What is last thursdayism?
Short answer
World was created last Thursday.
Long answer
Last Thursdayism (alternately Last Tuesdayism or Last Wednesdayism) is the idea that the universe was created last Thursday, but with the physical appearance of being billions of years old. Under Last Thursdayism, books, fossils, light already on the way from distant stars, and literally everything (including your memories of the time before last Thursday) were all formed at the time of creation (last Thursday) in a state such that they appear much older.
If the world was created 6000 years ago with the appearance of being made billions of years ago, what is there to stop us from claiming it was made Last Thursday? The debate on whether Last Thursdayism is true has raged ever since last Thursday.
Last Thursdayism is an unfalsifiable hypothesis; It’s an Omphalos hypothesis I.e the world was created recently. It’s possible to dismiss this using Occam’s razor. The idea that world was made Last Thursday is hard to believe as we perceive time to be like a river. One action follows another and we can recall the past. Once we get to next week Thursday, will the same question be posed? That is a troubling thought. It troubling because it makes the past seem like it never happened. The memories, the people in our lives, and history itself would be lies.
If history were a lie, the holocaust never happened, USA didn’t drop any atomic bombs on Japan and other key events never happened. The judicial system would fail as criminals who are incarcerated can get out of jail saying that since the crime that is perceived to have been purported never actually happened, they couldn’t be charged.
So it would be terrifying if it were true. The repercussions would be so bad that ignorance would be better.