If this is obvious, why don't you try Reality.
Human perception is based on an intricate process chain. First, the perception organ (eye, ear, tongue, etc.) receives input. This can be light reflecting off something and absorbed by the retina, sound transported by waves in the air, a chemical reaction on the tongue, etc. These in turn trigger nerve ends which transmit signals. Some of the signals are received by the brain which - and here it becomes a bit tricky - interprets the signal. This interpretation is based on a whole plethora of factors including experience, habit, immediately preceding signals, cross over with perceptions by other organs, status of the involved organs, the nerves etc. A highly complex thing. After this an act of recognition occurs. This is what happens in the mind. And remember that the mind is not the brain.
Picking on one perception specifically: We are very visual beings. There even is a saying that "Seeing is Believing". Now, this is interesting because believing is not knowing. Correct?
Another one says "You need to see something to believe it; visible facts cannot be denied." Any illusionist will teach you better. Plus all of the above processes chaining to produce a result should make us wary of our own perceptions. We think we can rely on our perceptions but actually, scientifically proven and undeniably: We cannot. All we have is sensory input of dubious accuracy and precision which is transmitted through biological relays underlying wear and tear and then interpreted by our mind based on previous experience.
http://www.newscientist.com/article/mg21729052.300-the-self-the-one-and-only-you.html?page=2 ...unfortunately this is not available for free. I will try to find replacement.
Consider a simple case, the "beta phenomenon" (see diagram and video above). If a bright spot is flashed onto the corner of a screen and is immediately followed by a similar spot in the opposite corner, it can appear as if there was a dot moving diagonally across the screen. This is easily explained: the brain often fills in elements of a scene using guesswork. But a tweak to this experiment produces a curious effect.
If the spots are different colours – for example a red spot followed by a green spot – observers see a moving spot that changes colour abruptly around the mid-point of the diagonal (see "Spotted trick"). This is very peculiar. If the brain is filling in the missing positions along the diagonal for the benefit of the self in the theatre, how does it know before the green spot has been observed that the colour will switch?