To paraphrase...
"Your perception determines your reality."
However...
"You can twist perception, but reality won't budge."
We can only understand reality through our own perceptions, and our reasoning will influence our perceptions greatly. However, the nature of reality is constant. It is what it is, whatever it is, whether or not we perceive it accurately. Of course, it doesn't matter, because reality to the individual is always filtered through our perceptions. Each person's take on reality is therefore unique.
Whether or not an individual's take on reality manifests into reality is another subject entirely. It is certainly proven that if we have the will to make our will manifest, we certainly can through physical effort. The extent to which one can affect reality through faith, belief, or simple assumption of reality would be difficult to measure.