i had thought that if the brain were so biological that it would have a sort of warning in it when it was providing its own sensory stimulation. not to mention i have to wonder how can it do that? how could it project something that would appear to be coming in from our eyes when it really wasnt? especially if it made the kinds of things people tell me they see when they take certain drugs-things that they have never seen before in real life, so how would the brain be able to manufacture those false perceptions when it had no memory to use and no frame of reference? and then be so bizarre as to not know it was doing it?
The example of drugs shows how both is possible:
Some drugs totally cut you off from reality one hundred percent, others don't.
For example you can deal with a horror trip on LSD remembering that "it's not real". This is what people on a horrortrip necessarily do.
Some drugs are not so userfriendly though.
Thornapple (scopolamine and adropine) will completely take away your memory of having taken a drug at all.
So while people wonder why you are naked in the street, having a discussion with a plant, for you everything seems totally normal. You wouldn't be suprised about a plant talking to you in a dream, so why should you now? And that's what you actually do: You dream while being awake.
When you know that you have taken LSD your brain normally works good enough to compare the input with the normal patterns of reality. You will (depending on the dosage) not have a problem to realize that the demonlike red glowing of the woman's eyes who is talking to you is far from being a typical pattern of your reality.
So you will easily be able to tell it's an illusion.
It may not always be so easy however. For example it may become difficult to tell if you are looking at a pile of fabric or a dead animal. Both is at least possible.
Personally i prefer a three stages kind of model for explaining the way we perceive reality:
First of all there is an outside reality. We know almost nothing about what it is like. This is the first stage of reality, Kant's world of things where the thing in itself does actually exist.
The second stage of reality is created by the means of informaton processing: A tiny part of the outside reality can be perceived by our senses. A tiny range of electromagnetical waves e.g. is sensed as light (and colors), particular waves created by events will be perceived as sound, and so on. Any of this information enters an extremely complex system of cameras, mirrors and filters which is your whole nervous system from your brain to your finger tips. Whatever happens in this (pretty huge) system is like a black box.
These cameras, mirrors and filters finally create the third stage of reality: Like in a cinema the pictures are projected to the cinema screen which is called our consciousness. You can imagine the screen like the inside of our skull. The pictures on the cinema screen are in fact the only thing we can refer to when we try to recognize reality.
We will never be able to see reality itself, the only thing we will ever see is the movie on the inside of our skull.
The second stage, at which the information is being processed in the black box is so relevant, that the output comes with a huge number of elements that haven't really been there in stage one.
For example when you go for a walk in a forest seeing the pictures: forest - man - woman - girl - little dog will be summed up to the output: Family.
On the inside of your skull there will be a family going for a walk. This may be close to the actual outside reality or it may not.
Now when you see a picture like the one provided by VideCorSpoon, a reprocessing takes place. That's nothing unusual actually. Something might make us wonder: Was that really a family? Maybe the guy was black and she was asian while the girl looked european. Something breaks up the common patterns.
The mentioned picture breaks up the common patterns when you print it and it still moves. Your experience tells you that it's impossible that a drawn picture moves in any way. so you question what you see and reprocess it.
You look at the projection on the inside of your skull and say: "I see it moving but i know that this is only the result of my mirrors and cameras being fooled."