Imagine that a mysterious race of aliens visit you, and leave you a mysterious black box as a gift.

You try poking and prodding the black box, but (as far as you can tell) you never succeed in eliciting a reaction.  You can’t make the black box produce gold coins or answer questions.  So you conclude that the black box is causally inactive:  ”For all X, the black box doesn’t do X.”  The black box is an effect, but not a cause; epiphenomenal; without causal potency.  In your mind, you test this general hypothesis to see if it is true in some trial cases, and it seems to be true - “Does the black box turn lead to gold?  No.  Does the black box boil water?  No.”

But you can see the black box; it absorbs light, and weighs heavy in your hand.  This, too, is part of the dance of causality.  If the black box were wholly outside the causal universe, you couldn’t see it; you would have no way to know it existed; you could not say, “Thanks for the black box.”  You didn’t think of this counterexample, when you formulated the general rule:  ”All X: Black box doesn’t do X”.  But it was there all along.

(Actually, the aliens left you another black box, this one purely epiphenomenal, and you haven’t the slightest clue that it’s there in your living room.  That was their joke.)

(from Zombies? by Eliezer Yudkowsky)