The IPU is commonly used to argue that supernatural beliefs are arbitrary by, for example, replacing the word God in any theistic statement with Invisible Pink Unicorn. A quotation from the alt.atheism FAQ sums up this use of the Invisible Pink Unicorn: "The point of this silliness is to prod the theist into remembering that their preaching is likely to be viewed by atheists as having all the credibility and seriousness of [the atheists'] preaching about the IPU.
It has become popular on atheist websites and online discussion forums to feign belief in the Invisible Pink Unicorn both for the sake of humor and to point out what they perceive as a logical flaw of theistic belief. These professions of faith intend to demonstrate the difficulty of refuting avowals of belief in phenomena outside human perception.
The IPU seems to have become notable primarily through online culture: in addition to alt.atheism, where IPU still frequently comes up in discussions, there are now a number of web sites dedicated to her. The earliest known written reference to the IPU was on July 7, 1990 on the Usenet discussion group alt.atheism. Other sources concerning IPU state that she was "revealed to us" on alt.atheism.
The concept was further developed by a group of college students from 1994 to 1995 on the ISCA -based BBS. The students created a manifesto that detailed a nonsensical (yet internally consistent) religion based on a multitude of invisible pink unicorns. It is from this document that the most famous quotation concerning IPUs originated:
"Invisible Pink Unicorns are beings of great spiritual power. We know this because they are capable of being invisible and pink at the same time. Like all religions, the Faith of the Invisible Pink Unicorns is based upon both logic and faith. We have faith that they are pink; we logically know that they are invisible because we can't see them." — Steve Eley
Eley's manifesto also spelled out the more whimsical articles of faith concerning IPUs, such as a fondness for raisin bread (which symbolizes the expanding universe) and the association with lost socks. Eley dubbed himself the "Chief Advocate and Spokesguy" of the religion, naming a succession of other High Priests and Priestesses (HPs), in accordance with a stated theory that the one who writes the gospels is the one with all the real power in any religion, and is never the one martyred. The first of these HPs was Natalie Overstreet, who popularized the above quotation in her Usenet sig.
In 1996 a similar concept—a unicorn that no one can see—was adapted as a teaching device at Camp Quest, the first free-thought summer camp for kids established in the United States, by Dr. L. Wilson. As reported years later in the July 21, 2006 Cincinnati Enquirer, "Campers must try to prove that imaginary unicorns—as a metaphor for God—don't exist. Richard Dawkins alluded to unicorns in this connection in his 2006 book The God Delusion, writing that "Russell's teapot, of course, stands for an infinite number of things whose existence is conceivable and cannot be disproved. [...] A philosophical favorite is the invisible, intangible, inaudible unicorn."
By 2007, the IPU had gained underground ubiquity as a symbol of atheism.
It is common when discussing the Invisible Pink Unicorn to point out that because she is invisible, no one can prove that she does not exist (or indeed that she is not pink). This is a parody of similar theistic claims about God—that God, as creator of the universe, is not subject to its laws and thus not materially detecting him tells us nothing about his existence or lack thereof. (It has likewise been said that trying to find God is like using a metal detector to search for unicorns in one's sock drawer.) The Invisible Pink Unicorn is an illustration which attempts to demonstrate the absurdity of citing attributes and a lack of evidence as proof of a deity's existence. Her two defining attributes, invisibility and color (pink), are inconsistent and contradictory; this is part of the satire. The paradox of something being invisible yet having visible characteristics (e.g., color) is reflected in some East Asian cultures, wherein an "invisible red string" is said to connect people who have a shared or linked destiny.
The IPU and similar ideas have been used as teaching devices in the past. In his essay The Dragon in my Garage from his book The Demon-Haunted World: Science As A Candle In the Dark, Carl Sagan uses the example of an invisible dragon breathing heatless fire that someone claims lives in his garage. The supposed dragon cannot be seen or heard or sensed in any way, nor does it leave footprints. We have no reason to believe this purported dragon exists. This raises the question: How does the claimant know that this is a dragon, rather than, for instance, a cat? For that matter, how can we know that the IPU is pink and has one horn instead of three horns, or none at all?
There are humorous mock-serious debates amongst her "followers" concerning her other attributes, such as whether she is completely invisible, or invisible to most, but visible to those who have faith in her (bearing similarities to "The Emperor's New Clothes"). Some of these debates are quite elaborate and tortuous, satirizing the disputatiousness and intricacy of many religions' theological debates.
"For I did see my unworthiness in Her sight, for I was a sinner, destined forever to spend existence in the presence of the unholy Purple Oyster, waxing his shell and massaging his most wretched and slimy feet. For lo, the Purple Oyster doth truly have feet, and the legs thereof, and the toes thereof, giving him dominion over all the clams of the seas, and allowing him to go unto the children of men, and tempt them unto destruction." — The Revelation of St. Bryce the Long-Winded (Partial), Chapter One, Verses 9 to 11
Adumbrations of Invisible Pink Unicorn commonly show either a fading pink unicorn, or simply nothing. Images representing "sightings" of her, showing an unremarkable image of a place where the invisible being supposedly was "seen", are also commonly presented as part of the joke. There is an Invisible Pink Unicorn logo that was created by frequenters of alt.atheism and adopted by others, and it is featured on T-shirts, coffee cups, and other paraphernalia. One website selling these items describes them as a subtle means for atheists to recognize one another without giving offense to non-atheists; this suggests that she has become a kind of emblem or mascot for atheists, particularly those who frequent online venues.
Epithets to the name of the Invisible Pink Unicorn in jocular discourse usually follow in brackets: "Blessed Be Her Holy Hooves", "Peace Be Unto Her", or "May Her Hooves Never Be Shod", which in turn are often shortened to "bbhhh", "pbuh", or "mhhnbs", respectively. These epithets recall, and are intended to satirize, the religious practice of adjoining epithets to the names of prophets, most famously Muhammad. (See peace be upon him and Islam and veneration for Muhammad.)