Arguably if you want to start getting technical, it seems hard to establish the existence or nature of anything other than the experience you are currently having right now at this exact moment.
There seems to be a distinction between "physical" and "mental" reality.
But the problem is you can never know "physical" reality except through "mental" reality.
You see this with Descartes.
A more appropriate form would be "I am thinking; Thinking exists".
But to make the leap from thinking to anything else is much more difficult.
E.g. say a glass bottle. You may perceive and experience a glass bottle but that doesn't say anything about the existence of the glass bottle outside of your perception.
It's like a paradox. How can you ever know about anything outside your own mind when your only means of interacting with it is your own mind?
It has already been shown that the human mind can conceivably experience a false reality.
Just look at the brain in the vat thought experiment:https://en.wikipedia.org/wiki/Brain_in_a_vat
Just think about when you are having a dream. Also realize that when you are having a dream you don't know that it's a dream, you think it's real. You are experiencing something in the conventional sense that "isn't real". It's just in your mind and when you wake up you realize that.
Whatever happened in your dream was just a illusion in your own mind it doesn't translate to "real life". "Real life" is meant to be the actual reality as separate from your mental reality.https://en.wikipedia.org/wiki/Dream_argument
The problem is there is nothing other than mental reality in the first place. A physical reality may exist but if there is no consciousness then the physical reality wouldn't matter because there would be no experience of it.
It's like the saying "if a tree falls in a forest and no one is around to hear it, does it make a sound?"https://en.wikipedia.org/wiki/If_a_tree ... n_a_forest