I’m not going to lie; initially I stupidly assumed it because it felt like a natural thing to assume. A biologically encoded assumption. I think all vertebrates assume this the moment they realise they’re receiving stimuli like the ones we receive.
Then in my teen years I became a hardcore sceptic agnostic philosopher kiddo and started questioning everything and then some, very certain that nothing is certain, including the notion that nothing is certain.
And now I am content with the position that while the existence of a world outside my mind is not completely entirely abso-freaking-lutely 100% certain, it’s still probable enough to make it reasonable to assume so.
The facts are simple – I am receiving stimuli. More interestingly, the stimuli I receive are not random, but highly coherent stimuli that always match exactly with the “there’s a world out there” explanation that seems to be hardwired into my brain.
Simplifying the possibilities into an ad-hoc dichotomy, I’m going to say that either there is a world out there that is the origin of all these stimuli, or they come from somewhere else. If the world I imagine them to come from is real, that’s a good explanation why the stimuli always match with that model. If there is not a world out there, then there must be some other origin(s) for the stimuli, plus a reason why the stimuli behave in this particular way.
Maybe that counts as an argument from lack of imagination, but I can’t think of any other way to explain the stimuli I’m receiving that is not more far-fetched than postulating the existence of a world out there.
I’ve seen the Matrix, thank you. That would be a more far-fetched explanation. Instead of postulating one complicated world, you’d postulate one equally complicated cyber-world embedded into another complicated world.