For example imagine I told you I own a pencil. Would you ask me to prove it or doubt me? Even you don't know or trust me, probably not. Why? Because pencils exist, it's easy to get one, and I have no reason to lie (in this scenario).
Now what if I told you I have an invisibility cloak. You'd call me a liar and want proof, hell probably an in person demonstration. Think about why.
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"I'm sure that a century ago, space was just something we would never reach."
Whether or not aliens exist is not the same as not reaching space. Say it's the year 1876-
1. Aliens may or may not exist, it's unknown
2. it's a true fact that humans haven't gone to space.
I get that you mean the amount of technological advancment in the last 300 years has been crazy.
The reason the burden of proof is on whether something occurred or exists is because the number of events that haven't occured and what doesn't exist is infinite.
Finally you also ask about who is to say and I assume you also mean "why not". There's always a downside to believing false things, maybe it's small for some people or maybe a crazy person adds aliens to the pile of "government lies" then decides to shoot up some government agency.
It's usually not a 50/50 percent probability on whether something is possible or exists.
ECREE is a good rule to follow when trying to come up with probabilities lacking direct evidence. https://en.m.wikipedia.org/wiki/Sagan_standard
For example imagine I told you I own a pencil. Would you ask me to prove it or doubt me? Even you don't know or trust me, probably not. Why? Because pencils exist, it's easy to get one, and I have no reason to lie (in this scenario).
Now what if I told you I have an invisibility cloak. You'd call me a liar and want proof, hell probably an in person demonstration. Think about why.
-------------- "I'm sure that a century ago, space was just something we would never reach."
Whether or not aliens exist is not the same as not reaching space. Say it's the year 1876- 1. Aliens may or may not exist, it's unknown 2. it's a true fact that humans haven't gone to space.
I get that you mean the amount of technological advancment in the last 300 years has been crazy.
The reason the burden of proof is on whether something occurred or exists is because the number of events that haven't occured and what doesn't exist is infinite.
Finally you also ask about who is to say and I assume you also mean "why not". There's always a downside to believing false things, maybe it's small for some people or maybe a crazy person adds aliens to the pile of "government lies" then decides to shoot up some government agency.