The Classical Skeptical Argument is something that Descartes takes as a threat. It goes like this: If I know p, and p logically entails q, then I know q. But if I don't know p then I don't know q. This basically says that if you don't know one thing then everything attached to it you also don't know, which nevertheless seems obvious. What makes it such a striking argument is the first part of the second sentence, "but if I don't know p". If reality becomes something that we all agree on, but still each see it from our perspective, then it is still subjective and not objective. It is not certain. The fact remains that we cannot look at the world outside our own minds, senses, and perceptions, and that makes we can't find out if what we experience and live isn't a dream, an illusion, or a deception. We could be a brain in a vat. We could be a ghost. We could be nothing. We could be part of one nonphysical conglomeration thoughts that all split up and unite as in our perception people live and die. We could have past life. We could be reincarnates. We could be programs in a machine.
There are infinite things to speculate on. Descartes tries to oppose it, he says if nothing there exists a thinking form of us which see the reality we call existence. I don't think this is adequate because supposed our thoughts just happen in the brain, yet if the brain doesn't exist then neither do the thoughts and the cogito fails. Yet there is knowledge at least in what we think we perceive. Natural rules, obvious sensual perceptions, and social conformism. These are what I call knowledge because we have to abide by them in order to be independent, capable, and free as we experience what our senses tell us is existence. There will always be doubt but I think we should overlook it so we can figure out this familiar but questionable world.