position of making arbitrary cuts on an unlabeled tree diagram; rather, itâs only after thousands of person-years of studying the natural world that people were able to infer things about evolutionary history and discover the the correct diagram.
It shouldnât be that surprising that the distinctions we notice in the natural world are both tied to the evolutionary history, but also donât always correspond to monophyletic clades. The continuity postulate in the evolutionary worldview imposes the desideratum that good categories should at least be a connected set on âphylogenetic spaceâ, not that we should never want to talk about âthis clade, except for these few sub-clades that picked up a lot of important differencesâ as a category of interestâespecially when talking about present-day creatures. (We talk about âlast common ancestorsâ, but no one has seen such creatures that lived millions of years ago; everything but the very leaves of the phylogenetic tree are inferred, not observed.)
The claim that dolphins shouldnât be considered âfishâ because the alleged âcourage of our convictionsâ should make us disdain paraphyletic categories only makes sense as an attempted reductio ad absurdum, not as a consistent argument on its own terms: putting dolphins and fish together would be polyphyletic! Thatâs even worse! But as has just been explained, the reductio fails because the alleged principle being allegedly violated was never actually a principle of category formulation.
You know what else are paraphyletic taxa? Monkeys (excludes apes, even though the common ancestor of monkeys and apes was a monkey). Reptiles (excludes birds, even though the common ancestor of birds was a reptile). Protists (excludes animals, plants, and fungi, even though their common ancestor would have been a protist). Prokaryotes (excludes eukaryotes, even though the common ancestor of eukaryotes would have been a prokaryote). These are pretty commonsensical categories that it makes sense to have words for! But because of the continuity of evolution, itâs not a coincidence that these commonsensical categories that people want words for ended up being connected sets in phylogenetic space.
The one replies:
Not all of them did, though! âFishâ used to just mean the swimmy animals: in the Bible, Jonah was swallowed by a âgreat fishâ, thought to be a whale. It was only after we figured genealogy that some pedants decided that whales didnât count.
The pattern generalizes. Some determined contrarians might be inclined to argue âbats are birdsâ (flappy flying animals) on the same grounds as âdolphins are fishâ (flappy swimmy animals). But did you know the German word for bat is Fledermaus (âflutter mouseâ), which dates back to fledarmĆ«s in Old High German? Apparently, people way back in the tenth century or so (also long before evolution was understood) already thought bats were like a mammal-that-happened-to-fly rather than a bird-that-happened-to-be-furry.
Similarly, we recognize ostriches and penguins as birds on the basis of overall similarity, even though they donât fly (although we may sometimes qualify them as âflightless birdsâ, in recognition of the fact that most birds fly). It would seem that âflappy flying animalsâ is not the common usage meaning of bird.
To be sure, convergent evolution is a thing, such that sometimes we might want short codewords that point to the cluster-structure-produced-by-convergent-evolution rather than the conditional-independence-structure-produced-by-connectedness-in-phylogenetic-spaceâtrees, and possibly crabs, are a case in point. But itâs important to notice the differenceâto see through to the inferences your concepts are buying youâand what gets lost when you try to reason in a domain where your concept falls apart.
But although the quest is an empirical oneâsomething that can only be achieved by studying whatâs out there, not just by writing blog posts about philosophyâit turns out that a little bit of philosophy is necessary to ground the rules of the investigation. Not much. Just the basics. The mapâterritory distinction. Probability, clustering. Conditional independence.
Maybe someday it could be possible to have a real science of psychiatry that reflects the actual structure of the mind, instead of doing the equivalent of lumping sprains and stress fractures together as hurtankles. Maybe even greater achievements are possible. Personally, Iâm not optimistic about humanityâs prospects.
(Thanks to Tailcalled for the âroot of the causal graphâ observation and John S. Wentworth for explaining the importance of conditional independence.)
Blood Is Thicker Than Water đŹ
Followup to: Where to Draw the Boundaries?
Without denying the obvious similarities that motivated the initial categorization
{salmon, guppies, sharks, dolphins, trout, ...}
, there is more structure in the world: to maximize the probability your world-model assigns to your observations of dolphins, you need to take into consideration the many aspects of reality in which the grouping{monkeys, squirrels, dolphins, horses ...}
makes more sense.The old category might have been âgood enoughâ for the purposes of the sailors of yore, but as humanity has learned more, as our model of Thingspace has expanded with more dimensions and more details, we can see the ways in which the original map failed to carve reality at the joints âŠ
So the one comes to youâa-gainâand says:
This one is going to take a few paragraphs.
