I had this AHA moment after reading the article "Unclassified" in the June 2010 issue of Discover magazine, where I was surprised to learn that there is no accepted universal definition of a biological species; there are at least 20+ competing definitions. I had thought that "being able to breed fertile offspring" was the definition, but that is only one (and of course it leaves out the vast majority of living things on earth that reproduce asexually).
After having read about all the conflicting ways to organize and cluster individuals into species, each one with its own way of looking at things, it begged the question: Why do you want to know? I.E. What is the purpose of knowing which species something is?" Depending on why you want to know, you choose one definition over all the others.
But of course as Darwin thought, this would mean that species are not "real". Instead of discovering pre-existing forms, we would merely be inventing arbitrary sets of attributes-in-common. Therefore, unlike the "teleology" of Plato and Aristotle where the "purpose" or "goal" of a species is internal to itself, it would seem that a possibly more important purpose is the one that WE have in wanting to place a particular into that species.
For example, Ponder all of the various shapes, sizes, forms, etc of things that you would want to call a "chair" [and do a Google image search of "chair"]. Now, ponder coming up with a universal definition of chair (such that all chairs would be recognized as such, and nothing else would), and you will see that it will be much easier if you can refer to the purpose we have for them; i.e. being able to sit (comfortably?) on them. Without that, it is hard to distinguish between a storage box (not a chair) and a storage bench (a chair). [Try it. Do a Google image search for storage box and then storage bench.]
SO, the lesson to learn is: When considering the purpose of a thing as a part of its definition, "purpose" is both its purpose, and our purpose for wanting to recognize one in the first place.