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The Münchhausen Trilemma is a problem in philosophy that all statements can be questioned and then need evidence. This problem has been well known in philosophy for thousands of years, but rarely gets addressed because it breaks the legs of philosophy, science, and any other possible approach to reality.
 Who was Münchhausen?
"Karl Friedrich Hieronymus, Freiherr von Münchhausen" was a German nobleman with a reputation for storytelling. Somewhat to his annoyance, he inspired a fictional collection of tall tales, by various authors, supposedly told by one Baron Munchausen. In one of those anecdotes, he claims to have pulled himself, and the horse he was sitting on, out of a swamp by his own hair.
 The problem
Some people would say that all statements need evidence to support them. Others would say that some statements can follow out of other statements. Most people belong to both categories, but all face one problem: what if there's no evidence for a fundamental statement and that statement also doesn't follow out of any other statements? Well as far as we know there are three options:
- Circular reasoning: X is because of Y, Y is because of X. We arrive at a state where statements make each other true. This has the inherent problem that a logical fallacy would serve as the basis of otherwise logically sound theories, which isn't all that bad but gives most people a stomach ache.
- Argumentum ad infinitum: The reasoning for a single statement regresses endlessly. While being the scientifically most sound option, this puts a bit of a damper on all discussions as humans tend to die at some point and occasionally need sleep.
- Self-evident argument: The argument is so clearly true that no supporting evidence is needed. This is practiced most of the time, but sometimes gets out of hand or one ends up insulting somebody's intelligence or arguing that most people know this to be true, therefore it is true.
There's also a less known fourth option:
 So how's that?
On multiple occasions in history, it has been asked how we can know for certain what is true and what is not. For example, it is possible that rather than being many billions of years old as the scientific evidence strongly suggests, the universe was actually made last Thursday. It is also possible that the entire world is the creation of your own mind, like a dream it takes you several decades to wake up from.
Attempts to attach any objective importance to speculations on this topic may alternatively be called bullshit.