Richard Dawkins would say that 'human', 'mind', and 'tree' are
intellectual pigeon holes rather than natural kinds. Check out Chapter
10 of "The Blind Watchmaker." (I just pulled my copy off the shelf and
will be rereading that chapter tonight. I'll say more on the topic
tomorrow.) We use the word 'atom' to lable the interactions between
sub-atomic entites which are anything but discrete objects with clearly
delineated boundaries. Memes seem to have less of a claim to being a
natural kind than any of the other things you named. Talking in terms of
memes allows us to identify patterns in events which might otherwise seem
unrelated. It allows us to pick out causal processes which would
otherwise remain opaque, but it's pretty hard to point at a meme and say,
"You're telling me that THAT RIGHT THERE isn't an instantiation of a
natural kind?" You could point to a human and say that, and my answer
would be, "It's useful to see it as such, but the boundaries which
determine what fits in that catagory and what doesn't are still a product
of our interests."
>Here's an analogy I find useful: Say that objective reality is a
>person and our model (theory, description, belief) about reality
>is a portrait of that person. There is no end to the number of
>different portraits that can be created: pencil sketch, oil painting,
>bronze bust, chalk drawing, marble statue, photograph, short story,
>X-ray picture, home video, artfully arranged vials of bodily fluids,
>caricature, mug shot, JPEG, 3-D computer model, Myers-Briggs type,
>etc., etc. They can be more or less accurate (an X-ray of the person
>is certainly more accurate than a drawing of a totally different
>person)
>and they can be more or less useful depending on the task at hand
>(a mug shot is more useful than a painting if you want to find the
>person, but a painting is more useful if you want to decorate your
>house). But which portrait is the True one? I hope you can see how
>the question doesn't even make sense, no matter how skillful and
>talented
>the artist, no matter what materials are used, a portrait can never
>become the portrayed. And the same is true of our models, beliefs
>and theories.
Good analogy.
Take care. -KMO