Think about it – how do we classify the world around us? Are the categories we use, like "gold" or "tiger," somehow built into the fabric of reality, or are they just useful human inventions? And how does the way we _talk_ about these categories, the very words we use, relate to these fundamental questions about what exists? That's what this collection of work explores, and it's a lively debate with lots of twists and turns. ### What Exactly _Are_ Natural Kinds Anyway? The Metaphysical Quest! One of the big reasons philosophers get so interested in natural kinds is a purely metaphysical one: are there real "joints" in nature that our classifications, whether in everyday life or sophisticated science, might be latching onto?. It feels intuitive, doesn't it? When you think about 'cat,' 'silver,' 'carbon,' or 'electron,' they seem to pick out a fundamentally different kind of category than something like 'object bigger than a car' (let's call those 'nargs'!) or 'carbonated drink'. But articulating _what_ makes something 'natural' in this sense is the tricky part. Philosophers have taken several swings at this. Some say the mark of a natural kind is its success in prediction and explanation. If a concept lets us make reliable predictions and explain things well, that's a good sign it's tracking something real. Richard Boyd's "homeostatic property cluster" account is a prominent example of this approach, suggesting natural kinds are clusters of properties that tend to hang together due to underlying causal mechanisms. However, many metaphysicians feel there must be something more fundamentally _distinctive_ about natural kinds than just their predictive power. They might argue that natural kinds are like universals, existing objectively in the world, or that they possess "essences" – those underlying properties that are necessary and sufficient for something to be a member of that kind. The challenge here is that predictive success comes in degrees. Where do you draw the line between a highly successful scientific category and a merely useful common-sense one? If you want natural kinds to have a special metaphysical status, predictive power alone might not be enough to define them. Some philosophers, like Brian Ellis, try a more a priori approach, setting out specific criteria that a kind _must_ meet to be considered natural. His list includes conditions like being objective (mind-independent), having clear, non-vague boundaries, being defined by intrinsic properties, allowing for variation within the kind, fitting into a species-to-genus hierarchy, and having an intrinsic essence necessary and sufficient for membership. This kind of strict approach can lead to interesting consequences! For instance, Ellis rules out biological species like 'tiger' as natural kinds because evolution suggests boundaries between species aren't always categorically distinct. He also doubts whether chemical kinds like 'protein' or 'enzyme' fit his hierarchy requirement, suggesting they might not be genuine natural kinds despite their importance in science. From a less strict, perhaps more naturalistic perspective, these issues with biological or chemical kinds might not be problems at all. If natural kindhood is tied to the explanatory and predictive work the concepts do in science, then there's no reason why all scientifically useful kinds should form a perfect hierarchy or have perfectly sharp boundaries. The fact that we might argue about exactly when one species became another doesn't diminish the predictive power of evolutionary biology, for example. This highlights a key tension: do we define natural kinds based on strict, pre-determined metaphysical criteria, or do we look to successful science to tell us what counts?. Now, thinking about this: How might different scientific disciplines (physics, chemistry, biology, social sciences) approach classifying nature? Would they necessarily agree on what the 'real' kinds are? Does the success of a science like sociology or economics mean the categories they use, like 'social class' or 'capitalism,' might also be considered natural kinds in some sense? These are big questions that touch on how we see the relationship between science and reality. ### Talking About Kinds: The Semantic Story Beyond the metaphysical puzzle, philosophers of language have looked at whether the _words_ we use for natural kinds have some special semantic properties. Could the difference between 'cat' and 'object bigger than a car' just be about language? One simple idea might be semantic complexity – 'cat' is simple, 'object bigger than a car' is complex. But this is easily shot down; we could invent a simple word, a 'narg,' for objects bigger than a car, and it wouldn't feel like we've suddenly discovered a natural kind. Plus, scientifically precise terms like 'H2O' are complex but arguably pick out natural kinds. A much more influential idea comes from Saul Kripke and Hilary Putnam, proposing a causal theory of reference for kind terms. They suggested that general terms for natural kinds have a special kinship with proper names. This "Kripke-Putnam account" has been hugely important, suggesting that these terms pick out kinds not by fitting a description we associate with them, but through a more direct, causal link established at an initial "baptism" or introduction of the term. On the surface, you might not expect a deep connection between the