I'm bad at writing/explaining, but here's my attempt:

Suppose you and a friend are looking at a painting of unknown origin that looks like a bunch of paint was thrown haphazardly at a piece of paper.
"Look," says your friend, "See how most of the red paint is concentrated in that one corner? This was clearly made by the famous artist Pablo Pretentious!"
"But," you protest, "Most of the canvas isn't even painted. There are blotches of white paint all over that look like they just dripped off a brush, and there's splotches of yellow all along the left side, which is the same color as that wall. This just looks like a piece of paper that was left on the ground when this room was remodeled."
"Laymen like us can't expect to understand the mind of a great artist! A Pablo painting is designed in a way that is beyond our comprehension--you may think that Pablo wouldn't paint ugly white dots, but there's no way we can truly understand the artistic decisions that Pablo Pretentious makes."
"But if that's the case," you reply, "Then we can't predict what a Pablo painting would look like, and that means that to us, any painting would have an equal chance of being produced by Pablo. And if any painting has an equal chance of being a Pablo, then seeing this painting doesn't tell us anything--it can't count as evidence either way."
"But I just told you, we know that it's a Pablo painting because of all that red in the corner!"
"You can't have it both ways. Either we have some idea what a Pablo painting looks like, in which case we can make good guesses about this painting's origin based on how it looks, or we don't know what a Pablo painting looks like, in which case no painting can tell us anything."