You have to remember that reality is a subjective concept.

It's all based on personal perception - and we typically agree that what most people think is real is in fact reality.

That's how we judge people to be psychotic meaning a break with reality that includes delusions, paranoia & hallucinations.

Because marijuana reacts with your neuro-receptors & brain chemistry & that's how you perceive reality, it's a no-brainer to say that it alters your perceptions of reality.
So do many other things from herbal supplements to prescription drugs.

It's a matter of science - whether you believe in a soul, that you're one with the universe or you're god incarnate makes no difference.