metaphysician's search for "joints in nature" and the semanticist's search for a special class of terms. Kripke himself used examples like 'heat,' 'sound,' and 'lightning' as natural kind terms, which aren't always considered natural kinds by metaphysicians. And, as we saw, some metaphysicians like Ellis might not even consider 'tiger' or perhaps even 'water' (due to impurities) as true natural kinds. This suggests that the semantic category of "natural kind terms" and the metaphysical category of "natural kinds" might only partially overlap. This raises another interesting question: If a term like 'phlogiston' or 'ether' was introduced with the intention of picking out a natural kind, but science later showed there was no such kind, what does that tell us about the term's meaning or semantic status?. ### Rigidity: A Special Semantic Feature? One key idea linking natural kind terms to proper names is _rigidity_. For proper names, rigidity means the name refers to the same individual in all possible worlds where that individual exists. The question then becomes: can we extend this idea to general terms like 'water' or 'tiger'?. It seems intuitive that 'water' refers to the same kind (H2O) in all possible worlds, whereas a descriptive term like 'the transparent liquid in lakes' might refer to something different (or nothing!) in another world. However, applying rigidity to general terms quickly runs into problems. The most significant is the "trivialization problem" or "overgeneralization problem". If 'water' designating the same kind (H2O) in all possible worlds makes it rigid, it seems that 'philosopher' designating the kind philosopher in all possible worlds would also make it rigid. If this is the case, then rigidity doesn't seem to pick out _natural_ kind terms as special; it seems to apply to many, perhaps all, general terms. This leads some philosophers, like Åsa Wikforss, to argue against treating natural kind terms as a special semantic category based on rigidity. How do proponents of special natural kind terms respond? Some argue that the overgeneralization isn't a problem; perhaps many or all simple general terms _are_ rigid in some sense, and there isn't a special semantic distinction between 'philosopher' and 'tungsten' at this level. Others, like Corine Besson, propose a more refined notion of rigidity: _de jure obstinate rigidity_. This idea is linked to the _metasemantics_ – how the term gets its meaning in the first place. Besson suggests that natural kind terms (and proper names) are distinctive because they are introduced via a "dubbing" or a name-acquiring transaction. This isn't necessarily a single historical event, but a theoretical way of understanding how the term's reference is fixed directly, rather than being mediated by a description. This direct, stipulated reference means they designate their referent across all possible worlds _as a matter of convention_ (de jure obstinate), unlike descriptive terms which might only happen to refer to something existing in all worlds (de facto obstinate). This introduces the fascinating idea that how a word is _introduced_ or how its meaning is _fixed_ might be key to its semantic properties. But what counts as a 'dubbing' for something like 'water'? Does it have to be a specific moment in history, or is it more of a theoretical construct based on how the term functions?. And does this metasemantic story really succeed in drawing a clear line between natural kind terms and all others? ### Semantics Meets Metaphysics: Necessity and Essence One of the most significant impacts of Kripke-Putnam semantics has been on metaphysics, particularly the idea of _necessary a posteriori_ truths. These are truths that are necessarily true (true in all possible worlds) but can only be known through empirical investigation, not just by thinking or analyzing concepts. Kripke famously argued that identity statements like "Hesperus is Phosphorus" (about planets) or "Water is H2O" are examples. Since 'water' and 'H2O' (or the description 'the element with atomic number 79' for gold) are argued to be rigid designators, if 'water' and 'H2O' refer to the same thing in the actual world, they must do so in all possible worlds, making the identity necessary. But discovering that water _is_ H2O was a scientific discovery, not something we knew just by analyzing the concept of water. Many metaphysicians have embraced this, linking Kripkean "natural kind terms" directly to their metaphysical notion of natural kinds that have essences. The idea is that scientific investigation reveals the essence of a natural kind, and statements linking the kind term to its essence are necessary a posteriori truths. For example, the statement 'gold is the element with atomic number 79' is seen as revealing the essence of gold, which is necessarily having 79 protons, and this was discovered empirically. This opens up a whole can of philosophical worms! For example, does this mean the "laws of nature" which describe how kinds behave are also necessary truths?. If electrons are _essentially_ negatively charged, then it's necessarily true that electrons repel each other in certain circumstances, even if we only discovered this empirically. This challenges the traditional Humean view that laws of nature are just contingent generalizations about what happens to be the case. However, there are major debates here. Some philosophers question whether