The argument from illusion is a set of ideas that propose what we perceive is not truly reality. We'll illustrate a simplified version of the argument with an example:
Bob is in a room, looking at what appears to be a blue box behind a glass wall. In Bob's mind, he has categorized the object as having a particular form -- a box -- and a particular color -- blue. But in reality, the box is just a folded piece of cardboard set against a mirror to give it the appearance of a box. In addition, the cardboard is white -- a blue light focused on the cardboard gives it the appearance of being blue.
This example shows that Bob thinks he is seeing one object when, in reality, he's looking at something else. The argument from illusion says that because Bob has no way of distinguishing between what he perceives and what's real, we can extrapolate this to mean there is no way to be sure anything we perceive is not just an illusion. Our own point of view is what some philosophers call "naïve realism" [source: Stanford Encyclopedia of Philosophy].
The argument from hallucination is similar to the argument from illusion. If it's possible for a subject to hallucinate a situation or object that seems real but doesn't exist independently from the subject's mind, then perception is unreliable and reality indefinable. Not all philosophers subscribe to this argument. Many take issue with the assumption that such hallucinations are really possible and indistinguishable from normal perception.
The debate continues in philosophical circles over whether perception and reality are one and the same. We do know that it's easy to trick the brain -- stage magicians and mentalists rely on that fact to make a living. But does that mean the mind is fundamentally unreliable?