Bertrand’s reference is the Invisible Pink Unicorn
The Invisible Pink Unicorn (IPU) is the goddess of a parody religion used to satirize theistic beliefs, taking the form of a unicorn that is paradoxically both invisible and pink. She is a rhetorical illustration used by atheists and other religious skeptics as a contemporary version of Russell's teapot, sometimes mentioned in conjunction with the Flying Spaghetti Monster.
The IPU is used to argue that supernatural beliefs are arbitrary by, for example, replacing the word God in any theistic statement with Invisible Pink Unicorn. The mutually exclusive attributes of pinkness and invisibility, coupled with the inability to disprove the IPU'...