My favorite quote from a calculous professor when I was in college was "Math is
beautiful, but not like a flower."
Even a mathematician needs to appreciate an aesthetic experience- or I bet no
one would bother becoming mathematicians.
But anyway, genius mathematicians and scientists are about as common as
artistic genius. That kid's statement that you can learn to be good in math,
etc. but not art may be true for him, but what he doesn't know is that what's
true for him is not true for everyone. That's about how an 8th grade brain
works. I'm just glad he had a theory of sorts. That's better than if he had
said art isn't a real subject because it doesn't matter. Or because his
parents don't mind if he gets poor grades in art.
I'd probably tell him that some people will not learn to be "good" in art, but
others will not be good in math, or science or whatever; but, everyone
learns to be "better" at whatever subject they apply themselves to.