This comes back to having shared goals; presumably the environmentalist and the nuclear engineer have the shared goal of human/environmental safety, so evaluati

This comes back to having shared goals; presumably the environmentalist and the nuclear engineer have the shared goal of human/environmental safety, so evaluation within that context is possible, but because the Harry Potter fan and the Bach scholar don't have shared ideas about what makes good art, each standard of what makes good art is just as good as another. The Bach scholar doesn't seem to have any reasoning behind the assertion that Bach's use of high techniques and synthesis of ideas means that his art is more valuable. On the other hand, because the nuclear engineer and the environmentalist agree on some desired state of the world, the environmentalist, lest he remain ignorant as to what is best for human/environmental safety, should listen. In short: the Harry Potter fan and the Bach scholar don't have a shared idea about the best way to judge art. The environmentalist and the nuclear engineer do have a shared idea of maximizing human/environmental safety. The environmentalist has no reason to listen to the nuclear engineer if the engineer's reason for nuclear power is something like "I like to see the number of reactions go up on the computer screen", because looking at numbers on a screen isn't a shared goal between the two parties.