I was going to post this to a phil community I found, but for some reason its not working. So I'll post it here and try again later.
I'm a little confused about what exactly it means for something to be a natural kind term. I have a basic idea, but I don't know to what extent this idea is flawed/incomplete.
As best I can see, a natural kind concept is something such that any instance of this concept in nature is a perfect example of any other instance of that concept. A non-natural kind concept is something whose various instances are not always identical, so long as they fall under basic guidelines of the necessary and sufficient application conditions of that concept.
For example, water is a natural kind term because if you were to produce a glass of water, anything you said about the water in that glass could justifiably said about any other instance of water in your environment. (except of course the property of being in a glass or in your possession, etc.) For example, you could look at that water and determine that water is a clear liquid.
Something else, like a couch, is not a natural kind term because you could not look at an instance of a couch and determine anything about the nature of other couches in your environment. Were you to encounter enough couches, you would eventually determine what is required for an object to be considered a couch, but that wouldn't allow you to deduce any knowledge of other couches from seeing a random love-seat. Things like jade are also non-natural kinds for the same reason. Jade is a name applied to two chemically distinct substances, jadeite and nephrite. So if you were looking at a chunk of jade that just so happened to be nephrite, you couldn't say that based on your observation of this one chunk, all jade is nephrite.
A flaw I just noticed in my definition is that there do appear to be different kinds of water in the environment that would prohibit you from being able to determine facts about water from an encounter with a particular instance. For example, if you were to be confronted with a glass of ice-water, and you might be inclined to say that all water is tasteless. If however, you were confronted with a glass of saltwater, you would be inclined to say that all water tastes salty. When someone is thinking about the concept of water, does this concept only include only fresh 100% pure (liquid?) H20? or does it also include saltwater and river water and ice, etc? If not, would a person looking at a lake and thinking that water is wet have a different thought content from someone looking at the ocean and thinking that water is wet?
So this is what I've got let me know your thoughts.
Thanks!