Skepticism is a fun way to think. Global skepticism is a philosophical stance according which we can know pretty much nothing. Everyone who is not a skepticist tries, of course, to show that the skepticist is wrong. I have not seen a tight proof along those lines, as of yet. I think it is pretty futile to try, due to skepticism being right. There is precisely one thing we can know with certainty, though reasoning must be assumed to work (if it doesn’t, this entire exercise if futile, but so are any and all responses and claims of futility, so I find myself justified in assuming that reasoning does work).
The argument goes thusly: I think, therefore a thought exists.
Do note that “I” does not necessarily exist. Or the thought might be momentary; time and other measures of change may be illusions.
The skeptic can’t really touch that argument. Neither is it particularly strong argument. The existence of a world in which we live would be nice to know, for example, and would be a lot stronger claim. I just can’t figure out a way to prove it without nontrivial assumptions. This is why I will assume it and the capability of know things about it. There are further justifications for that assumption, gratefully.
It is a fact that I perceive something around me. I will call the immediate source (as opposed to the ultimate or final source, if there is such) of these perceptions a world or a reality or some word that is practically synonym thereof.
If there actually is no world, I will lose little by assuming it, because I can’t perceive whatever else there may exist (otherwise it would, by definition, be part of world, which is a contradiction). If there exists a totally irrational and random world (defined as one about which useful knowledge can’t be gained), I likewise lose little, because no matter what I assume or don’t assume, there is nothing useful I can know about it. At least trying to find patterns keeps me well amused. If a world about which something useful can be known exists, it is smart to assume so, because it is true. A world which works so that all human assumptions about it are false is contradictory, because one could assume that all human assumptions about it all false, which leads a and not a, where “a” means “all human assumptions about it are false”.
Hence, it is justified to assume that there is a world about which one can know things.