scientific discoveries like "Water is H2O" are discoveries of pre-existing, necessary essences, or if they involve an element of _stipulation_ or _refinement_ in how we use the terms. Joseph LaPorte argues that vernacular terms like 'water' have "open texture" – a kind of vagueness that means there's no determinate fact about their boundaries until scientists make decisions about how to refine their use. When scientists decide that heavy water _is_ a kind of water, or that the definition of 'element' is based on atomic number, they are making choices that fix the meaning and make statements like 'water is H2O' necessarily true, but it's an _empirically motivated stipulation_, not just a pure discovery of a hidden essence. Alexander Bird presents an alternative view, suggesting that such data might show conceptual shifts rather than the precisification of open texture, but he agrees that many essentialist claims (like 'the Ceylon spiny mouse is not a rabbit') can be discovered even if the full essence isn't known. Another challenge comes from questioning how broadly Kripke's idea of the necessary a posteriori can be applied. Helen Beebee and Nigel Sabbarton-Leary argue that philosophers like Brian Ellis are too quick to assume that _all_ of the things he calls "natural kinds" (including processes like refraction or properties like solubility) generate necessary a posteriori truths simply because they are scientifically important or have "essences" in his sense. They point out that many scientific "identities," like 'ununbium is the element with atomic number 112' or 'phosphorus trichloride is PCl3,' seem more like _analytic_ truths – knowable just from understanding chemical nomenclature – than necessary a posteriori truths discoverable through deep empirical investigation. If these are analytic truths about kinds, and analytic truths are supposedly not about "real essences" (on Ellis's view), then he faces a problem: either these kinds aren't natural kinds, or his account of necessity and natural kinds is inconsistent. This highlights the difficulty in linking semantic properties (like rigidity or analyticity) directly to deep metaphysical claims about necessity and essence. It also makes you wonder: How do we decide if a truth is analytic (true by meaning) or necessary a posteriori (true by empirical discovery of a necessary link)? Are there cases that blur the lines? And what are the implications for scientific practice if some fundamental scientific statements are seen as partly conventional? ### The Big Picture: Interacting Fields and Ongoing Debates As you can see, the topic of natural kinds is a vibrant crossroads for philosophy. Metaphysicians are asking about the structure of reality and what makes kinds distinct. Philosophers of language are puzzling over how our words pick out these kinds and what special semantic properties they might have. And philosophers of science are considering how scientific practice classifies the world and what kind of reality scientific categories track. These areas aren't working in isolation; ideas from one field deeply influence the others. The Kripke-Putnam account from philosophy of language has significantly shaped metaphysical views on essences and necessity. Debates in the metaphysics of science about classification and realism (like Boyd's accommodationism or Dupré's pluralism) challenge semantic accounts or offer alternative ways to understand the success of science without relying on strict essentialism. And the detailed examination of scientific examples, like chemical nomenclature or biological classification, puts pressure on philosophical theories about the criteria for natural kinds or the nature of necessary truths. Many fundamental questions remain hotly debated: - Is there really a distinct _semantic_ category of natural kind terms, or is their specialness due to their _scientific_ role in explanation and prediction?. - Do natural kinds have metaphysically robust essences, and if so, are statements about these essences discoverable necessary a posteriori truths, or are they in part products of our decisions and classifications?. - How does scientific classification relate to the 'real' divisions in nature? Are scientifically useful categories necessarily mind-independent, or can they be, in some sense, discipline-relative or even "social constructions" while still being real?. - How do philosophical concepts like rigidity, necessity, and essence apply, or fail to apply, across different scientific domains (physics, chemistry, biology, social sciences)?. The journey through the semantics and metaphysics of natural kinds is far from over. It requires careful attention to linguistic usage, deep metaphysical reflection, and a close look at how science actually operates. For further exploration, you might find yourself wondering: - How does the historical development of scientific terms (like 'element' or 'species') fit with the philosophical accounts of reference-fixing and semantic change?. - If natural kinds support inductive inferences, what does this mean for areas like social science or psychology? Are the categories they use also natural kinds in some sense, and what criteria would they need to meet?. - How do different metaphysical views on properties (like whether they have causal powers as part of their identity) bear on the debate between Humean and non-Humean accounts of necessity in science?