There is a set part of reality as a whole that it is possible to know exists. I define this part in five steps.
1: That which exists exists.
If I don't include this tautological statement, I risk getting my argument derailed by someone claiming nothing is true, which would simply eat up time and prove rather pointless to argue against, as by definition of that position, they cannot be wrong as there is no right. Moving on...
2: I think, therefor I exist.
As I, by thinking, attain at least that one property which is not included in the properties of non-existence, I must exist. I think therefor I am is an extremely powerful statement which can only be argued against if you believe you might not exist, and hopefully anyone who believes that would not have gotten past the first step.
3: I have access to a perception of reality. This perception must therefor exist either as an independent entity, or as a part of myself.
As long as I can be said to exist, my perception of reality must also exist. This doesn't mean it's accurate independent of myself. But whether I'm a human being on Earth, or a brain in a jar on SR388, I still have some perception of what I would call reality. Whether or not my reality is independent of myself is irrelevant to my worldview, as I have no way of telling for sure one way or the other.
4: All that I can observe directly or indirectly is part of my perception of reality, and must therefor exist either as an independent entity, or as a part of myself.
Essentially, anything I can note the existence of must exist. Even if I hallucinated a flying teapot, that teapot would exist in some way. Hallucinations certainly exist, they are just limited in who can observe them barring a level of neuroscience we currently do not have access to. Even if I know something to be a figment of my imagination, I cannot say that it does not exist. Rather, I can say it exists solely within myself and/or the physical structure that makes up myself (which may only exist in my mind as well, but that's already covered honestly).
5: I cannot objectively and conclusively say that anything I cannot observe directly or indirectly exists.
To put it more simply, I only know for a fact that reality exists as part of my perception of it. I believe that it exists independently of myself, but that's a matter of faith to be honest. If there's an invisible, intangible unicorn floating five feet in front of my face that cannot interact with the observable universe in any way, I can't say that I know it exists. Whether or not it does exist is irrelevant as far as my reaction to it, and it doesn't effect my worldview in any way.
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