Last Thursdayism is a philosophical theorem that can be traced back to the 1990s. This theorem satirically exposes a premise espoused primarily in Judeo-Christian creationism: creationists theorize that the world, which scientists interpret as several billion years old, was created with all aspects of that age in one fell swoop only a few millennia ago.
In Last Thursdayism, this theory is satirically exaggerated: The entire world as we perceive it at the moment was actually created only last Thursday - together with all people and their memories, which make the world seem much older to them. Thursday itself has only a symbolic meaning and is supposed to represent that it happened "recently".
The religious premise under attack is known in research as the Omphalos hypothesis, named after Philip Henry Gosse's book "Omphalos: An Attempt to Untie the Geological Knot" (London 1857), which, two years before Charles Darwin's work "On the Origin of Species", attempted to explain the fossils, which by now were thought to be evidence of a longer history of the earth, as objects of divine creation. God, they argued, had created the world along with these fossils, as well as with everything that science considered to be evidence of a higher age of the earth.
Although Gosse's original Omphalos hypothesis specifies a popular creation story, others have suggested that this idea does not rule out a creation only five minutes ago, including memories of times before that creation in place. This idea is sometimes referred to by its opponents as Last Thursday ideology, along the lines of, "The world might as well have been created last Thursday."
The philosophical aporia whereby we might theoretically imagine the past in error is familiar as a dream experience and part of the more complex solipsism model. It is played out in Bertrand Russell's The Analysis of Mind: "There is no logical impossibility in the hypothesis that the world sprang into being five minutes ago, exactly as it then was, with a population that 'remembered' a wholly unreal past."
From a scientific viewpoint, however, the concept is not testable by any conceivable scientific study and is thus not falsifiable - in other words, it is impossible to conclude that the hypothesis is true, since it presupposes that the empirical data themselves have been arbitrarily created to look like they do at every observable level of detail.
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