Comment by TooSmugToFail

I would have bet it’s a Terry Pratchett quote, and it kinda is:

“"The turtle moves," said Didactylos. "The turtle is a giant reptile that swims through space. It doesn't have to stand on anything. Swimming is what turtles do. The idea that it has to stand on another turtle, and that turtle has to stand on another turtle, is just silly. It's turtles all the way down, and that's a logical absurdity."

Small Gods, 1992