Ultimately, all our experiences are subjective and we can't be sure that any of them are authentic. Perception is reality. LSD really makes this fact stand out.
Perception isn't reality. Incorrectly perceiving an oncoming car will prove this.
There are perceptions and there are facts. This is true whether the facts are easily knowable or not, and extends as much to the existence of God and other such questions as it does to the sum of two numbers.
