Things in nature can only exist as kinds of things:
What this proposition means is that any sense-observable entity or individual, animate or inanimate, man-made or naturally occurring, presents certain distinguishing characteristics to our senses and understandings, characteristics which it necessarily shares with other individuals. No individual can exist at all without sharing a set of necessary characteristics with other individuals, and together these individuals constitute a kind.
That is the meaning of the proposition, which I claim does not require an argument to be accepted, but only an inventory of our thoughts and perceptions, each individual conducting this inventory for him-or-herself.
The particular table I am seated at now presents certain properties which also belong to other individuals and group these individuals under the kind "table." Whatever a table may be, and even though the concept of a table has been determined by invention and convention, individuals must present to our senses and cognition certain characteristics in order for the sortal term "table" to be legitimately applied to them.
But a table is an "artifact," so it is neither necessary nor sensible to conduct an investigation to discover the properties that tables present in common. Humans have constructed the idea of a table, and the idea (Greek eidon) of a table exists more or less perfectly in the mind of the craftsman before he builds a table.
Artifacts are generally opposed to "natural kinds" in the philosophical tradition that traces its origins to Plato and Aristotle.
A philosophical "realist" about the sense-accessible world defends the position that naturally occurring things animate and inanimate have "natures" or "internal structures" which make them what they are independently of our human "conceptions" or "representations" of them. Thus a partial conception of a naturally occurring kind can be made more complete through empirical investigation, as scientists in the eighteenth century improved our concept of water by discovering its molecular structure.
A realist philosopher would argue that when a human being observes (for example) a flock of starlings, having previously observed blue jays, mockingbirds, and many similar animate beings, "flock" (indeed a constructed sortal) is not the only sortal he uses to organize or cognize his perception of starlings on this particular occasion. No, says the philosophical realist, he also groups them into the species starling and the genus bird: he perceives them as starlings and birds even if he is unacquainted with these terms. Furthermore, argues the realist, this sorting into genus and species is not only natural but inevitable. That is, it is necessary for us to group starlings that way because that is how they really exist, and our perceptual and cognitive apparatus must be adapted to perceive and conceive things as they really are; otherwise our perceptions and cognitions could not be accurate or true.
In recent years a body of evidence has been developed from psychological experiments on young children that human beings naturally group objects or entities in the way that philosophical realism would predict.
I hope you will not take offense if I admonish you that no English speaker has the capacity to use some English word as he chooses and thereby change the meaning of the word in a way that excludes a meaning which is present in the general culture of English speakers.
Thus you write: "I agree that things are either natural or man-made (or man-caused, as in planting an apple tree). However, as I'm using the term 'kinds' to mean man-made categories for organizing 'things,' there are no 'natural kinds' (as you put it). You'll have to clarify what you were saying before I can give much more of a response."
Here you seem to have made your argument depend on your privately determined definition of a common word, "kind," which leaves it a very implausible argument indeed. You cannot credibly assert that you will use the word "kind" to mean only man-made categories in order to conclude against centuries of tradition that there are no natural kinds.
With respect to the standard by which we can judge the truth of a proposition which has pretensions to self-evidence: general acceptance is good but not conclusive evidence, and experimental psychology (as I have said) now provides a somewhat stronger kind of generalization from experience that human beings cannot do otherwise than organize the world of the senses according to the natural categories of genera and species.
While even the evidence from experimental psychology is not conclusive in the sense of deductive certainty, that is not relevant to the sort of evidentiary appeal that I am making in our discussion. I am arguing that a supposed self-evident truth appeals to the reason of the individual rational being. When that individual says, I cannot conceive the falsehood of this proposition, he takes that as evidence that it cannot be false. That is, that individual rational being cannot conceive of any rational being conceiving that proposition to be false.
You, Simplicimus, are able to form the assertion, at the purely verbal level, that you could conceive every individual human being as a kind in his or her own right. I say as kindly as I know how that you are misunderstanding a feature of your own rationality. In fact you cannot conceive every individual human being as one of a kind; in fact you necessarily put any individual that you now call a human being into the species human being and you cannot do otherwise. But the evidence for this must be the conclusion you reach when you examine your own cognition.
Elenchus should always be the goal in a discussion like this (noble elenchus, not self-serving elenchus). An alcoholic who lives in a homeless camp outside of Sacramento and can no longer satisfy his addiction with any strong drink other than vanilla extract will always be an exaggerator, good Simplicimus. But I hope I know my place as a professing Christian; I have not tried to produce the desired elenchus by speaking to you in parables.
Your reasonable friend, Pseudo Dionysus