Reality is only the result of our chemical perception to it.

Ergo it is variable to change:

Human minds are bound, unaware, by human instinct.

How does one know that others don't view the world in a completely different way?
Having only ever known their own perceptions of reality?

Through this we see that what we perceive as reality is only based on our own inbuilt idea of "reality" and our notion that it is the same for everyone else.

You may hold something in your hand and say 'this is real'

But it is only the chemicals in your brain telling you its real;

Change those chemicals and your perception of reality changes.

Through this:
the definition of 'reality' becomes variable to change.

We see our most prominent examples in schizophrenic patients and users of hallucinogenic drugs:

The disorder or the drug creates an imbalance in the persons chemical reaction to the world. Changing their perception of the world and allowing them to perceive their hallucinations as 'real'

Ergo 'reality' and our perception of it becomes subject to change depending on how our brains perceive our world through chemical reaction.

~Smog