Focusing on evolutionary relatedness is not an arbitrary ĂŠsthetic whim because evolution actually happened. Evolution isnât just a story that our Societyâs specialists happen to have chosen because they liked it; they chose it because it predicts what we see in the world. You canât choose a substantively different theory and make the same predictions about the real world. (At most, youâd end up with an isomorphic theory with additional epiphenominal elements, asserting that an allele rose in frequency âbecauseâ the angels willed it, without an account of why the angelsâ will happens to line up with what would have transpired if there were no angels.) Similarly, category definitions represent hidden probabilistic inferences; you canât âredrawâ the âboundariesâ of the categories your mind actually uses and still make the same predictions about the real world. Accordingly, it shouldnât be surprising that our knowledge of evolution turns out to have implications for how we should categorize organismsânot as an ĂŠsthetic choice, but for structural reasons that can be understood mechanistically.
One element of the evolutionary worldview is a âcontinuityâ postulate: all else being equal, creatures that are more closely related are more similar in general. Creationists sometimes try to discredit evolution by ridiculing the absurdity of the idea that a monkey could give birth to a person. But actually, evolutionary biologists agree on the absurdity of that specific scenario. Monkeys donât suddenly give birth to humans in a single generation; if they did, that would utterly falsify our understanding of evolution! Rather, monkeys and humans had a common ancestor forty million years ago, with the separate lines of descent leading to present-day monkeys and present-day humans each accumulating their own differences one mutation at a time.
The fact that evolution persists information in the genome creates a regularity in the world that can be exploited by cognitive algorithms that know about phylogeny. In terms of the formalization of causality with directed acyclic graphs pioneered by Judea Pearl and others, an organismâs genome is at the root of the causal graph underlying all other features of an organism:
In the language of causal graphs, conditioning on the âdolphin DNAâ node in the diagram d-separates the paths between the âblowholeâ and âflippersâ nodes that run through the âdolphin DNAâ node. That means thatâassuming there arenât any other paths between âblowholeâ and âflippersâ that donât go through âdolphin DNAâââblowholeâ and âflippersâ become conditionally independent given âdolphin DNAâ: when I see a creature with a blowhole, that makes me more likely to think itâs a dolphin, which makes me more likely to think it has flippers, but given that I already know something is a dolphin, learning more about its flippers doesnât change my predictions about its blowhole.
But conditional independence assertions of this kind are exactly what makes âcategorizingâ a useful AI technique in the first place. Itâs often helpful to visualize this by claiming that entities in the same category belong to a cluster in some configuration space, but this handy visual metaphor is lacking in rigor and well-definedness.
What space? What do the dimensions of this space represent? âFeaturesâ? But there are no pre-existing âfeaturesâ in the world. Assuming the existence of a âspaceâ up front is punting on most of the actual AI challenge. âThereâs conditional independence structure in the causal graphâ is a meaningfully deeper explanation than âThereâs a cluster in configuration spaceâ, because conditional independence is what what makes it possible to construct a âspaceâ such that there are clusters. (Though this isnât a complete explanation: we still need to figure out where the âvariablesâ in the causal graph come from.)
Going beyond the configuration space metaphor is important because it lets us understand how we can learn new things about dolphins that we donât already know. Dolphins are complicated! Dolphins are complicated in a very specific way. Dolphins are fragile: the shortest computer program that simulates a dolphin requires many bits of initial information, and if you changed some of the bits, you wouldnât have a dolphin anymore. Complex functional adaptations are universal within a species because each beneficial allele has to reach fixation before there can be selection pressure for the next incremental improvement. Thatâs why itâs possible to claim that there are 206 bones in âtheâ human skeleton, even if most humans havenât had their bones counted. I havenât been able to find a citation on how many bones dolphins have, but Iâm confident that itâs the same number for all or nearly-all members of a particular dolphin species.
