Anna: Did you know that rainbows always occur opposite the sun, appearing high in the sky when the sun is low, and low in the sky when the sun is high? The Roman scholar Pliny the Elder claimed that this was so, in the first century A. D. William: His claim cannot be correct. After all, Pliny the Elder wrote that there are tribes of dogheaded people and beings with no heads or necks but with eyes on their shoulders, and said that smearing snails on your forehead cures headaches!
William's argument against Anna's claims about rainbows is most vulnerable to criticism because it illicitly infers that, because Pliny the Elder made some incorrect assertions, Pliny the Elder's assertions about rainbows are also incorrect.