Does Reality Matter?

My question is how much does reality matter? I ask on a few levels, but first let me tell a story. A few years ago I took a trip to NY with a few friends, 8 of us in total. One night 4 of us, 2 guys and 2 girls, sat in my room and talked over a few drinks. We all wound up sleeping in the 2 beds in my room. When I got back home my girlfreind was insisting I had hooked up with the person I slept next to. (How she found out about it prior to me making it home is a story for another day). It was the beginning of the continuous fight that became our end. This is one example of the reality of me not cheating on my girl didn’t matter because she beleived I did.

Another example is clinical drug trials. There are often instances where the group taking the placebo reports having their condition improve in the same manner as the group taking the experimental drug. Their belief in the drug enabled them to improve when the reality was the drug didn’t do anything for them. In other words, reality did not matter. Some aspects of dreams come to mind. I’ve had many dreams that were so realistic they influenced my actions in the real world. How many other scenarios does this apply to?