But ânumber of bonesâ wasnât one of the dimensions of the âspaceâ that we originally noticed the dolphin cluster in! Thatâs what the âcarving reality at the jointsâ metaphor means: genetic relatedness is an underlying generator of similarities, that includes the âfinned swimmy animalsâ properties that dolphins and fish have in common, but also includes many more high-dimensional details: how dolphins are warm-blooded, how dolphins have eyelids, the way female dolphins nurse their live-born young, the way male dolphins sometimes gang-rape female dolphins, the way dolphins sleep with only half their brain at a time, the specific bones in the (the!) dolphin skeleton (however many there turn out to be), the way dolphins swim in a circle to trick fish into jumping and being eaten, &c.
In contrast, âfinned swimmy animalsâ is an intrinsically less cohesive subject matter: there are similarities between them due to convergent evolution to the aquatic habitat, and it probably makes sense to want a short word or phrase (perhaps, âsea creaturesâ) to describe those similarities in contexts where only those similarities are relevant.
But that category âfalls apartâ very quickly as you consider more and more aspects of the creatures: the finned-swimmy-animals-with-gills are systematically different from the finned-swimmy-animals-with-a-blowhole, in more ways than just the ârespiratory organâ feature that Iâm using in this sentence to point to the two groups.
A âdefinitionâ is just a description that helps someone else pick out âthe sameâ natural abstraction in their own world-model: you canât pack everything there is to know about dolphins into the definition of the word âdolphinâ, in part because we donât know everything there is to know about dolphins as an empirical regularity in the real world. The âfinned swimmy animalsâ category less useful to the extent that it fails to compress more information than is contained in its definition. Blood is thicker than water (that is, the similarities induced by shared blood cluster in a âthickerâ (higher-dimensional) configuration space than the similarities induced by living in the water).
The one replies:
In some sense, sure: ignorance isnât better than knowledge if you donât care about knowing things. If you live in human civilization and donât need to carve up the world of aquatic life in much detailâif your use-case for thinking about aquatic animals is watching a nature documentary (for entertainment??) rather than living and working with them every day, then you might think the deeper causal structure isnât buying you anything. And for you and your extremely limited use-case, maybe it isnât. But you would likely change your mind if you were a veterinarian or a zoologist who actually had skin in the game in robustly describing this part of the world.
When people have skin in the game, they care about the underlying mechanisms and want short codewords for them, because the underlying mechanisms sometimes have decision-relevant implications. If you hurt your ankle while running, you would probably be interested to know whether it was a sprain or a stress fracture because that affects your decisions about how to recover. You wouldnât say, âWell, all I know is that my ankle hurtsâthatâs all a child would knowâso Iâm going to call it a hurtankle; I donât care about anatomy.â
You may not be intrinsically curious about anatomy, but even if the only thing you care about is relief from pain and recovering your mobility, you still benefit from living in a Society whose shared ontology distinguishes sprains and stress fractures being different things in the territory, even if they compress to the same point in your map of how much your ankle hurts right now. And you probably also benefit from living in a Society that can stabilize a shared map of living things based on the facts of evolutionary history, which we can all agree on in the limit of good science, unlike the vagaries of what I personally think tastes good on pizza.
When you think about it, it makes sense that our shared language ends up being optimized for robustly describing reality, rather than catering to the ignorance of people who donât have reasons to care about whether a particular distinction is actually robust. Personally, I confess I donât know the difference between alligators and crocodiles, and I donât particularly need to know: Iâm not likely to encounter either outside of a zoo or a nature documentary. But precisely because I donât need to know, you donât see me demanding that the rest of the world redefine one of these words as a hypernym that includes both. The people who write encyclopedias seem to think thereâs a difference, and since they probably know what theyâre doing, it makes sense for their opinion to have more weight on English language common usage than mineâat least until I were to start regularly ending up in situations where I need to point to an alligator-or-crocodile in my environment and I still didnât notice any differences.
