I like this question, because it is impossible to answer. It reminds me of that old story about three blind men trying to figure out what an elephant is: one man feels its leg, and is sure it's a tree. One feels its trunk and is sure it is a large snake. One feels the elephant's side, and says it is a wall. Now, you could say that "objectively," everyone who can see can agree that it is an elephant. But, even those people are only "there" in the sense that YOU perceive them to be. And how can you ever be sure they are seeing what you are seeing, smelling what you are smelling, and feeling what you are feeling? Perhaps you are all perceiving some quite different, but each calling it "elephant." We have absolutely no way to confirm reality through another person, because we can never leave our own reality to do so.

Many philosophers have opined that the mind is really all there is. As Descartes said, "I think therefore I am." While, reasonably, we may believe that we, and the rest of reality, exists in the most conventional sense of the world, we cannot really prove it. There is nothing you could do to prove that this world really exists, and is not some cosmic "coma dream."

Perception of reality is in ones brain, reality itself is not. While the world may be one way to one person and another to someone else, it's still the same world. For your brain to perceive reality there must be some input and hence an external reality. Other wise, you are much like a computer without a motherboard.

In Plato's dialogue Theaetetus, Socrates considers a number of theories as to what knowledge is, the last being that knowledge is true belief that has been "given an account of" - meaning explained or defined in some way. According to the theory that knowledge is justified true belief, in order to know that a given proposition is true, one must not only believe the relevant true proposition, but one must also have a good reason for doing so. One implication of this would be that no one would gain knowledge just by believing something that happened to be true. For example, an ill person with no medical training, but a generally optimistic attitude, might believe that they will recover from their illness quickly. Nevertheless, even if this belief turned out to be true, the patient would not have known that they would get well since their belief lacked justification. The definition of knowledge as justified true belief was widely accepted until the 1960s. At this time, a paper written by the American philosopher Edmund Gettier provoked widespread discussion. See theories of justification for other views on the idea.