Natural kinds

Natural kinds are widely understood to be the real classifications of things that actually exist in the world. Natural kinds are the categories we tend to aim for when we seek to understand the world, as it really is. Discovering what these real classifications are is often considered to be the project of scientific research in many fields from astronomy and agronomy to zoology and zymurgy. When we discover something unfamiliar to us and we want to know what sort of thing it is, we might ask: ‘what kind is that?’ For instance, in a physics class, we might ask: ‘what kind of quark is that?’, where the answer might be: ‘that is a charm quark’. In biology, we might ask: ‘what kind of plant is that?’, with the answer being: ‘it is a Venus flytrap (Dionaea muscipula)’. Or, in chemistry, we might ask: ‘what element is that?’, with the answer being: ‘lithium’. Knowing that the thing we asked about is a member of that particular kind tells us a lot about it if it is a natural kind. Membership in a natural kind tells us that the thing in question shares many important characteristics with other things that are in the same natural kind category. For example, consider the category of Venus flytraps. All plants that belong to that category share many important characteristics; among these include: perennial flowering, carnivorous eating habits, capable of thigmonastic responses (closing their ‘trap’ when prey alight on their trichomes), ability to photosynthesise, belonging to the family Droseraceae and the kingdom Plantae. Some of these important characteristics and properties have been referred to as ‘essential’ by philosophers because they are the properties that are thought to be necessary for the thing to be a member of that natural kind. That means that if the thing does not have those necessary properties, then it cannot be a member of that natural kind.

When we ask the question: ‘what kind is that?’, we do not always discover natural kinds. Sometimes when we ask: ‘what kind of thing is that?’, we find out, for instance, that this thing that we are asking about is green. This means we find out that it belongs to the category of green things. The category of green things is a kind category, but it is not a natural kind. The category of all green things includes the Venus flytrap but also green traffic lights, green tea, guacamole, collards, and dark jade-painted 1978 Ford Mustang sportscars. What all of these things have in common is that they are all green. However, they do not share any other properties or characteristics apart from being green. Green kinds of things are not natural kinds like those mentioned earlier. The kinds that are picked out by the classifications of charm quark, Venus flytrap, and lithium are considered to be very different from the classification of green things. Whilst all charm quarks, all Venus flytraps, and all samples of lithium are each considered to be classifications of natural kinds, the category of green things is not. The philosophical question that arises is: ‘what makes classifications like that of the natural kind that includes all charm quarks natural and classifications like that of all green things not natural?’ Put a different way: ‘what makes something a natural kind and how can we tell the difference between natural kinds and what we might call “artificial kinds”, like the grouping of green things?’ A popular answer to this question is that natural kinds pick out natural groupings whose existence in the world is not dependent upon human interests or activities, whereas artificial kinds pick out groupings whose existence in the world is dependent upon human interests or activities. However, others have provided substantial evidence challenging this claim, arguing that there are at least some natural kinds that are dependent upon human activities and practices for their existence.

In addition to questions concerning what qualifies as naturalness in natural kinds and what is the distinction between natural and artificial kinds, philosophical discussion also focuses on the metaphysics of natural kinds and the epistemic value of natural kinds. A perennial question widely debated is whether the classifications used in scientific disciplines – physics, chemistry, biology, neuroscience, geology, linguistics, anthropology, and more –really do map on to a natural classification that really exists in the world. That is, are the ways we partition elements in chemistry, organisms in biology, or quarks in physics, the same partitionings that naturally exist? A lot of the literature on natural kinds relies on using examples that are thought to be quintessential natural kinds, like biological species and chemical elements. But others argue that there is clear evidence that many, if not most, biological species and chemical elements are not natural kinds, especially if membership within a natural kind requires possession of an essential property. Within the discussion of natural kinds, there are also questions with regard to the conditions of membership that challenge the view that natural kinds membership is determined by the possession of a particular essence. Instead of the possession of a particular essence, some argue that membership in a natural kind may instead be determined by the possession of a cluster of properties, a relationship, or something else. In many of these discussions, Plato’s metaphor of carving nature at its joints is used to describe the mapping of natural classifications onto natural kinds by the implied comparison to the butchering of an animal along its natural divisions (knuckles, limbs, etc.) rather than partitioning it in a way that does not coincide with the animal’s body structure. Whilst the metaphor helps explain the nature of natural kinds, it does so by assuming nature is that which is pre-partitioned.