Some animals that I do see in my local environment sometimes are cats and dogs, because people often keep them as pets. I benefit from having separate words (in my map) for cats and dogs, because I can see that cats and dogs are actually different (in the territory). If my pen pal from a faraway land that had no cats were to visit America and encounter a cat for the first time, he might remark, âWhat a strange dog!â If I were to reply, âActually, thatâs a cat; theyâre not the same thing as dogsâ, it would be pretty obnoxious if he were to snap back, âWhat kind of definitional gymnastics is this? Itâs a four-legged furry animal with a tail! As far as Iâm concerned, itâs a dog.â
Itâs true that dogs and cats are both four-legged furry animal with a tail. If you had never seen a cat before, or you didnât spend much time around four-legged furry tailed animals at all, it might not be immediately obvious why someone might want to allocate two words for these subcategories, or why anyone might oppose just using dog to refer to the supercategory. And yet thereâs some sense in which my countrymen who think cats and dogs are different things know what theyâre doing. My âActually, thatâs a catâ claim represented an attempt to convey information about the statistical structure of creatures in the real world, and my foreign friendâs insistence that he can define a word any way he wantsâto suit his ignorance, to avoid challenges to his current ontologyâfunctions to shut down that transfer of information.
But if you donât know what a better ontology can buy youâif you donât know that there are mathematical laws governing the use of categories in a rational mindâyou may not know what youâre missing. As part of a review of a book on post-traumatic stress disorder, psychiatrist Scott Alexander casually mentions the American Psychiatric Associationâs âphilosophical commitment to categorizing by symptoms rather than causeâ: â[w]hen the APA decides not to [recognize developmental trauma disorder], theyâre not necessarily rejecting the seriousness of child abuse, only saying itâs not the kind of thing they build their categories around.â
In a sane world, this would be utterly discrediting to the APA. The cognitive function of categories is to group relevantly similar things together in order to make similar predictions and decisions about them. But for the decisions involved in treating a condition, causes are of supreme relevance! Medical doctors understand this: we consider bacterial and viral infections to be different categories of disease even when they cause similar symptoms, because antibiotics can treat the former but not the latter. No matter what words are used to describe it, at some point your decision algorithm needs to categorize by cause in order to compute the correct treatment: for example, to give antibiotics to the patients with bacterial diseases and antivirals to the patients with viral diseases. If the authoritative body of professional psychiatrists has a âphilosophical commitmentâ against this, that means we donât have a science of psychiatry.
In short, if you care about making high-quality decisions, mechanisms matter and causality matters, and mechanisms and causality arenât necessarily pinned down by whatever particular high-level surface analogy happens to seem most salient to a particular human.
The one replies:
But itâs not that weâve âdecidedâ that we âwantâ to define animal words based on phylogeny. Definitions are uninteresting; you canât change reality by choosing a different definition! When we find structure in the distribution of animals in the world, and we want to come up with a âdefinitionâ of a category in order to efficiently point to the structure to someone who doesnât already know what the words in our language refer to, weâre likely to end up talking about phylogenetics as a convenience, because the creatures that are actually all-around similar are actually related to each other for non-accidental reasons. But there is no principle that it would be hypocritical to betray, that definitions need to be monophyletic clades.
Itâs true that paraphyletic groups like fish are evolutionary non-events: thereâs no inherited feature that all fish share, that isnât also shared by the tetrapods. That doesnât mean we somehow canât or shouldnât talk about fish! Paraphyletic categoriesâdescendants of a common ancestor, but excluding one or more monophyletic groupsâcan make sense when the excluded groups have picked up some salient features not shared by the other âbranchesâ of the family. Tetrapods picked up a lot of adaptations specific to living on land; itâs not crazy to want to talk about their cousins that didnât do that, even if that means that some fish are more recently related to some tetrapods than they are to some other fish.
Noticing the relevance of evolutionary relatedness to optimal categorization doesnât mean being slavishly committed to taking âyears since last common ancestorâ as our only criterion for which creatures are relevantly similar. âYears since last common ancestorâ correlates with overall similarity, all other things being equal, but sometimes not all other things are equal, and people who arenât committed to the fallacy that words need to have a simple definition can take the other things into account.
If someone handed you a phylogenetic tree diagram of the development of life on some alien planet, and the diagram was only labeled with years and species names, without any other information about these alien creatures, you wouldnât have enough information to âcarve it at the jointsâ. You wouldnât spontaneously invent a paraphyletic groupingâbut you also also wouldnât know which monophyletic groups are most significant.
In contrast, when classifying life on Earth, weâre not in the position of making arbitrary cuts on an unlabeled tree diagram; rather, itâs only after thousands of person-years of studying the natural world that people were able to infer things about evolutionary history and discover the the correct diagram.
