The Babel fish is small, yellow, leech-like, and probably the oddest thing in the universe. It feeds on brain wave energy, absorbing all unconscious frequencies and then excreting telepathically a matrix formed from the conscious frequencies and nerve signals picked up from the speech centres of the brain, the practical upshot of which is that if you stick one in your ear, you can instantly understand anything said to you in any form of language: the speech you hear decodes the brain wave matrix. It is a universal translator which simultaneously translates from one spoken language to another. It takes the brainwaves of the other body and what they are thinking then transmits the thoughts to the speech centres of the host's brain, the speech heard by the ear decodes the brainwave matrix. When inserted into the ear, its nutrition processes convert unconscious sound waves into conscious brain waves, neatly crossing the language divide between any species.
The book points out that the Babel fish could not possibly have developed naturally, and therefore both proves and disproves the existence of God:
Now it is such a bizarrely improbable coincidence that anything so mindbogglingly useful could evolve purely by chance that some thinkers have chosen to see it as a final and clinching proof of the non-existence of God. The argument goes something like this:
"I refuse to prove that I exist," says God, "for proof denies faith, and without faith I am nothing".
"But," says man, "the Babel fish is a dead giveaway, isn't it? It proves you exist and so therefore you don't. QED."
"Oh dear," says God, "I hadn't thought of that," and promptly vanishes in a puff of logic.
"Oh, that was easy," says man, and for an encore goes on to prove that black is white, and gets killed on the next zebra crossing.