It shouldnât be that surprising that the distinctions we notice in the natural world are both tied to the evolutionary history, but also donât always correspond to monophyletic clades. The continuity postulate in the evolutionary worldview imposes the desideratum that good categories should at least be a connected set on âphylogenetic spaceâ, not that we should never want to talk about âthis clade, except for these few sub-clades that picked up a lot of important differencesâ as a category of interestâespecially when talking about present-day creatures. (We talk about âlast common ancestorsâ, but no one has seen such creatures that lived millions of years ago; everything but the very leaves of the phylogenetic tree are inferred, not observed.)
The claim that dolphins shouldnât be considered âfishâ because the alleged âcourage of our convictionsâ should make us disdain paraphyletic categories only makes sense as an attempted reductio ad absurdum, not as a consistent argument on its own terms: putting dolphins and fish together would be polyphyletic! Thatâs even worse! But as has just been explained, the reductio fails because the alleged principle being allegedly violated was never actually a principle of category formulation.
You know what else are paraphyletic taxa? Monkeys (excludes apes, even though the common ancestor of monkeys and apes was a monkey). Reptiles (excludes birds, even though the common ancestor of birds was a reptile). Protists (excludes animals, plants, and fungi, even though their common ancestor would have been a protist). Prokaryotes (excludes eukaryotes, even though the common ancestor of eukaryotes would have been a prokaryote). These are pretty commonsensical categories that it makes sense to have words for! But because of the continuity of evolution, itâs not a coincidence that these commonsensical categories that people want words for ended up being connected sets in phylogenetic space.
The one replies:
But the claim that the distinction between fish and cetaceans (dolphins and whales) was only recognized after their differing evolutionary histories were discovered is just false to historical fact. Aristotle, writing in the fourth century BCE, already distinguished cetaceans from fish (âVery extensive genera of animals, into which other subdivisions fall, are the following: one, of birds; one, of fishes; and another, of cetaceansâ). Aristotle was not being a phylogenetics pedant, because Aristotle did not know about evolution! He actually noticed the differences!
The pattern generalizes. Some determined contrarians might be inclined to argue âbats are birdsâ (flappy flying animals) on the same grounds as âdolphins are fishâ (flappy swimmy animals). But did you know the German word for bat is Fledermaus (âflutter mouseâ), which dates back to fledarmĆ«s in Old High German? Apparently, people way back in the tenth century or so (also long before evolution was understood) already thought bats were like a mammal-that-happened-to-fly rather than a bird-that-happened-to-be-furry.
Similarly, we recognize ostriches and penguins as birds on the basis of overall similarity, even though they donât fly (although we may sometimes qualify them as âflightless birdsâ, in recognition of the fact that most birds fly). It would seem that âflappy flying animalsâ is not the common usage meaning of bird.
To be sure, convergent evolution is a thing, such that sometimes we might want short codewords that point to the cluster-structure-produced-by-convergent-evolution rather than the conditional-independence-structure-produced-by-connectedness-in-phylogenetic-spaceâtrees, and possibly crabs, are a case in point. But itâs important to notice the differenceâto see through to the inferences your concepts are buying youâand what gets lost when you try to reason in a domain where your concept falls apart.
The power to define concepts is the power to delimit thought, to determine what kinds of inferences are easily representable. Finding the right concepts to explain and control the world we see is a fundamentally empirical challenge, a scientific challengeâto see the difference between things that seem similar and to see the similarities between things which seem different.
But although the quest is an empirical oneâsomething that can only be achieved by studying whatâs out there, not just by writing blog posts about philosophyâit turns out that a little bit of philosophy is necessary to ground the rules of the investigation. Not much. Just the basics. The mapâterritory distinction. Probability, clustering. Conditional independence.
Maybe someday it could be possible to have a real science of psychiatry that reflects the actual structure of the mind, instead of doing the equivalent of lumping sprains and stress fractures together as hurtankles. Maybe even greater achievements are possible. Personally, Iâm not optimistic about humanityâs prospects.
Iâm sure of one thing, though. If there is a better world out there, a way to unlock the secrets of the universe and wield them in the service of our values, itâs only possible if we stop playing nitwit games and admit that dolphins donât belong on the fish list.
(Thanks to Tailcalled for the âroot of the causal graphâ observation and John S. Wentworth for explaining the importance of conditional independence.)